The Web of Things is made of entities (Things) that can describe their capabilities in a machine-interpretable Thing Description (TD) and expose these capabilities through the WoT Interface, that is, network interactions modeled as Properties (for reading and writing values), Actions (to execute remote procedures with or without return values) and Events (for signaling notifications).

The main Web of Things (WoT) concepts are described in the [[[WOT-ARCHITECTURE]]] specification.

Scripting is an optional building block in WoT and it is typically used in gateways or browsers that are able to run a WoT Runtime and script management, providing a convenient way to extend WoT support to new types of endpoints and implement WoT applications such as Thing Directory.

This specification describes an application programming interface (API) representing the WoT Interface that allows scripts to discover, operate Things and to expose locally defined Things characterized by WoT Interactions specified by a script.

The APIs defined in this document deliberately follow the [[[WOT-TD]]] specification closely. It is possible to implement more abstract APIs on top of them, or implementing directly the WoT network facing interface (i.e. the WoT Interface).

This specification is implemented at least by the Eclipse Thingweb project also known as node-wot, which is considered the reference open source implementation at the moment. Check its source code, including examples.

Implementers need to be aware that this specification is considered unstable. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation phase should subscribe to the repository and take part in the discussions.

Please contribute to this draft using the GitHub Issues page of the WoT Scripting API repository. For feedback on security and privacy considerations, please use the WoT Security and Privacy Issues.

Introduction

WoT provides layered interoperability based on how Things are used: "consumed" and "exposed", as defined in the [[[WOT-ARCHITECTURE]]] terminology.

By consuming a TD, a client Thing creates a local runtime resource model that allows accessing the Properties, Actions and Events exposed by the server Thing on a remote device.

Exposing a Thing requires: This specification describes how to expose and consume Things by a script. Also, it defines a generic API for Thing discovery.

Typically scripts are meant to be used on bridges or gateways that expose and control simpler devices as WoT Things and have means to handle (e.g. install, uninstall, update etc.) and run scripts.

This specification does not make assumptions on how the WoT Runtime handles and runs scripts, including single or multiple tenancy, script deployment and lifecycle management. The API already supports the generic mechanisms that make it possible to implement script management, for instance by exposing a manager Thing whose Actions (action handlers) implement script lifecycle management operations.

Use Cases

The following scripting use cases are supported in this specification:

Consuming a Thing

Exposing a Thing

Discovery

This specification describes the conformance criteria for the following classes of user agent (UA).

Due to requirements of small embedded implementations, splitting WoT client and server interfaces was needed. Then, discovery is a distributed application, but typical scenarios have been covered by a generic discovery API in this specification. This resulted in using 3 conformance classes for a UA that implements this API, one for client, one for server, and one for discovery. An application that uses this API can introspect for the presence of the consume(), produce() and discover() methods on the WoT API object in order to determine which conformance class the UA implements.

WoT Consumer UA

Implementations of this conformance class MUST implement the {{ConsumedThing}} interface and the consume() method on the WoT API object.

WoT Producer UA

Implementations of this conformance class MUST implement {{ExposedThing}} interface and the produce() method on the WoT API object.

WoT Discovery UA

Implementations of this conformance class MUST implement the ThingDiscovery interface and the discover() method on the WoT API object.

These conformance classes MAY be implemented in a single UA.

This specification can be used for implementing the WoT Scripting API in multiple programming languages. The interface definitions are specified in [[!WEBIDL]].

The UA may be implemented in the browser, or in a separate runtime environment, such as Node.js or in small embedded runtimes.

Implementations that use ECMAScript executed in a browser to implement the APIs defined in this document MUST implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]].

Implementations that use TypeScript or ECMAScript in a runtime to implement the APIs defined in this document MUST implement them in a manner consistent with the TypeScript Bindings defined in the TypeScript specification [[!TYPESCRIPT]].

The ThingDescription type

      typedef object ThingDescription;
    

Represents a Thing Description (TD) as defined in [[!WOT-TD]]. It is expected to be a parsed JSON object that is validated using JSON Schema validation.

Fetching a Thing Description

Fetching a TD given a URL should be done with an external method, such as the Fetch API or a HTTP client library, which offer already standardized options on specifying fetch details.

        try {
          let res = await fetch('https://tds.mythings.biz/sensor11');
          // ... additional checks possible on res.headers
          let td = await res.json();
          let thing = await WOT.consume(td);
          console.log("Thing name: " + thing.getThingDescription().title);
        } catch (err) {
          console.log("Fetching TD failed", err.message);
        }
      

Expanding a Thing Description

Note that the [[[WOT-TD]]] specification allows using a shortened Thing Description by the means of defaults and requiring clients to expand them with default values specified in the [[[WOT-TD]]] specification for the properties that are not explicitly defined in a given TD.

To expand a TD given |td:ThingDescription|, run the following steps:
  1. For each item in the TD default values table from [[!WOT-TD]], if the term is not defined in |td|, add the term definition with the default value specified in [[!WOT-TD]].

Validating a Thing Description

The [[!WOT-TD]] specification defines how a TD should be validated. Therefore, this API expects the {{ThingDescription}} objects be validated before used as parameters. This specification defines a basic TD validation as follows.

To validate a TD given |td:ThingDescription|, run the following steps:
  1. If JSON Schema validation fails on |td|, [= exception/throw =] a {{"TypeError"}} and abort these steps.

Additional steps may be added to fill the default values of mandatory fields.

The WOT namespace

Defines the WoT API object as a singleton and contains the API methods, grouped by conformance classes.

    [SecureContext, Exposed=(Window,Worker)]
    namespace WOT {
      // methods defined in UA conformance classes
    };
  

The consume() method

      partial namespace WOT {
        Promise<ConsumedThing> consume(ThingDescription td);
      };
    
Belongs to the WoT Consumer conformance class. Expects an |td:ThingDescription| argument and returns a {{Promise}} that resolves with a {{ConsumedThing}} object that represents a client interface to operate with the Thing. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Let |thing:ConsumedThing| be a new {{ConsumedThing}} object constructed from |td|.
  4. Set up the WoT Interactions based on introspecting td as explained in [[!WOT-TD]] and [[!WOT-PROTOCOL-BINDINGS]]. Make a request to the underlying platform to initialize the Protocol Bindings.

    Implementations encapsulate the complexity of how to use the Protocol Bindings for implementing WoT interactions. In the future elements of that could be standardized.

  5. Resolve |promise| with |thing|.

Note the difference between constructing ConsumedThing and using the consume() method: the latter also initializes the protocol bindings, whereas a simple constructed object will not have WoT Interactions initialized until they are invoked.

The produce() method

      partial namespace WOT {
        Promise<ExposedThing> produce(ThingDescription td);
      };
    
Belongs to the WoT Producer conformance class. Expects a |td:ThingDescription| argument and returns a {{Promise}} that resolves with an {{ExposedThing}} object that extends {{ConsumedThing}} with a server interface, i.e. the ability to define request handlers. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Let |thing:ExposedThing| be a new {{ExposedThing}} object constructed with |td|.
  4. Resolve |promise| with |thing|.

The discover() method

      partial namespace WOT {
        ThingDiscovery discover(optional ThingFilter filter = null);
      };
    
Belongs to the WoT Discovery conformance class. Starts the discovery process that will provide {{ThingDescription}} objects for Thing Descriptions that match an optional |filter:ThingFilter| argument of type {{ThingFilter}}. The method MUST run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{"SecurityError"}} and abort these steps.
  2. Construct a ThingDiscovery object |discovery:ThingDiscovery| with |filter|.
  3. Invoke the discovery.start() method.
  4. Return |discovery|.

Handling interaction data

As specified in the [[[WOT-TD]]] specification, WoT interactions extend DataSchema and include a number of possible Forms, out of which one is selected for the interaction. The Form contains a `contentType` to describe the data. For certain content types, a DataSchema is defined, based on JSON Schema, making possible to represent these contents as JavaScript types and eventually set range constraints on the data.

The InteractionInput type

      typedef any DataSchemaValue;
      typedef (ReadableStream or DataSchemaValue) InteractionInput;
    

Belongs to the WoT Consumer conformance class and represents the WoT Interaction data provided by application scripts to the UA.

DataSchemaValue is an ECMAScript value that is accepted for DataSchema defined in [[WoT-TD]] (i.e. null, boolean, number, string, array, or object).

{{ReadableStream}} is meant to be used for WoT Interactions that don't have a DataSchema in the Thing Description, only a {{Form}}'s `contentType` that can be represented by a stream.

In practice, any ECMAScript value may be used for WoT Interactions that have a DataSchema defined in the Thing Description, or which can be mapped by implementations to the {{Form}}'s `contentType` defined in the Thing Description.

The algorithms in this document specify how exactly input data is used in WoT Interactions.

The InteractionOutput interface

Belongs to the WoT Consumer conformance class. An {{InteractionOutput}} object is always created by the implementations and exposes the data returned from WoT Interactions to application scripts.

This interface exposes a convenience function which should cover the vast majority of IoT use cases: the value() function. Its implementation will inspect the data, parse it if adheres to a DataSchema, or otherwise fail early, leaving the underlying stream undisturbed so that application scripts could attempt reading the stream themselves, or handling the data as {{ArrayBuffer}}.

      [SecureContext, Exposed=(Window,Worker)]
      interface InteractionOutput {
        readonly attribute ReadableStream? data;
        readonly attribute boolean dataUsed;
        readonly attribute Form? form;
        readonly attribute DataSchema? schema;
        Promise<ArrayBuffer> arrayBuffer();
        Promise<any> value();
      };
    

The data property represents the raw payload in WoT Interactions as a {{ReadableStream}}, initially `null`.

The dataUsed property tells whether the data stream has been disturbed. Initially `false`.

The form attribute represents the Form selected from the Thing Description for this WoT Interaction, initially `null`.

The schema attribute represents the DataSchema (defined in [[WoT-TD]]) of the payload as a {{JSON}} object, initially `null`.

The [[\value]] internal slot represents the parsed value of the WoT Interaction, initially `undefined` (note that `null` is a valid value).

