This specification extends the High Resolution Time specification [[HR-TIME-2]] by providing methods to store and retrieve high resolution performance metric data.

Performance Timeline Level 2 replaces the first version of [[PERFORMANCE-TIMELINE]] and includes:

Introduction

Accurately measuring performance characteristics of web applications is an important aspect of making web applications faster. This specification defines the necessary Performance Timeline primitives that enable web developers to access, instrument, and retrieve various performance metrics from the full lifecycle of a web application.

[[NAVIGATION-TIMING-2]], [[RESOURCE-TIMING-2]], and [[USER-TIMING-2]] are examples of specifications that define timing information related to the navigation of the document, resources on the page, and developer scripts, respectively. Together these and other performance interfaces define performance metrics that describe the Performance Timeline of a web application. For example, the following script shows how a developer can access the Performance Timeline to obtain performance metrics related to the navigation of the document, resources on the page, and developer scripts:

      <!doctype html>
      <html>
      <head></head>
      <body onload="init()">
        <img id="image0" src="https://www.w3.org/Icons/w3c_main.png" />
        <script>
          function init() {
            // see [[USER-TIMING-2]]
            performance.mark("startWork");
            doWork(); // Some developer code
            performance.mark("endWork");
            measurePerf();
          }
          function measurePerf() {
            performance
              .getEntries()
              .map(entry => JSON.stringify(entry, null, 2))
              .forEach(json => console.log(json));
          }
        </script>
        </body>
      </html>
    

Alternatively, the developer can observe the Performance Timeline and be notified of new performance metrics and, optionally, previously buffered performance metrics of specified type, via the PerformanceObserver interface:

    <!doctype html>
    <html>
    <head></head>
    <body>
    <img id="image0" src="https://www.w3.org/Icons/w3c_main.png" />
    <script>
    // Know when the entry types we would like to use are not supported.
    function detectSupport(entryTypes) {
      for (const entryType of entryTypes) {
        if (!PerformanceObserver.supportedEntryTypes.includes(entryType)) {
          // Indicate to client-side analytics that |entryType| is not supported.
        }
      }
    }
    detectSupport(["resource", "mark", "measure"]);
    const userTimingObserver = new PerformanceObserver(list => {
      list
        .getEntries()
        // Get the values we are interested in
        .map(({ name, entryType, startTime, duration }) => {
          const obj = {
            "Duration": duration,
            "Entry Type": entryType,
            "Name": name,
            "Start Time": startTime,
          };
          return JSON.stringify(obj, null, 2);
        })
        // Display them to the console.
        .forEach(console.log);
      // Disconnect after processing the events.
      userTimingObserver.disconnect();
    });
    // Subscribe to new events for User-Timing.
    userTimingObserver.observe({entryTypes: ["mark", "measure"]});
    const resourceObserver = new PerformanceObserver(list => {
      list
        .getEntries()
        // Get the values we are interested in
        .map(({ name, startTime, fetchStart, responseStart, responseEnd }) => {
          const obj = {
            "Name": name,
            "Start Time": startTime,
            "Fetch Start": fetchStart,
            "Response Start": responseStart,
            "Response End": responseEnd,
          };
          return JSON.stringify(obj, null, 2);
        })
        // Display them to the console.
        .forEach(console.log);
      // Disconnect after processing the events.
      resourceObserver.disconnect();
    });
    // Retrieve buffered events and subscribe to newer events for Resource Timing.
    resourceObserver.observe({type: "resource", buffered: true});
    </script>
    </body>
    </html>
    

The PerformanceObserver interface was added in Performance Timeline Level 2 and is designed to address limitations of the buffer-based approach shown in the first example. By using the PerformanceObserver interface, the application can:

The developer is encouraged to use PerformanceObserver where possible. Further, new performance API's and metrics may only be available through the PerformanceObserver interface.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

Performance Timeline

Each ECMAScript global environment has:

The relevant performance entry buffer is the performance entry buffer associated with the relevant global object.