The value() function

Parses the data returned by the WoT Interaction and returns a value with the type described by the interaction DataSchema if that exists, or by the `contentType` of the interaction Form. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If the value of the [[\value]] internal slot is not `undefined`, resolve |promise| with that value and abort these steps.
  3. If the value of the |data| property is not a {{ReadableStream}} or if |dataUsed| is `true`, or if |form| is `null` or if |schema| or its |type| are `null` or `undefined`, reject |promise| with {{NotReadableError}} and abort these steps.
  4. If |form|'s |contentType| is not `application/json` and if a mapping is not available in the Protocol Bindings from |form|'s |contentType| to [[!JSON-SCHEMA]], reject |promise| with {{NotSupportedError}} and abort these steps.
  5. Let |reader| be the result of getting a reader from |data|. If that threw an exception, reject |promise| with that exception and abort these steps.
  6. Let |bytes| be the result of reading all bytes from |data| with |reader|.
  7. Set |dataUsed| to `true`.
  8. If |form|'s |contentType| is not `application/json` and if a mapping is available in the Protocol Bindings from |form|'s |contentType| to [[!JSON-SCHEMA]], transform |bytes| with that mapping.
  9. Let |json| be the result of running parse JSON from bytes on |bytes|. If that throws, reject |promise| with that exception and abort these steps.
  10. Set [[\value]] to the result of running check data schema on |json| and |schema|. If that throws, reject |promise| with that exception and abort these steps.
  11. Resolve |promise| with [[\value]].

The arrayBuffer() function

When invoked, MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If |data| is not {{ReadableStream}} or if |dataUsed| is `true`, reject |promise| with {{NotReadableError}} and abort these steps.
  3. Let |reader| be the result of getting a reader from |data|. If that threw an exception, reject |promise| with that exception and abort these steps.
  4. Let |bytes| be the result of reading all bytes from |data| with |reader|.
  5. Set |dataUsed| to `true`.
  6. Let |arrayBuffer| be a new {{ArrayBuffer}} whose contents are |bytes|. If that throws, reject |promise| with that exception and abort these steps.
  7. Resolve |promise| with |arrayBuffer|.

The check data schema algorithm

To run the check data schema steps on |payload| and |schema:object|,
  1. Let |type| be |schema|'s |type|.
  2. If |type| is `"null"` and if |payload| is not `null`, throw {{TypeError}} and abort these steps, otherwise return `null`.
  3. If |type| is `"boolean"` and |payload| is a falsey value or its byte length is 0, return `false`, otherwise return `true`.
  4. If |type| is `"integer"` or `"number"`,
    1. If |payload| is not a number, throw {{TypeError}} and abort these steps.
    2. If |form|'s |minimum| is defined and |payload| is smaller, or if |form|'s |maximum| is defined and |payload| is bigger, throw {{RangeError}} and abort these steps.
  5. If |type| is `"string"`, return |payload|.
  6. If |type| is `"array"`, run these sub-steps:
    1. If |payload| is not an array, throw {{TypeError}} and abort these steps.
    2. If |form|'s |minItems| is defined and |payload|'s |length| is less than that, or if |form|'s |maxItems| is defined and |payload|'s |length| is more than that, throw {{RangeError}} and abort these steps.
    3. Let |payload| be an array of items obtained by running the check data schema steps on each element |item| of |payload| and |schema|'s |items|. If this throws at any stage, re-throw that exception and abort these steps.
  7. If |type| is `"object"`, run these sub-steps:
    1. If |payload| or |schema|'s |properties| is not an object, throw {{TypeError}} and abort these steps.
    2. For each property |key| in |payload|,
      1. Let |prop| be the value of |key|.
      2. Let |propSchema| be the value of |key| in |interaction|'s |properties|.
      3. Let |prop| be the result of running the check data schema steps on |prop| and |propSchema|. If this throws, re-throw that exception and abort these steps.
    3. Let |required| be |schema|'s |required| if that is an array or an empty array otherwise.
    4. For each |key| in |required|, if |key| is not present in |payload|, throw {{SyntaxError}} and abort these steps.
  8. Return |payload|.

The create interaction request algorithm

For a given ConsumedThing object |thing:ConsumedThing|, in order to create interaction request given a |source: InteractionInput|, |form:Form| and |schema:object|, run these steps:
  1. Let |idata| be a new an {{InteractionOutput}} object whose |form| is set to |form|, whose |schema| is set to |schema|, whose [[\value]] internal slot is `undefined` and whose |data| is `null`.
  2. If |source| is a {{ReadableStream}} object, let |idata|'s |data| be |source|, return |idata| and abort these steps.
  3. If |schema| and its |type| are defined and not `null`, run these sub-steps:
    1. If |type| is `"null"` and |source| is not, throw {{TypeError}} and abort these steps.
    2. If |type| is `"boolean"` and |source| is a falsy value, set |idata|'s [[\value]] |value| to `false`, otherwise to `true`.
    3. If |type| is `"integer"` or `"number"` and |source| is not a number, or if |form|'s |minimum| is defined and |source| is smaller, or if |form|'s |maximum| is defined and |source| is bigger, throw {{RangeError}} and abort these steps.
    4. If |type| is `"string"` and |source| is not a string, let |idata|'s [[\value]] be the result of running serialize JSON to bytes on |source|. If that is failure, throw {{SyntaxError}} and abort these steps.
    5. If |type| is `"array"`, run these sub-steps:
      1. If |source| is not an array, throw a {{TypeError}} and abort these steps.
      2. Let |length| be the length of |source|.
      3. If |form|'s |minItems| is defined and |length| is less than that, or if |form|'s |maxItems| is defined and |length| is more than that, throw {{RangeError}} and abort these steps.
      4. For each |item| in |source|, let |itemschema| be |schema|'s |items| and let |item| be the result of running the create interaction request steps on |item|, |form| and |itemschema|. If this throws, re-throw that exception and abort these steps.
      5. Set |data|'s [[\value]] to |source|.
    6. If |type| is `"object"`, run these sub-steps:
      1. If |source| is not an object, throw {{TypeError}} and abort these steps.
      2. If |schema|'s |properties| is not an object, throw {{TypeError}} and abort these steps.
      3. For each property |key| in |source|,
        1. Let |value| be the value of |key|.
        2. Let |propschema| be the value of |key| in |properties|.
        3. Let |value| be the result of running the create interaction request steps on |value|, |form| and |propschema|. If this throws, re-throw that exception and abort these steps.
      4. If |schema|'s |required| is an array, for each |item| in |required| check if |item| is a property name in |source|. If an |item| is not found in |source|, throw {{SyntaxError}} and abort these steps.
      5. Set |data|'s [[\value]] to |source|.
  4. Set |idata|'s |data| to a new {{ReadableStream}} created from |idata|'s [[\value]] internal slot as its underlying source.
  5. Return |idata|.

The parse interaction response algorithm

For a given ConsumedThing object |thing:ConsumedThing|, in order to parse interaction response given |response|, |form:Form| and |schema:object|, run these steps:
  1. Let |result| be a new {{InteractionOutput}} object.
  2. Let |result|'s |schema| be |schema|.
  3. Let |result|'s |form| be |form|.
  4. Let |result|'s |data| be a new {{ReadableStream}} with the payload data of |response| as its underlying source.
  5. Let |result|'s |dataUsed| be `false`.
  6. Return |result|.

Using {{InteractionInput}} and {{InteractionOutput}}

As illustrated in the next pictures, the {{InteractionOutput}} interface is used every time implementations provide data to scripts, while {{InteractionInput}} is used when the scripts pass data to the implementation.

Data structures used when reading data

When a {{ConsumedThing}} reads data, it receives it from the implementation as an {{InteractionOutput}} object.

An {{ExposedThing}} read handler provides the read data to the implementation as {{InteractionInput}}.

Data structures used when writing data

When a {{ConsumedThing}} writes data, it provides it to the implementation as {{InteractionInput}}.

An {{ExposedThing}} write handler receives data from to implementation as an {{InteractionOutput}} object.

Data structures used when invoking an Action

When a {{ConsumedThing}} invokes an Action data, it provides the parameters as {{InteractionInput}} and receives the output of the Action as an {{InteractionOutput}} object.

An {{ExposedThing}} action handler receives arguments from the implementation as an {{InteractionOutput}} object and provides Action output as {{InteractionInput}} to the implementation.

Error handling

The algorithms in this API define the errors to be reported to application scripts.

The errors reported to the other communication end are mapped and encapsulated by the Protocol Bindings.

Error handling in WoT interactions

This topic is still being discussed in Issue #200. A standardized error mapping would be needed in order to ensure consistency in mapping script errors to protocol errors and vice versa. In particular, when algorithms say "error received from the Protocol Bindings", that will be factored out as an explicit error mapping algorithm. Currently, that is encapsulated by implementations.

The ConsumedThing interface

Represents a client API to operate a Thing. Belongs to the WoT Consumer conformance class.

      [SecureContext, Exposed=(Window,Worker)]
      interface ConsumedThing {
        constructor(ThingDescription td);
        Promise<InteractionOutput> readProperty(DOMString propertyName,
                                    optional InteractionOptions options = null);
        Promise<PropertyMap> readAllProperties(optional InteractionOptions options = null);
        Promise<PropertyMap> readMultipleProperties(
                                    sequence<DOMString> propertyNames,
                                    optional InteractionOptions options = null);
        Promise<undefined> writeProperty(DOMString propertyName,
                                    InteractionInput value,
                                    optional InteractionOptions options = null);
        Promise<undefined> writeMultipleProperties(PropertyMap valueMap,
                                    optional InteractionOptions options = null);
        /*Promise<undefined> writeAllProperties(PropertyMap valueMap,
                                    optional InteractionOptions options = null);*/
        Promise<InteractionOutput> invokeAction(DOMString actionName,
                                    optional InteractionInput params = null,
                                    optional InteractionOptions options = null);
        Promise<Subscription> observeProperty(DOMString name,
                                    InteractionListener listener,
                                    optional ErrorListener onerror,
                                    optional InteractionOptions options = null);
        Promise<Subscription> subscribeEvent(DOMString name,
                                    InteractionListener listener,
                                    optional ErrorListener onerror,
                                    optional InteractionOptions options = null);
        ThingDescription getThingDescription();
      };

      dictionary InteractionOptions {
        unsigned long formIndex;
        object uriVariables;
        any data;
      };

      [SecureContext, Exposed=(Window,Worker)]
      interface Subscription {
        readonly attribute boolean active;
        Promise<undefined> stop(optional InteractionOptions options = null);
      };

      typedef object PropertyMap;

      callback InteractionListener = undefined(InteractionOutput data);
      callback ErrorListener = undefined(Error error);
    

The writeAllProperties() method is still under discussion. Meanwhile, use the writeMultipleProperties() method instead.

Internal slots for ConsumedThing

A {{ConsumedThing}} object has the following internal slots:

Internal Slot Initial value Description (non-normative)
[[\td]] `null` The Thing Description of the {{ConsumedThing}}.

Constructing ConsumedThing

After fetching a Thing Description as a JSON object, one can create a {{ConsumedThing}} object.

To create {{ConsumedThing}} with the {{ThingDescription}} |td:ThingDescription|, run the following steps:
  1. Run the validate a TD steps on |td|. If that fails, [= exception/throw =] {{SyntaxError}} and abort these steps.
  2. Run the expand a TD steps on |td|. If that fails, re-[= exception/throw =] the error and abort these steps.
  3. Let |thing:ConsumedThing| be a new {{ConsumedThing}} object.
  4. Set the internal slot [[\td]] of |thing| to |td|.
  5. Return |thing|.

The getThingDescription() method

Returns the internal slot [[\td]] of the {{ConsumedThing}} object that represents the Thing Description of the {{ConsumedThing}}. Applications may consult the Thing metadata stored in [[\td]] in order to introspect its capabilities before interacting with it.

The readProperty() method

Reads a Property value. Takes as arguments |propertyName:string| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves with a Property value represented as an {{InteractionOutput}} object or rejects on error. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |properties|'s |propertyName|.
  4. If |option|'s |formIndex| is defined, let |form| be the Form associated with |formIndex| in |interaction|'s |forms| array, otherwise let |form| be the first Form in |interaction|'s |forms| whose |op| is `readproperty`.
  5. If |form| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  6. Make a request to the underlying platform (via the Protocol Bindings) to retrieve the value of the Property given by |propertyName| using |form| and the optional URI templates given in |options|' |uriVariables|.
  7. If the request fails, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  8. Let |response| be the response received to the request.
  9. Let |data| be the result of running parse interaction response on |response|, |form| and |interaction|. If that fails, reject |promise| with a {{SyntaxError}} and abort these steps.
  10. Resolve |promise| with |data|.

The readMultipleProperties() method

Reads multiple Property values with one request. Takes as arguments |propertyNames: string sequence| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves with a {{PropertyMap}} object that maps keys from |propertyNames| to values returned by this algorithm. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. If |option|'s |formIndex| is defined, let |form| be the Form associated with |formIndex| in [[\td]]'s |forms| array, otherwise let |form| be the first Form in [[\td]]'s |forms| array whose |op| is `readmultipleproperties`.
  4. If |form| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  5. Let |result:object| be an object and for each string |name:string| in |propertyNames| add a property with key |name| and the value `null`.
  6. Make a request to the underlying platform (via the Protocol Bindings) to retrieve the Property values given by |propertyNames| with |form| and optional URI templates given in |options|' |uriVariables|.
  7. If this cannot be done with a single request with the Protocol Bindings, reject |promise| with a {{NotSupportedError}} and abort these steps.
  8. Process the response and for each |key| in |result|, run the following sub-steps:
    1. Let |value| be the value of |result|'s |key|.
    2. Let |schema| be the value of [[\td]]'s |properties|'s |key|.
    3. Let |property| be the result of running parse interaction response on |value|, |form| and |schema|.
  9. If the above step throws at any point, reject |promise| with that exception and abort these steps.
  10. Resolve |promise| with |result|.

The readAllProperties() method

Reads all properties of the Thing with one request. Takes |options:InteractionOptions| as optional argument. It returns a {{Promise}} that resolves with a {{PropertyMap}} object that maps keys from Property names to values returned by this algorithm. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. If |option|'s |formIndex| is defined, let |form| be the Form associated with |formIndex| in [[\td]]'s |forms| array, otherwise let |form| be the first Form in [[\td]]'s |forms| array whose |op| is `readallproperties`.
  4. If |form| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  5. Make a request to the underlying platform (via the Protocol Bindings) to retrieve the value of the all the Property definitions from the TD with |form| and optional URI templates given in |options|' |uriVariables|.
  6. If this cannot be done with a single request with the Protocol Bindings of the Thing, then reject |promise| with a {{NotSupportedError}} and abort these steps.
  7. If the request fails, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  8. Process the reply and let |result:object| be an object with the keys and values obtained in the reply.
  9. Process the response and for each |key| in |result|, run the following sub-steps:
    1. Let |value| be the value of |result|'s |key|.
    2. Let |schema| be the value of [[\td]]'s |properties|'s |key|.
    3. Let |property| be the result of running parse interaction response on |value|, |form| and |schema|.
  10. Resolve |promise| with |result|.

The writeProperty() method

Writes a single Property. Takes as arguments |propertyName:string|, |value:InteractionInput| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves on success and rejects on failure. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |properties|'s |propertyName|.
  4. If |option|'s |formIndex| is defined, let |form| be the Form associated with |formIndex| in |interaction|'s |forms| array, otherwise let |form| be the first Form in |interaction|'s |forms| whose |op| is `writeproperty`.
  5. If |form| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  6. Let |data| be the result of running the create interaction request steps on |value|, |form| and |interaction|. If that throws, reject promise with that exception and abort these steps.
  7. Make a request to the underlying platform (via the Protocol Bindings) to write the Property given by |propertyName| using |data:InteractionOutput| and the optional URI templates given in |options|' |uriVariables|.
  8. If the request fails, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  9. Otherwise resolve |promise|.

As discussed in Issue #193, the design decision is that write interactions only return success or error, not the written value (optionally). TDs should capture the schema of the Property values, including precision and alternative formats. When a return value is expected from the interaction, an Action should be used instead of a Property.

The writeMultipleProperties() method

Writes a multiple Property values with one request. Takes as arguments |properties:object| - as an object with keys being Property names and values as Property values - and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves on success and rejects on failure. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. If |option|'s |formIndex| is defined, let |form| be the Form associated with |formIndex| in [[\td]]'s |forms| array, otherwise let |form| be the first Form in [[\td]]'s |forms| array whose |op| is `writemultipleproperties`.
  4. If |form| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  5. Let |propertyNames| be an array of |string| with as elements the keys of the |properties| object.
  6. For each |name:string| in |propertyNames|, let |property| be the value of [[\td]]'s |properties|'s |name|. If |property| is `null` or `undefined` or is not `writeable` reject |promise| with {{NotSupportedError}} and abort these steps.
  7. Let |result:object| be an object and for each string |name:string| in |propertyNames| add a property with key |name| and let its value be `null`.
  8. Let |schemas:object| be an object and for each string |name:string| in |propertyNames| add a property with key |name| and let its value be the value of [[\td]]'s |properties|'s |name|.
  9. For each key |key:string| in |properties|, take its value as |value| and run the create interaction request steps on |value|, |form| and the value for |schema|'s |key|. If that throws for any |name|, reject promise with that exception and abort these steps.
  10. Make a single request to the underlying platform (via the Protocol Bindings) to write each Property provided in |properties| with optional URI templates given in |options|' |uriVariables|.
  11. If this cannot be done with a single request with the Protocol Bindings of the Thing, then reject |promise| with a {{NotSupportedError}} and abort these steps.
  12. If the request fails, return the error received from the Protocol Bindings and abort these steps.
  13. Otherwise resolve |promise|.

The observeProperty() method

Makes a request for Property value change notifications. Takes as arguments |propertyName:string|, |listener:InteractionListener| and optionally |onerror:ErrorListener| and |options:InteractionOptions|. It returns a {{Promise}} that resolves on success and rejects on failure. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. If |listener| is not a {{Function}}, reject |promise| with a {{TypeError}} and abort these steps.
  4. If |onerror| is not `null` and is not a {{Function}}, reject |promise| with a {{TypeError}} and abort these steps.
  5. Let |subscription| be a new {{Subscription}} object with its internal slots set as follows:
    • Let |subscription|'s [[\type]] be `"property"`.
    • Let |subscription|'s [[\name]] be the value of |propertyName|.
    • Let |subscription|'s [[\interaction]] be the value of [[\td]]'s |properties|'s |propertyName|.
    • Let |subscription|'s [[\form]] be the Form associated with |formIndex| in [[\interaction]]'s |forms| array if |option|'s |formIndex| is defined, otherwise let [[\form]] be the first Form in [[\interaction]]'s |forms| array whose |op| is `"observeproperty"`.
    • If |subscription|'s [[\form]] is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  6. Make a request to the underlying platform to observe the Property identified by |propertyName| with |form| and optional URI templates given in |options|' |uriVariables|.
  7. If the request fails, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  8. Otherwise resolve |promise|.
  9. Whenever the underlying platform detects a notification for this subscription with a new Property value |value|, run the following sub-steps:
    • Let |reply| be the result of running parse interaction response with |value|, [[\form]] and [[\interaction]]. If that throws, reject |promise| with that exception and abort these steps.
    • Invoke |listener| with |reply|.
  10. Whenever the underlying platform detects an error for this subscription, run the following sub-steps:
    • If the error is irrecoverable and stops the subscription, set |subscription|'s |active| to `false` and suppress further notifications.
    • Let |error| be a new {{NetworkError}} and set its |message| to reflect the underlying error condition.
    • If |onerror| is a {{Function}}, invoke it with |error|.

The invokeAction() method

Makes a request for invoking an Action and return the result. Takes as arguments |actionName:string|, optionally |params:InteractionInput| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves with the result of the Action represented as an {{InteractionOutput}} object, or rejects with an error. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |actions|'s |actionName|.
  4. If |option|'s |formIndex| is defined, let |form| be the Form associated with |formIndex| in |interaction|'s |forms| array, otherwise let |form| be the first Form in |interaction|'s |forms| array whose |op| is `invokeaction`.
  5. If |form| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  6. Let |args| be the result of running the create interaction request steps on |params|, |form| and |interaction|. If that throws, reject promise with that exception and abort these steps.
  7. Make a request to the underlying platform (via the Protocol Bindings) to invoke the Action identified by |actionName| with parameters provided in |args| and optional URI templates given in |options|'s |uriVariables|.
  8. If the request fails locally or returns an error over the network, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  9. Let |value| be the reply returned in the reply.
  10. Let |result| be the result of running parse interaction response with |value|, |form| and |interaction|. If that throws, reject |promise| with that exception and abort these steps.
  11. Resolve |promise| with |result|.

The subscribeEvent() method

Makes a request for subscribing to Event notifications. Takes as arguments |eventName:string|, |listener:WoTListener| and optionally |onerror:ErrorListener| and |options:InteractionOptions|. It returns a {{Promise}} to signal success or failure. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. If |listener| is not a {{Function}}, reject |promise| with a {{TypeError}} and abort these steps.
  4. If |onerror| is not `null` and is not a {{Function}}, reject |promise| with a {{TypeError}} and abort these steps.
  5. Let |subscription| be a new {{Subscription}} object with its internal slots set as follows:
    • Let |subscription|'s [[\type]] be `"event"`.
    • Let |subscription|'s [[\name]] be the value of |eventName|.
    • Let |subscription|'s [[\interaction]] be the value of [[\td]]'s |events|'s |eventName|.
    • Let |subscription|'s [[\form]] be the Form associated with |formIndex| in [[\interaction]]'s |forms| array if |option|'s |formIndex| is defined, otherwise let [[\form]] be the first Form in [[\interaction]]'s |forms| array whose |op| is `"subscribeevent"`.
    • If |subscription|'s [[\form]] is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  6. Make a request to the underlying platform via the Protocol Bindings to subscribe to an Event identified by |eventName:string| with [[\form]], optional URI templates given in |options|' |uriVariables| and optional subscription data given in |options|'s |data|.
  7. If the request fails, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  8. Otherwise resolve |promise|.
  9. Whenever the underlying platform detects a notification for this Event subscription, run the following sub-steps:
    1. Invoke |listener| with the result of running parse interaction response on the data provided with the Event, [[\form]] and [[\interaction]].
  10. Whenever the underlying platform detects an error for this subscription, run the following sub-steps:
    • If the error is irrecoverable and stops the subscription, set |subscription|'s |active| to `false` and suppress further notifications.
    • Let |error| be a new {{NetworkError}} and set its |message| to reflect the underlying error condition.
    • If |onerror| is a {{Function}}, invoke it with |error|.

The InteractionOptions dictionary

Holds the interaction options that need to be exposed for application scripts according to the Thing Description.

The formIndex property, if defined, represents an application hint for which Form definition, identified by this index, of the TD to use for the given WoT interaction. Implementations SHOULD use the Form with this index for making the interaction, but MAY override this value if the index is not found or not valid. If not defined, implementations SHOULD attempt to use the Form definitions in order of appearance as listed in the TD for the given Wot Interaction.

The uriVariables property if defined, represents the URI template variables to be used with the WoT Interaction that are represented as parsed JSON objects defined in [[!WOT-TD]].

The support for URI variables comes from the need, exposed by the [[[WOT-TD]]] specification, to be able to describe existing REST-ful endpoints that use them. However, it should be possible to write a Thing Description that would use Actions for representing this kind of interactions and model the URI variables as action parameters. In that case, implementations can serialize the parameters as URI variables, and therefore, the |options| parameter could be dismissed.

The data property if defined, represents additional opaque data that needs to be passed to the interaction.

The PropertyMap type

Represents a map of Property names as strings to a value that the Property can take. It is used as a property bag for interactions that involve multiple Properties at once.

It could be defined in Web IDL (as well as {{ThingDescription}}) as a maplike interface from string to any.

The InteractionListener callback

User provided callback that is given an argument of type {{InteractionOutput}} and is used for observing Property changes and handling Event notifications. Since subscribing to Events are WoT interactions and might take options or even data, they are not modelled with software events.

The ErrorListener callback

User provided callback that is given an argument of type {{Error}} and is used for conveying critical and non-critical errors from the Protocol Bindings to applications.

The Subscription interface

Represents a subscription to Property change and Event interactions.

The active boolean property denotes if the subscription is active, i.e. it is not stopped because of an error or because of invocation of the stop() method.

Internal slots for {{Subscription}}

A {{Subscription}} object has the following internal slots:
Internal Slot Initial value Description (non-normative)
[[\type]] `null` Indicates what WoT Interaction the {{Subscription}} refers to. The value can be either `"property"` or `"event"` or `null`.
[[\name]] `null` The Property or Event name.
[[\interaction]] `null` The Thing Description fragment that describes the WoT interaction.
[[\form]] `null` The Form associated with the subscription.

The stop() method

Stops delivering notifications for the subscription. It takes an optional parameter |options:InteractionOptions| and returns a {{Promise}}. When invoked, the method MUST execute the following steps:

  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. If |options|' |formIndex| is defined, let |unsubscribeForm| be the Form associated with |formIndex| in [[\interaction]]'s |forms| array.
  4. Otherwise let |unsubscribeForm| be the result of running the find a matching unsubscribe form algorithm given [[\form]].
  5. If |unsubscribeForm| is failure, reject |promise| with a {{SyntaxError}} and abort these steps.
  6. If [[\type]] is `"property"`, make a request to the underlying platform via the Protocol Bindings to stop observing the Property identified by [[\name]] with |unsubscribeForm| and optional URI templates given in |options|' |uriVariables|.
  7. Otherwise, if [[\type]] is `"event"`, make a request to the underlying platform via the Protocol Bindings to unsubscribe from the Event identified by [[\name]] with |unsubscribeForm|, with optional URI templates given in |options|' |uriVariables| and optional unsubscribe data given in |options|'s |data|.
  8. If the request fails, reject |promise| with the error received from the Protocol Bindings and abort these steps.
  9. Otherwise set active to `false` and resolve |promise|.
  10. If the underlying platform receives further notifications for this subscription, implementations SHOULD silently suppress them.

Finding an unsubscribe Form

To find a matching unsubscribe form given |subscribeForm| in the context of a {{Subscription}} object, run the following steps:

    This algorithm is under development and is non-normative. Implementations MAY choose another algorithm to find a matching `unsubscribe` Form to a given `subscribe` Form.

  1. Let |results| be an empty array.
  2. For each |form| in [[\interaction]]'s |forms| array,
    1. Add an internal slot [[\matchLevel]] on |form| and set its value to `0`.
    2. If |form|'s |op| is `"unobserveproperty"` if [[\type]] is `"property"` or if |form|'s |op| is `"unsubscribeevent"` if [[\type]] is`"event"`,
      1. Set the internal slot [[\matchLevel]] on |form| to 1 and add |form| to |results|.
      2. If |form|'s |href| and [[\subscribeForm]]'s |href| are same origin-domain, increment |form|'s [[\matchLevel]].
      3. If |form|'s |contentType| is equal to [[\subscribeForm]]'s |contentType| and |form|'s [[\matchLevel]] is greater than 2, increment |form|'s [[\matchLevel]].
  3. If |results| is empty, return `null` and terminate these steps.
  4. Return the first |form| in |results| that has the highest [[\matchLevel]] value.

ConsumedThing Examples

The next example illustrates how to fetch a TD by URL, create a {{ConsumedThing}}, read metadata (title), read property value, subscribe to property change, subscribe to a WoT event, unsubscribe.

        try {
          let res = await fetch("https://tds.mythings.org/sensor11");
          let td = res.json();
          let thing = new ConsumedThing(td);
          console.log("Thing " + thing.getThingDescription().title + " consumed.");
        } catch (e) {
          console.log("TD fetch error: " + e.message);
        };

        try {
          // subscribe to property change for “temperature”
          await thing.observeProperty("temperature", async (data) => {
            try {
              console.log("Temperature changed to: " + await data.value());
            } catch (error) {
              console.error("Cannot read the observed property temperature");
              console.error(error);
            }
          });
          // subscribe to the “ready” event defined in the TD
          await thing.subscribeEvent("ready", async (eventData) => {
            try {
              console.log("Ready; index: " + await eventData.value());
              // run the “startMeasurement” action defined by TD
              await thing.invokeAction("startMeasurement", { units: "Celsius" });
              console.log("Measurement started.");
            } catch (error) {
              console.error("Cannot read the ready event or startMeasurement failed");
              console.error(error)
            }
          });
        } catch (e) {
          console.log("Error starting measurement.");
        }

        setTimeout(async () => {
          try {
            const temperatureData = await thing.readProperty("temperature")
            const temperature = await temperatureData.value();
            console.log("Temperature: " + temperature);

            await thing.unsubscribe("ready");
            console.log("Unsubscribed from the ‘ready’ event.");
          } catch (error) {
            console.log("Error in the cleanup function");
          }
        }, 10000);
      

The following shows an advance usage of {{InteractionOutput}} to read a property without a {{DataSchema}}.

        /*
        * takePicture affordance form:
        * "form": {
        *   "op": "invokeaction",
        *   "href" : "http://camera.example.com:5683/takePicture",
        *   "response": {
        *     "contentType": "image/jpeg",
        *     "contentCoding": "gzip"
        *   }
        *}
        * See https://www.w3.org/TR/wot-thing-description/#example-23
        */
        let response;
        let image;
        try {
          response = await thing.invokeAction(“takePicture”));
          image = await response.value() // throws NotReadableError --> schema not defined
        } catch(ex) {
          image = await response.arrayBuffer();
          // image: ArrayBuffer [0x1 0x2 0x3 0x5 0x15 0x23 ...]
        }
      

Finally, the next two examples shows the usage of a {{ReadableStream}} from an {{InteractionOutput}}.

        /*{
        "video": {
          "description" : "the video stream of this camera",
          "forms": [
            {
              "op": "readproperty",
              "href": "http://camera.example.com/live",
              "subprotocol": "hls"
              "contentType": "video/mp4"
            }
          ]
        }}*/

        const video = await thing.readProperty("video")
        const reader = video.data.getReader()
        reader.read().then(function processVideo({ done, value }) {
          if (done) {
            console.log("live video stoped");
            return;
          }
          const decoded = decode(value)
          UI.show(decoded)
          // Read some more, and call this function again
          return reader.read().then(processText);
        });
      

Here consider that the JSON object is too big to be read wholly in the memory. Therefore, we use streaming processing to get the total number of the events recorded by the remote Web Thing.

        /*
        * "eventHistory":
        * {
        *   "description" : "A long list of the events recorderd by this thing",
        *   "type": "array",
        *   "forms": [
        *     {
        *       "op": "readproperty",
        *       "href": "http://recorder.example.com/eventHistory",
        *     }
        *   ]
        * }
        */

        // Example of streaming processing: counting json objects
        let objectCounter = 0
        const parser = new Parser() //User library for json streaming parsing (i.e. https://github.com/uhop/stream-json/wiki/Parser)

        parser.on('data', data => data.name === 'startObject' && ++objectCounter);
        parser.on('end', () => console.log(`Found ${objectCounter} objects.`));

        const response = await thing.readProperty(“eventHistory”)
        await response.data.pipeTo(parser);

        // Found N objects
      

The ExposedThing interface

The {{ExposedThing}} interface is the server API to operate the Thing that allows defining request handlers, Property, Action, and Event interactions.

      [SecureContext, Exposed=(Window,Worker)]
      interface ExposedThing: ConsumedThing {
        ExposedThing setPropertyReadHandler(DOMString name,
                PropertyReadHandler handler);
        ExposedThing setPropertyWriteHandler(DOMString name,
                PropertyWriteHandler handler);
        ExposedThing setPropertyObserveHandler(DOMString name,
                PropertyReadHandler handler);
        ExposedThing setPropertyUnobserveHandler(DOMString name,
                PropertyReadHandler handler);

        ExposedThing setActionHandler(DOMString name, ActionHandler action);

        ExposedThing setEventSubscribeHandler(DOMString name,
                EventSubscriptionHandler handler);
        ExposedThing setEventUnsubscribeHandler(DOMString name,
                EventSubscriptionHandler handler);
        ExposedThing setEventHandler(DOMString name,
                EventListenerHandler eventHandler);
        Promise<undefined> emitEvent(DOMString name,
                InteractionInput data);

        Promise<undefined> expose();
        Promise<undefined> destroy();
      };

      callback PropertyReadHandler = Promise<any>(
              optional InteractionOptions options = null);

      callback PropertyWriteHandler = Promise<undefined>(
              InteractionOutput value,
              optional InteractionOptions options = null);

      callback ActionHandler = Promise<InteractionInput>(
              InteractionOutput params,
              optional InteractionOptions options = null);

      callback EventSubscriptionHandler = Promise<undefined>(
              optional InteractionOptions options = null);

      callback EventListenerHandler = Promise<InteractionInput>();
    

Constructing {{ExposedThing}}

The {{ExposedThing}} interface extends {{ConsumedThing}}. It is constructed from a full or partial {{ThingDescription}} object.

Note that an existing {{ThingDescription}} object can be optionally modified (for instance by adding or removing elements on its |properties|, |actions| and |events| internal properties) and the resulting object can used for constructing an {{ExposedThing}} object. This is the current way of adding and removing Property, Action and Event definitions, as illustrated in the examples.

Before invoking expose(), the {{ExposedThing}} object does not serve any requests. This allows first constructing {{ExposedThing}} and then initialize its Properties and service handlers before starting serving requests.

To construct an {{ExposedThing}} with the {{ThingDescription}} |td:ThingDescription|, run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Run the expand a TD steps on |td|. If that fails, re-[= exception/throw =] the error and abort these steps.
  3. Let |thing:ExposedThing| be a new {{ExposedThing}} object.
  4. Set the internal slot [[\td]] of |thing| to |td|.
  5. Return |thing|.

Methods inherited from {{ConsumedThing}}

The readProperty(), readMultipleProperties(), readAllProperties(), writeProperty(), writeMultipleProperties(), writeAllProperties() methods have the same algorithmic steps as described in ConsumedThing, with the difference that making a request to the underlying platform MAY be implemented with local methods or libraries and don't necessarily need to involve network operations.

The implementation of {{ConsumedThing}} interface in an {{ExposedThing}} provide the default methods to interact with the {{ExposedThing}}.

After constructing an {{ExposedThing}}, a script can initialize its Properties and can set up the optional read, write and action request handlers (the default ones are provided by the implementation). The script provided handlers MAY use the default handlers, thereby extending the default behavior, but they can also bypass them, overriding the default behavior. Finally, the script would call expose() on the {{ExposedThing}} in order to start serving external requests.

The request handlers actually implement the behavior and it is the responsibility of the developers to keep the Thing Description defined in {{ExposedThing}} synchronized with the implementation of the request handlers.

The PropertyReadHandler callback

A function that is called when an external request for reading a Property is received and defines what to do with such requests. It returns a {{Promise}} and resolves with an {{ReadableStream}} object or an ECMAScript value conforming to DataSchema, or rejects with an error.

The setPropertyReadHandler() method

Takes as arguments |name:string| and |handler:PropertyReadHandler|. Sets the service handler that defines what to do when a request is received for reading the specified Property matched by |name|. Throws on error. Returns a reference to |this| object for supporting chaining.

Note that there is no need to register handlers for handling requests for reading multiple or all Properties. The request and reply are transmitted in a single network request, but the ExposedThing may implement them using multiple calls to the single read handler.

The |handler| callback function should implement reading a Property and SHOULD be called by implementations when a request for reading a Property is received from the underlying platform.

There MUST be at most one handler for any given Property, so newly added handlers MUST replace the previous handlers. If no handler is initialized for any given Property, implementations SHOULD implement a default property read handler based on the Thing Description provided in the [[\td]] internal slot.

When the method is invoked given |name:string| and |handler:PropertyReadHandler|, implementations MUST run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
  3. If a Property interaction with |name| is not found, [= exception/throw =] {{NotFoundError}} and abort these steps.
  4. Add the internal slot [[\readHandler]] to |interaction| and set it to |handler|.

Handling requests for reading a Property

When a network request for reading Property |name:string| is received by the implementation with |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |value| be the result of running the read server property steps with |name:string| and |options:InteractionOptions|:
    1. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
    2. If a Property with |name| does not exist, throw {{NotFoundError}} and abort these steps.
    3. Let |handler:function| be `null`.
    4. If there is a user provided internal slot [[\readHandler]] on |interaction|, let |handler| be that.
    5. Otherwise, if there is a default read handler provided by the implementation, let |handler| be that.
    6. If |handler| is `null`, throw {{NotSupportedError}} and abort these steps.
    7. Let |value| be the result of invoking |handler| with |options|. If that fails, throw the error and abort these steps.
    8. Return |value|.

      The |value| returned here SHOULD either conform to DataSchema or it SHOULD be an {{ReadableStream}} object created by the |handler|.

  4. If the previous step has thrown an error, send the error back with the reply created by following the Protocol Bindings and abort these steps.
  5. Serialize and add the returned |value| to the reply created by following the Protocol Bindings.

Handling requests for reading multiple Properties

When a network request for reading multiple Properties given in an object |propertyNames| is received with |options:InteractionOptions|, run the following read multiple properties steps on |propertyNames| and |options|:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. For each property with key |name| defined in |propertyNames|,
    1. Let |value| be the result of running the read server property steps on |name| and |options|. If that throws, send back the error in the reply created by following the Protocol Bindings and abort these steps.
    2. Set the value of |propertyNames|'s |name| to |value|.
  4. Reply to the request by sending a single reply created from |propertyNames| according to the Protocol Bindings.

Handling requests for reading all Properties

When a network request for reading all Properties is received with |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |properties| be an object created with all properties defined in the Thing with values set to `null`.
  4. Run the read multiple properties steps on |properties| and |options|.

The setPropertyObserveHandler() method

Takes as arguments |name:string| and |handler:PropertyReadHandler|. Sets the service handler that defines what to do when a request is received for observing the specified Property matched by |name|. Throws on error. Returns a reference to |this| object for supporting chaining.

The |handler| callback function should implement reading a Property and resolve with an {{InteractionOutput}} object or reject with an error.

There MUST be at most one handler for any given Property, so newly added handlers MUST replace the previous handlers. If no handler is initialized for any given Property, implementations SHOULD implement a default property read handler based on the Thing Description.

When the method is invoked given |name:string| and |handler:PropertyReadHandler|, implementations MUST run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
  3. If a Property interaction with |name| is not found, [= exception/throw =] {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\observeHandler]] on |interaction| to |handler|.

Handling Property observe requests

When a network request for observing a Property |name:string| is received by the implementation with |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |property| be the value of [[\td]]'s |properties|'s |name|. If it does not exist, send back a {{NotFoundError}} in the reply and abort these steps.
  4. Save the request sender information together with |options| to |property|'s internal observer list, in order to be able to notify about Property value changes.
  5. Every time the value of |property| changes, run the following sub-steps:
    1. Let |handler:function| be `null.`
    2. If there is an [[\observeHandler]] internal slot associated with |name| on |property|, let |handler| be that.
    3. Otherwise, if there is a [[\readHandler]] internal slot associated with |name| on |property|, let |handler| be that.
    4. If |handler| is `null`, abort these steps.
    5. Let |promise| be the result of invoking |handler| wih |options|.
    6. If |promise| rejects, abort these steps.
    7. If |promise| resolves with |data|, then for each |observer| in |property|'s internal observer list, run the following sub-steps:
      1. Let |options| be the interaction options saved with |observer|.
      2. Create a |reply| from |data| and |options| according to the Protocol Bindings.
      3. Send |reply| to |observer|.

The setPropertyUnobserveHandler() method

Takes as arguments |name:string| and |handler:PropertyReadHandler|. Sets the service handler that defines what to do when a request is received for unobserving the specified Property matched by |name|. Throws on error. Returns a reference to |this| object for supporting chaining.

The |handler| callback function should implement what to do when an unobserve request is received by the implementation.

There MUST be at most one handler for any given Property, so newly added handlers MUST replace the previous handlers. If no handler is initialized for any given Property, implementations SHOULD implement a default handler based on the Thing Description.

When the method is invoked given |name:string| and |handler:PropertyReadHandler|, implementations MUST run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
  3. If a Property interaction with |name| is not found, [= exception/throw =] {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\unobserveHandler]] on |interaction| to |handler|.

Handling Property unobserve requests

When a network request for unobserving a Property |name:string| with |options:InteractionOptions| is received by the implementation, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |property| be the value of [[\td]]'s |properties|'s |name|. If it does not exist, send back a {{NotFoundError}} in the reply and abort these steps.
  4. If there is an [[\unobserveHandler]] of type {{Function}} defined for |name| on |property|, invoke that with |options|, send back a reply following the Protocol Bindings and abort these steps.
  5. Let |sender| be the matching observer found in |property|'s internal observer list. If not found, send back a {{NotFoundError}} in the reply and abort these steps.
  6. Remove |sender| from |property|'s internal observer list and send back a reply following the Protocol Bindings.

The PropertyWriteHandler callback

A function that is called when an external request for writing a Property is received and defines what to do with such requests. Takes as argument |value:InteractionOutput| and returns a {{Promise}}, resolved when the value of the Property - identified by the name provided when setting the handler has been updated -, or rejects with an error if the property is not found or the value cannot be updated.

Note that the code in this callback function can read the property before updating it in order to find out the old value, if needed. Therefore the old value is not provided to this function.

The value is provided by implementations as an {{InteractionOutput}} object in order to be able to represent values that are not described by a DataSchema, such as streams.

The setPropertyWriteHandler() method

Takes as arguments |name:string| and |handler:PropertyWriteHandler|. Sets the service handler that defines what to do when a request is received for writing the Property matched by |name| given when setting the handler. Throws on error. Returns a reference to |this| object for supporting chaining.

Note that even for readonly Properties it is possible to specify a write handler, as explained in Issue 199. In this case, the write handler may define in an application-specific way to fail the request.

There MUST be at most one write handler for any given Property, so newly added handlers MUST replace the previous handlers. If no write handler is initialized for any given Property, implementations SHOULD implement default property update if the Property is writeable and notifying observers on change if the Property is observable, based on the Thing Description.

When the method is invoked given |name:string| and |handler:PropertyWriteHandler|, implementations MUST run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
  3. If a Property interaction with |name| is not found, [= exception/throw =] {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\writeHandler]] on |interaction| to |handler|.

Handling requests for writing a Property

When a network request for writing a Property |name:string| with a new value |value:InteractionOutput| and |options:InteractionOptions| is received, implementations MUST run the following update property steps, given |name|, |value|, |options| and |mode| set to `"single"`:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
  4. If a Property with |name| does not exist, return a {{NotFoundError}} in the reply and abort these steps.
  5. Let |handler:function| be `null`.
  6. If there is a user provided internal slot [[\writeHandler]] on |interaction|, let |handler| be that.
  7. Otherwise, if there is a default write handler provided by the implementation, let |handler| be that.
  8. Otherwise, if |handler| is `null`, send back a {{NotSupportedError}} with the reply and abort these steps.
  9. Let |promise| be the result of invoking |handler| with |name| and |options|. If it fails, return the error in the reply and abort these steps.
  10. If |mode| is `"single"`, reply to the request reporting success, following the Protocol Bindings and abort these steps.

Handling requests for writing multiple Properties

When a network request for writing multiple Properties given in an object |propertyNames| is received with |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. For each property with key |name| and value |value| defined in |propertyNames|, run the update property steps with |name|, |value|, |options| and |mode| set to `"multiple"`. If that fails, reply to the request with that error and abort these steps.
  4. Reply to the request by sending a single reply according to the Protocol Bindings.

The ActionHandler callback

A function that is called when an external request for invoking an Action is received and defines what to do with such requests. It is invoked with |params:InteractionOutput| and optionally with an |options:InteractionOptions| object. It returns a {{Promise}} that rejects with an error or resolves with the value returned by the Action as {{InteractionInput}}.

Application scripts MAY return a {{ReadableStream}} object from an {{ActionHandler}}. Implementations will then use the stream for constructing the Action's response.

The setActionHandler() method

Takes as arguments |name:string| and |action:ActionHandler|. Sets the handler function that defines what to do when a request is received to invoke the Action matched by |name|. Throws on error. Returns a reference to |this| object for supporting chaining.

The |action| callback function will implement an Action and SHOULD be called by implementations when a request for invoking the Action is received from the underlying platform.

There MUST be at most one handler for any given Action, so newly added handlers MUST replace the previous handlers.

When the method is invoked given |name:string| and |action:ActionHandler|, run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |actions|'s |name|.
  3. If an Action with name |name| is not found, [= exception/throw =] a {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\actionHandler]] on |interaction| to |action|.

Handling Action requests

When a network request for invoking the Action identified by |name:string| is received with |inputs| and optionally with |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |properties|'s |name|.
  4. If an Action identified by |name| does not exist, return a {{NotFoundError}} in the reply and abort these steps.
  5. Let |handler:function| be `null`.
  6. If there is a user provided internal slot [[\actionHandler]] on |interaction|, let |handler| be its value.
  7. If |handler| is `null`, return a {{NotSupportedError}} with the reply created by following the Protocol Bindings and abort these steps.
  8. Let |promise| be the result of invoking |handler| with |name|, |inputs| and |options|.
  9. If |promise| rejects, send the error with the reply and abort these steps.
  10. When |promise| resolves with |data:InteractionInput|, use |data| to create and send the reply according to the Protocol Bindings.

The EventListenerHandler callback

A function that is called when an associated Event is triggered and provides the data to be sent with the Event to subscribers. Returns a {{Promise}} that resolves with {{InteractionInput}} value that represents the Event data, or rejects with an error.

Applications MAY return {{ReadableStream}} from an {{EventListenerHandler}} Implementations will then use the stream provided in {{InteractionOutput}} when constructing the event notification.

The EventSubscriptionHandler callback

A function that is called when an external request for subscribing to an Event is received and defines what to do with such requests. It is invoked with an |options:InteractionOptions| object provided by the implementation and coming from subscribers. It returns a {{Promise}} that rejects with an error or resolves when the subscription is accepted.

The setEventSubscribeHandler() method

Takes as arguments |name:string| and |handler:EventSubscriptionHandler|. Sets the handler function that defines what to do when a subscription request is received for the specified Event matched by |name|. Throws on error. Returns a reference to |this| object for supporting chaining.

The |handler| callback function SHOULD implement what to do when an subscribe request is received, for instance necessary initializations. Note that the handler for emitting Events is set separately.

There MUST be at most one event subscribe handler for any given Event, so newly added handlers MUST replace the previous handlers.

When the method is invoked given |name:string| and |handler:EventSubscriptionHandler|, run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  3. If an Event with the name |name| is not found, [= exception/throw =] a {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\subscribeHandler]] on |interaction| to |handler|.
  5. Return `this`.

Handling Event subscribe requests

When an Event subscription request for |name| is received by the underlying platform with optional |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  4. If an Event with the name |name| is not found, send back a {{NotFoundError}} and abort these steps.
  5. If |name| has an associated [[\subscribeHandler]] internal slot, invoke it with |options| and abort these steps.
  6. Otherwise, if no [[\subscribeHandler]] is defined, then implement the default subscriber mechanism:
    1. Let |subscriber| be a tuple formed of |options| (from which |uriVariables| and |data| may be used) and the subscriber information needed to create an Event notification response.
    2. Add |subscriber| to the internal listener list of |interaction|.

The setEventUnsubscribeHandler() method

Takes as arguments |name:string| and |handler:EventSubscriptionHandler|. Sets the handler function that defines what to do when the specified Event matched by |name| is unsubscribed from. Throws on error. Returns a reference to |this| object for supporting chaining.

The |handler| callback function SHOULD implement what to do when an unsubscribe request is received.

There MUST be at most one handler for any given Event, so newly added handlers MUST replace the previous handlers.

When the method is invoked with |name:string| and |handler:EventSubscriptionHandler|, run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  3. If an Event with the name |name| is not found, [= exception/throw =] a {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\unsubscribeHandler]] on |interaction| to |handler|.
  5. Return `this`.

Handling Event unsubscribe requests

When an Event unsubscribe request for |name| is received by the underlying platform optionally with |options:InteractionOptions|, run the following steps:
  1. If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and abort these steps.
  2. If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  4. If an Event with the name |name| is not found, send back a {{NotFoundError}} and abort these steps.
  5. If |name| has an associated [[\unsubscribeHandler]] internal slot that is a function, invoke it with |options| and abort these steps.
  6. Otherwise let |subscriber| be the tuple saved in |interaction|'s internal listener list.
  7. Remove |subscriber| from |interaction|'s internal listener list.
  8. Return `this`.

The setEventHandler() method

Takes as arguments |name:string| and |eventHandler:EventListenerHandler|. Sets the event handler function for the specified Event matched by |name|. Throws on error. Returns a reference to |this| object for supporting chaining.

The |eventHandler| callback function will implement what to do when the event is emitted. It SHOULD resolve with a value that represents the Event data, or reject with an error.

There MUST be at most one handler for any given Event, so newly added handlers MUST replace the previous handlers.

When the method is invoked with |name:string| and |eventHandler:EventListenerHandler|, run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and abort these steps.
  2. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  3. If an Event with the name |name| is not found, [= exception/throw =] a {{NotFoundError}} and abort these steps.
  4. Set the internal slot [[\eventHandler]] of |interaction| to |eventHandler|.
  5. Return `this`.

Handling Events

When an Event with name |name| is emitted with |data:InteractionInput| either by the underlying platform or by the emitEvent() method, run the following steps:
  1. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  2. If |data| is not defined or `null`,
    1. Let |eventHandler| be the value of the [[\eventHandler]] internal slot of |interaction|.
    2. If |eventHandler| is failure, abort these steps.
    3. Let |data| be the result of awaiting to resolve the invocation of |eventHandler|. If it rejects, abort these steps.
  3. For each |subscriber| in the internal listener list of |interaction|, run the following sub-steps:
    1. Create an Event notification |response| according to the Protocol Bindings from |data| and |subscriber|, including its |options|. If the underlying protocol stack permits conveying event errors and if an error condition has been detected by the UA, create |response| as an error notification according to the Protocol Bindings, using |data|, |subscriber| and its |options|.

      The error reporting is protocol specific and it is encapsulated by implementations. On the client end, the error listener passed with the subscription will be invoked if the client UA detects the error.

    2. Send |response| to the subscriber identified by |subscriber|.

The emitEvent() method

Takes as arguments |name:string| denoting an Event name and |data:InteractionInput|. Triggers emitting the Event with the given data. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Let |interaction| be the value of [[\td]]'s |events|'s |name|.
  4. If an Event with the name |name| is not found, reject |promise| with {{NotFoundError}} and abort these steps.
  5. Make a request to the underlying platform to emit an Event with |data|.

    This will trigger the handling events steps.

The expose() method

Start serving external requests for the Thing, so that WoT Interactions using Properties, Actions and Events will be possible. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Run the expand a TD steps on the internal slot [[\td]].
  4. Run the validate a TD on [[\td]]. If that fails, reject |promise| with a {{TypeError}} and abort these steps.
  5. For each Property definition in [[\td]]'s |properties|, initialize an |internal observer list| internal slot in order to store observe request data needed to notify the observers on value changes.
  6. For each Event definition is [[\td]]'s |events|, initialize an |internal listener list| internal slot in order to store subscription request data needed to notify the Event listeners.
  7. Set up the WoT Interactions based on introspecting [[\td]] as explained in [[!WOT-TD]] and [[!WOT-PROTOCOL-BINDINGS]]. Make a request to the underlying platform to initialize the Protocol Bindings and then start serving external requests for WoT Interactions (read, write and observe Properties, invoke Actions and manage Event subscriptions), based on the Protocol Bindings. Implementations MAY reject this step for any reason (e.g. if they want to enforce further checks and constraints on interaction forms).
  8. If there was an error during the request, reject |promise| with an {{Error}} object |error| with |error|'s |message| set to the error code seen by the Protocol Bindings and abort these steps.
  9. Otherwise resolve |promise| and abort these steps.

The destroy() method

Stop serving external requests for the Thing and destroy the object. Note that eventual unregistering should be done before invoking this method. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If invoking this method is not allowed for the current scripting context for security reasons, reject |promise| with a {{SecurityError}} and abort these steps.
  3. Make a request to the underlying platform to stop serving external requests for WoT Interactions, based on the Protocol Bindings.
  4. If there was an error during the request, reject |promise| with an {{Error}} object |error| with its |message| set to the error code seen by the Protocol Bindings and abort these steps.
  5. Otherwise resolve |promise| and abort these steps.

ExposedThing Examples

The next example illustrates how to create an {{ExposedThing}} based on a partial TD object constructed beforehands.

        try {
          let temperaturePropertyDefinition = {
            type: "number",
            minimum: -50,
            maximum: 10000
          };
          let tdFragment = {
            properties: {
              temperature: temperaturePropertyDefinition
            },
            actions: {
              reset: {
                description: "Reset the temperature sensor",
                input: {
                  temperature: temperatureValueDefinition
                },
                output: null,
                forms: []
              },
            },
            events: {
              onchange: temperatureValueDefinition
            }
          };
          let thing1 = await WOT.produce(tdFragment);
          // initialize Properties
          await thing1.writeProperty("temperature", 0);
          // add service handlers
          thing1.setPropertyReadHandler("temperature", () => {
             return readLocalTemperatureSensor();  // Promise
          });
          // start serving requests
          await thing1.expose();
        } catch (err) {
           console.log("Error creating ExposedThing: " + err);
        }
      

The next example illustrates how to add or modify a Property definition on an existing {{ExposedThing}}: take its |td| property, add or modify it, then create another {{ExposedThing}} with that.

        try {
          // create a deep copy of thing1's TD
          let instance = JSON.parse(JSON.stringify(thing1.td));
          const statusValueDefinition = {
            type: "object",
            properties: {
              brightness: {
                type: "number",
                minimum: 0.0,
                maximum: 100.0,
                required: true
              },
              rgb: {
                type: "array",
                "minItems": 3,
                "maxItems": 3,
                items : {
                    "type" : "number",
                    "minimum": 0,
                    "maximum": 255
                }
              }
          };
          instance["name"] = "mySensor";
          instance.properties["brightness"] = {
            type: "number",
            minimum: 0.0,
            maximum: 100.0,
            required: true,
          };
          instance.properties["status"] = statusValueDefinition;
          instance.actions["getStatus"] = {
            description: "Get status object",
            input: null,
            output: {
              status : statusValueDefinition;
            },
            forms: [...]
          };
          instance.events["onstatuschange"] = statusValueDefinition;
          instance.forms = [...];  // update
          var thing2 = new ExposedThing(instance);
          // TODO: add service handlers
          await thing2.expose();
          });
        } catch (err) {
           console.log("Error creating ExposedThing: " + err);
        }
      

The ThingDiscovery interface

Discovery is a distributed application that requires provisioning and support from participating network nodes (clients, servers, directory services). This API models the client side of typical discovery schemes supported by various IoT deployments.

The {{ThingDiscovery}} object is constructed given a filter and provides the properties and methods controlling the discovery process.

      [SecureContext, Exposed=(Window,Worker)]
      interface ThingDiscovery {
        constructor(optional ThingFilter filter = null);
        readonly attribute ThingFilter? filter;
        readonly attribute boolean active;
        readonly attribute boolean done;
        readonly attribute Error? error;
        undefined start();
        Promise<ThingDescription> next();
        undefined stop();
      };
    

The {{ThingDiscovery}} interface has a next() method and a done property, but it is not an Iterable. Look into Issue 177 for rationale.

The discovery results internal slot is an internal queue for temporarily storing the found {{ThingDescription}} objects until they are consumed by the application using the next() method. Implementations MAY optimize the size of this queue based on e.g. the available resources and the frequency of invoking the next() method.

The filter property represents the discovery filter of type ThingFilter specified for the discovery.

The active property is `true` when the discovery is actively ongoing on protocol level (i.e. new TDs may still arrive) and `false` otherwise.

The done property is `true` if the discovery has been completed with no more results to report and discovery results is also empty.

The error property represents the last error that occurred during the discovery process. Typically used for critical errors that stop discovery.

Constructing {{ThingDiscovery}}

To create {{ThingDiscovery}} with a |filter:ThingFilter| or type {{ThingFilter}}, run the following steps:
  1. If |filter| is not an object or `null`, [= exception/throw =] a {{TypeError}} and abort these steps.
  2. Let |discovery:ThingDiscovery| be a new {{ThingDiscovery}} object.
  3. Set the filter property to |filter|.
  4. Set the active and done properties to `false`. Set the error property to `null`.
  5. Return |discovery|.

The start() method sets active to `true`. The stop() method sets the active property to |false|, but done may be still `false` if there are {{ThingDescription}} objects in the discovery results not yet consumed with next().

During successive calls of next(), the active property may be `true` or `false`, but the done property is set to `false` by next() only when both the active property is `false` and discovery results is empty.

The DiscoveryMethod enumeration

        typedef DOMString DiscoveryMethod;
      

Represents the discovery type to be used:

The ThingFilter dictionary

Represents an object containing the constraints for discovering Things as key-value pairs.

        dictionary ThingFilter {
          (DiscoveryMethod or DOMString) method = "any";
          USVString? url;
          USVString? query;
          object? fragment;
        };
      

The method property represents the discovery type that should be used in the discovery process. The possible values are defined by the DiscoveryMethod enumeration that MAY be extended by string values defined by solutions (with no guarantee of interoperability).

The url property represents additional information for the discovery method, such as the URL of the target entity serving the discovery request, for instance the URL of a Thing Directory (if method is `"directory"`), or otherwise the URL of a directly targeted Thing.

The query property represents a query string accepted by the implementation, for instance a SPARQL or JSON query. Support may be implemented locally in the WoT Runtime or remotely as a service in a Thing Directory.

The fragment property represents a template object used for matching property by property against discovered Things.

The start() method

Starts the discovery process. The method MUST run the following steps:
  1. If invoking this method is not allowed for the current scripting context for security reasons, set the error property to a {{SecurityError}} and abort these steps.
  2. If discovery is not supported by the implementation, set the error property to {{NotSupportedError}} and abort these steps.
  3. Let |filter| denote the filter property.
  4. If the |filter| is defined,
    • If |filter|'s |query| is defined, pass it as an opaque string to the underlying implementation to be matched against discovered items. The underlying implementation is responsible to parse it e.g. as a SPARQL or JSON query and match it against the Thing Descriptions found during the discovery process. If queries are not supported, set |this.error| to {{NotSupportedError}} and abort these steps.
  5. Create the discovery results internal slot for storing discovered {{ThingDescription}} objects.
  6. Request the underlying platform to start the discovery process, with the following parameters:
    • If |filter|s |method| is not defined or the value is `"any"`, use the widest discovery method supported by the underlying platform.
    • Otherwise if |filter|s |method| is `"local"`, use the local Thing Directory for discovery. Usually that defines Things deployed in the same device, or connected to the device in slave mode (e.g. sensors connected via Bluetooth or a serial connection).
    • Otherwise if |filter|s |method| is `"directory"`, use the remote Thing Directory specified in |filter.url|.
    • Otherwise if |filter|s |method| is `"multicast"`, use all the multicast discovery protocols supported by the underlying platform.
  7. When the underlying platform has started the discovery process, set the active property to `true`.
  8. Whenever a new Thing Description |td:ThingDescription| is discovered by the underlying platform, run the following sub-steps:
    1. Fetch |td| as a JSON object |json|. If that fails, set the error property to {{SyntaxError}}, discard |td| and continue the discovery process.
    2. If |filter|'s |query| is defined, check if |json| is a match for the query. The matching algorithm is encapsulated by implementations. If that returns `false`, discard |td| and continue the discovery process.
    3. If |filter|'s |fragment| is defined, for each property defined in it, check if that property exists in |json|'s properties and has the same value. If this is `false` in any checks, discard |td| and continue the discovery process.
    4. Otherwise add |td| to the discovery results.
    5. At this point implementations MAY control the flow of the discovery process (depending on memory constraints, for instance temporarily stop discovery if the queue is getting too large, or resume discovery when the queue is emptied sufficiently).
  9. Whenever an error occurs during the discovery process,
    1. Let |error| be a new {{Error}} object. Set |error|'s |name| to `"DiscoveryError"`.
    2. If there was an error code or message provided by the Protocol Bindings, set |error|'s |message| to that value as string.
    3. Set error property to |error|.
    4. If the error is irrecoverable and discovery has been stopped by the underlying platform, set the active property to `false`.
  10. When the underlying platform reports the discovery process has completed, set the active property to `false`.

The next() method

Provides the next discovered {{ThingDescription}} object. The method MUST run the following steps:
  1. Return a {{Promise}} |promise:Promise| and execute the next steps in parallel.
  2. If the active property is `true`, wait until the discovery results internal slot is not empty.
  3. If discovery results is empty and the active property is `false`, set the done property to `true` and reject |promise|.
  4. Remove the first {{ThingDescription}} object |td| from discovery results.
  5. Resolve |promise| with |td| and abort these steps.

The stop() method

Stops or suppresses the discovery process. It might not be supported by all discovery methods and endpoints, however, any further discovery results or errors will be discarded and the discovery is marked inactive. The method MUST run the following steps:
  1. Request the underlying platform to stop the discovery process. If this returns an error, or if it is not possible, for instance when discovery is based on open ended multicast requests, the implementation SHOULD discard subsequent discovered items.
  2. Set the active property to `false`.

Discovery Examples

The following example finds {{ThingDescription}} objects of Things that are exposed by local hardware, regardless how many instances of WoT Runtime it is running. Note that the discovery can end (become inactive) before the internal discovery results queue is emptied, so we need to continue reading {{ThingDescription}} objects until done. This is typical with local and directory type discoveries.

        let discovery = new ThingDiscovery({ method: "local" });
        do {
          let td = await discovery.next();
          console.log("Found Thing Description for " + td.title);
          let thing = new ConsumedThing(td);
          console.log("Thing name: " + thing.getThingDescription().title);
        } while (!discovery.done);
      

The next example finds {{ThingDescription}} objects of Things listed in a Thing Directory service. We set a timeout for safety.

        let discoveryFilter = {
          method: "directory",
          url: "http://directory.wotservice.org"
        };
        let discovery = new ThingDiscovery(discoveryFilter);
        setTimeout( () => {
            discovery.stop();
            console.log("Discovery stopped after timeout.");
          },
          3000);
        do {
          let td = await discovery.next();
          console.log("Found Thing Description for " + td.title);
          let thing = new ConsumedThing(td);
          console.log("Thing name: " + thing.getThingDescription().title);
        } while (!discovery.done);
        if (discovery.error) {
          console.log("Discovery stopped because of an error: " + error.message);
        }
      

The next example is for an open-ended multicast discovery, which likely won't complete soon (depending on the underlying protocol), so stopping it with a timeout is a good idea. It will likely deliver results one by one.

        let discovery = new ThingDiscovery({ method: "multicast" });
        setTimeout( () => {
            discovery.stop();
            console.log("Stopped open-ended discovery");
          },
          10000);
        do {
          let td = await discovery.next();
          let thing = new ConsumedThing(td);
          console.log("Thing name: " + thing.getThingDescription().title);
        } while (!discovery.done);
      

Security and Privacy

A detailed discussion of security and privacy considerations for the Web of Things, including a threat model that can be adapted to various circumstances, is presented in the informative document [[!WOT-SECURITY]]. This section discusses only security and privacy risks and possible mitigations directly relevant to the scripts and WoT Scripting API.

A suggested set of best practices to improve security for WoT devices and services has been documented in [[!WOT-SECURITY]]. That document may be updated as security measures evolve. Following these practices does not guarantee security, but it might help avoid common known vulnerabilities.

The WoT security risks and possible mitigations are concerning the following groups:

Scripting Runtime Security and Privacy Risks

This section is normative and contains specific risks relevant for the WoT Scripting Runtime.

Corrupted Input Security and Privacy Risk

A typical way to compromise any process is to send it a corrupted input via one of the exposed interfaces. This can be done to a script instance using WoT interface it exposes.

Mitigation:
Implementors of this API SHOULD perform validation on all script inputs. In addition to input validation, fuzzing should be used to verify that the input processing is done correctly. There are many tools and techniques in existence to do such validation. More details can be found in [[!WOT-SECURITY]].

Physical Device Direct Access Security and Privacy Risk

In case a script is compromised or misbehaving, the underlying physical device (and potentially surrounded environment) can be damaged if a script can use directly exposed native device interfaces. If such interfaces lack safety checks on their inputs, they might bring the underlying physical device (or environment) to an unsafe state (i.e. device overheats and explodes).

Mitigation:
The WoT Scripting Runtime SHOULD avoid directly exposing the native device interfaces to the script developers. Instead, a WoT Scripting Runtime should provide a hardware abstraction layer for accessing the native device interfaces. Such hardware abstraction layer should refuse to execute commands that might put the device (or environment) to an unsafe state. Additionally, in order to reduce the damage to a physical WoT device in cases a script gets compromised, it is important to minimize the number of interfaces that are exposed or accessible to a particular script based on its functionality.

Provisioning and Update Security Risk

If the WoT Scripting Runtime supports post-manufacturing provisioning or updates of scripts, WoT Scripting Runtime or any related data (including security credentials), it can be a major attack vector. An attacker can try to modify any above described element during the update or provisioning process or simply provision attacker's code and data directly.

Mitigation:
Post-manufacturing provisioning or update of scripts, WoT Scripting Runtime or any related data should be done in a secure fashion. A set of recommendations for secure update and post-manufacturing provisioning can be found in [[!WOT-SECURITY]].

Security Credentials Storage Security and Privacy Risk

Typically the WoT Scripting Runtime needs to store the security credentials that are provisioned to a WoT device to operate in WoT network. If an attacker can compromise the confidentiality or integrity of these credentials, then it can obtain access to the WoT assets, impersonate WoT things or devices or create Denial-Of-Service (DoS) attacks.

Mitigation:
The WoT Scripting Runtime should securely store the provisioned security credentials, guaranteeing their integrity and confidentiality. In case there are more than one tenant on a single WoT-enabled device, a WoT Scripting Runtime should guarantee isolation of each tenant provisioned security credentials. Additionally, in order to minimize a risk that provisioned security credentials get compromised, the WoT Scripting Runtime should not expose any API for scripts to query the provisioned security credentials.

Script Security and Privacy Risks

This section describes specific risks relevant for script developers.

Corrupted Script Input Security and Privacy Risk

A script instance may receive data formats defined by the TD, or data formats defined by the applications. While the WoT Scripting Runtime SHOULD perform validation on all input fields defined by the TD, scripts may be still exploited by input data.

Mitigation:
Script developers should perform validation on all application defined script inputs. In addition to input validation, fuzzing could be used to verify that the input processing is done correctly. There are many tools and techniques in existence to do such validation. More details can be found in [[!WOT-SECURITY]].

Denial Of Service Security Risk

If a script performs a heavy functional processing on received requests before the request is authenticated, it presents a great risk for Denial-Of-Service (DOS) attacks.

Mitigation:
Scripts should avoid heavy functional processing without prior successful authentication of requestor. The set of recommended authentication mechanisms can be found in [[!WOT-SECURITY]].

Terminology and conventions

The generic WoT terminology is defined in [[!WOT-ARCHITECTURE]]: Thing, Thing Description (in short TD), Web of Things (in short WoT), WoT Interface, Protocol Bindings, WoT Runtime, Consuming a Thing Description, Thing Directory, Property, Action, Event, DataSchema, Form etc.

WoT Interaction is a synonym for Interaction Affordance. An Interaction Affordance (or shortly, affordance) is the term used in [[!WOT-TD]] when referring to Thing capabilities, as explained in TD issue 282. However, this term is not well understood outside the TD semantic context. Hence for the sake of readability, this document will use the previous term WoT interaction or, simply, interaction instead.

WoT network interface synonym for WoT Interface

JSON-LD is defined in [[!JSON-LD]] as a JSON document that is augmented with support for Linked Data.

JSON Schema is defined in these specifications.

The terms URL, URL scheme, URL host, URL path, URL record, parse a URL, absolute-URL string, path-absolute-URL string, basic URL parser are defined in [[!URL]].

The terms MIME type, Parsing a MIME type, Serializing a MIME type, valid MIME type string, JSON MIME type are defined in [[!MIMESNIFF]].

The terms UTF-8 encoding, UTF-8 decode, encode, decode are defined in [[!ENCODING]].

string, parse JSON from bytes and serialize JSON to bytes, are defined in [[!INFRA]].

{{Promise}}, Error, JSON, JSON.stringify, JSON.parse, internal method and internal slot are defined in [[!ECMASCRIPT]].

The terms browsing context, top-level browsing context, global object, current settings object, executing algorithms in parallel are defined in [[!HTML5]] and are used in the context of browser implementations.

The term secure context is defined in [[!WEBAPPSEC]].

IANA media types (formerly known as MIME types) are defined in RFC2046.

The terms hyperlink reference and relation type are defined in [[!HTML5]] and RFC8288.

API design rationale

API rationale usually belongs to a separate document, but in the WoT case the complexity of the context justifies including basic rationale here.

Approaches to WoT application development

The WoT Interest Group and Working Group have explored different approaches to application development for WoT that have been all implemented and tested.

No Scripting API

It is possible to develop WoT applications that only use the WoT network interface, typically exposed by a WoT gateway that presents a REST-ful API towards clients and implements IoT protocol plugins that communicate with supported IoT deployments. One such implementation is the Mozilla WebThings platform.

Simple Scripting API

WoT Things show good synergy with software objects, so a Thing can be represented as a software object, with Properties represented as object properties, Actions as methods, and Events as events. In addition, metadata is stored in special properties. Consuming and exposing is done with factory methods that produce a software object that directly represents a remote Thing and its interactions. One such implementation is the Arena Web Hub project.

In the next example, a Thing that represents interactions with a lock would look like the following: the |status| property and the open() method are directly exposed on the object.

          let lock = await WoT.consume(‘https://td.my.com/lock-00123’);
          console.log(lock.status);
          lock.open('withThisKey');
        

This API, aligned with the [[[WOT-TD]]] specification

Since the direct mapping of Things to software objects have had some challenges, this specification takes another approach that exposes software objects to represent the Thing metadata as data property and the WoT interactions as methods. One implementation is node-wot in the the Eclipse ThingWeb project, which is the current reference implementation of the API specified in this document.

The same example now would look like the following: the |status| property and the open() method are represented indirectly.

          let res = await fetch(‘https://td.my.com/lock-00123’);
          let td = await res.json();
          let lock = new ConsumedThing(td);
          console.log(lock.readProperty(‘status’));
          lock.invokeAction(‘open’, 'withThisKey');
        

In conclusion, the WoT WG decided to explore the third option that closely follows the [[[WOT-TD]]] specification. Based on this, a simple API can also be implemented. Since Scripting is an optional module in WoT, this leaves room for applications that only use the WoT network interface. Therefore all three approaches above are supported by the [[[WOT-TD]]] specification.

Moreover, the WoT network interface can be implemented in many languages and runtimes. Consider this API an example for what needs to be taken into consideration when designing a Scripting API for WoT.

Fetching and validating a TD

The fetch(url) method has been part of this API in earlier versions. However, now fetching a TD given a URL should be done with an external method, such as the Fetch API or a HTTP client library, which offer already standardized options on specifying fetch details. The reason is that while simple fetch operations (covering most use cases) could be done in this API, when various fetch options were needed, there was no point in duplicating existing work to re-expose those options in this API.

Since fetching a TD has been scoped out, and TD validation is defined externally in the [[[WOT-TD]]] specification, that is scoped out, too. This specification expects a TD as parsed JSON object that has been validated according to the [[[WOT-TD]]] specification.

Factory vs constructors

The factory methods for consuming and exposing Things are asynchronous and fully validate the input TD. In addition, one can also construct {{ConsumedThing}} and {{ExposedThing}} by providing a parsed and validated TD. Platform initialization is then done when needed during the WoT interactions. So applications that prefer validating a TD themselves, may use the constructors, whereas applications that leave validation to implementations and prefer interactions initialized up front SHOULD use the factory methods on the WoT API object.

Observers

Earlier drafts used the Observer construct, but since it has not become standard, a new design was needed that was light enough for embedded implementations. Therefore observing Property changes and handling WoT Events is done with callback registrations.

Using Events

This API ended up not using software events at all, for the following reasons:
  • Subscription to WoT Events may be different from handling software events (subscription might need parameters, might involve security tokens etc).
  • Most implementations are for Node.js and browser implementations will likely be libraries (because possible dependency management issues in native implementations), using Events has been challenging.
  • Observing Property changes and handling WoT Events is done with the solution above.

Polymorphic functions

The reason to use function names like readProperty(), readMultipleProperties() etc. instead of a generic polymorphic read() function is that the current names map exactly to the "op" vocabulary from the Form definition in the [[[WOT-TD]]] specification.

Changes

The following is a list of major changes to the document. Major versions of this specification are the following:

For a complete list of changes, see the github change log. You can also view the recently closed issues.

Full Web IDL

Acknowledgements

Special thanks to former editor Johannes Hund (until August 2017, when at Siemens AG) and Kazuaki Nimura (until December 2018) for developing this specification. Also, the editors would like to thank Dave Raggett, Matthias Kovatsch, Michael Koster, Elena Reshetova, Michael McCool as well as the other WoT WG members for their comments, contributions and guidance.