Extensions to the Performance interface

This extends the Performance interface [[HR-TIME-2]] and hosts performance related attributes and methods used to retrieve the performance metric data from the Performance Timeline.

      partial interface Performance {
        PerformanceEntryList getEntries ();
        PerformanceEntryList getEntriesByType (DOMString type);
        PerformanceEntryList getEntriesByName (DOMString name, optional DOMString type);
      };
      typedef sequence<PerformanceEntry> PerformanceEntryList;
      

The PerformanceEntryList represents a sequence of PerformanceEntry, providing developers with all the convenience methods found on JavaScript arrays.

getEntries() method

Returns a PerformanceEntryList object returned by algorithm with buffer set to relevant performance entry buffer, and name and type set to null.

getEntriesByType() method

Returns a PerformanceEntryList object returned by algorithm with buffer set to relevant performance entry buffer, name set to null, and type set to `type`.

getEntriesByName() method

Returns a PerformanceEntryList object returned by algorithm with buffer set to relevant performance entry buffer, name set to `name`, and type set to null if optional `entryType` is omitted, and type set to `type` otherwise.

The PerformanceEntry interface

The PerformanceEntry interface hosts the performance data of various metrics.

      [Exposed=(Window,Worker)]
      interface PerformanceEntry {
        readonly    attribute DOMString           name;
        readonly    attribute DOMString           entryType;
        readonly    attribute DOMHighResTimeStamp startTime;
        readonly    attribute DOMHighResTimeStamp duration;
        [Default] object toJSON();
      };
name
This attribute MUST return an identifier for this PerformanceEntry object. This identifier does not have to be unique.
entryType
This attribute MUST return the type of the interface represented by this PerformanceEntry object.

Example `entryType` values defined by other specifications include: "mark" and "measure" [[USER-TIMING-2]], "navigation" [[NAVIGATION-TIMING-2]], "resource" [[RESOURCE-TIMING-2]], and "longtask".

startTime
This attribute MUST return the time value of the first recorded timestamp of this performance metric. If the startTime concept doesn't apply, a performance metric may choose to return a `startTime` of 0.
duration
This attribute MUST return the time value of the duration of the entire event being recorded by this PerformanceEntry. Typically, this would be the time difference between the last recorded timestamp and the first recorded timestamp of this PerformanceEntry. If the duration concept doesn't apply, a performance metric may choose to return a `duration` of 0.

When toJSON is called, run [[WebIDL]]'s default toJSON operation.

The PerformanceObserver interface

The PerformanceObserver interface can be used to observe the Performance Timeline to be notified of new performance metrics as they are recorded, and optionally buffered performance metrics.

Each PerformanceObserver has these associated concepts:

The `PerformanceObserver(callback)` constructor must create a new PerformanceObserver object with PerformanceObserverCallback set to callback and then return it.

A registered performance observer is a struct consisting of an observer member (a PerformanceObserver object) and an options list member (a list of PerformanceObserverInit dictionaries).

      callback PerformanceObserverCallback = void (PerformanceObserverEntryList entries,
                                                   PerformanceObserver observer);
      [Constructor(PerformanceObserverCallback callback), Exposed=(Window,Worker)]
      interface PerformanceObserver {
        void observe (PerformanceObserverInit options);
        void disconnect ();
        PerformanceEntryList takeRecords();
        static readonly attribute FrozenArray<DOMString> supportedEntryTypes;
      };
      

To keep the performance overhead to minimum the application ought to only subscribe to event types that it is interested in, and disconnect the observer once it no longer needs to observe the performance data. Filtering by name is not supported, as it would implicitly require a subscription for all event types — this is possible, but discouraged, as it will generate a significant volume of events.

observe() method

The observe() method instructs the user agent to register the observer and must run these steps:

  1. Let observer be the context object.
  2. If options's entryTypes and type members are both omitted, then throw a SyntaxError.
  3. If options's entryTypes is present and any other member is also present, then throw a SyntaxError.
  4. Update or check observer's observer type by running these steps:
    1. If observer's observer type is "undefined":
      1. If options's entryTypes member is present, then set observer's observer type to "multiple".
      2. If options's type member is present, then set observer's observer type to "single".
    2. If observer's observer type is "single" and options's entryTypes member is present, then throw an InvalidModificationError.
    3. If observer's observer type is "multiple" and options's type member is present, then throw an InvalidModificationError.
  5. If observer's observer type is "multiple", run the following steps:
    1. Let entry types be options's entryTypes sequence.
    2. Remove all types from entry types that are not contained in supported entry types. The user agent SHOULD notify developers if entry types is modified. For example, a console warning listing removed types might be appropriate.
    3. If the resulting entry types sequence is an empty sequence, abort these steps. The user agent SHOULD notify developers when the steps are aborted to notify that registration has been aborted. For example, a console warning might be appropriate.
    4. If the list of registered performance observer objects of relevant global object contains a registered performance observer whose observer is the context object, replace its options list with a list containing options as its only item.
    5. Otherwise, create and append a registered performance observer object to the list of registered performance observer objects of relevant global object, with observer set to the context object and options list set to a list containing options as its only item.
  6. Otherwise, run the following steps:
    1. Assert that observer's observer type is "single".
    2. If options's type is not contained in supported entry types, abort these steps. The user agent SHOULD notify developers when this happens, for instance via a console warning.
    3. If the list of registered performance observer objects of relevant global object contains a registered performance observer obs whose observer is the context object:
      1. If obs's options list contains a PerformanceObserverInit item currentOptions whose type is equal to options's type, replace currentOptions with options in obs's options list.
      2. Otherwise, append options to obs's options list.
    4. Otherwise, create and append a registered performance observer object to the list of registered performance observer objects of relevant global object, with observer set to the context object and options list set to a list containing options as its only item.
    5. If options's buffered flag is set:
      1. Let entries be the PerformanceEntryList object returned by the algorithm with buffer set to relevant performance entry buffer, name set to null and type set to options's type.
      2. Append entries to the context object's observer buffer.

A PerformanceObserver object needs to always call observe() with options's entryTypes set OR always call observe() with options's type set. If one PerformanceObserver calls observe() with entryTypes and also calls observe with type, then an exception is thrown. This is meant to avoid confusion with how calls would stack. When using entryTypes, no other parameters in PerformanceObserverInit can be used. In addition, multiple observe() calls will override for backwards compatibility and because a single call should suffice in this case. On the other hand, when using type, calls will stack because a single call can only specify one type. Calling observe() with a repeated type will also override.

PerformanceObserverInit dictionary

          dictionary PerformanceObserverInit {
            sequence<DOMString> entryTypes;
            DOMString type;
            boolean buffered;
          };
          
entryTypes
A list of entry types to be observed. If present, the list MUST NOT be empty and all other members MUST NOT be present. Types not recognized by the user agent MUST be ignored.
type
A single entry type to be observed. A type that is not recognized by the user agent MUST be ignored. Other members may be present.
buffered
A flag to indicate whether buffered entries should be queued into observer's buffer.

PerformanceObserverEntryList interface

          [Exposed=(Window,Worker)]
          interface PerformanceObserverEntryList {
            PerformanceEntryList getEntries();
            PerformanceEntryList getEntriesByType (DOMString type);
            PerformanceEntryList getEntriesByName (DOMString name, optional DOMString type);
          };
          

getEntries() method

Returns a PerformanceEntryList object returned by algorithm with buffer set to observer buffer, and name and type set to null.

getEntriesByType() method

Returns a PerformanceEntryList object returned by algorithm with buffer set to observer buffer, name set to null, and type set to `type`.

getEntriesByName() method

Returns a PerformanceEntryList object returned by algorithm with buffer set to observer buffer, name set to `name`, and type set to null if optional `entryType` is omitted, and type set to `type` otherwise.

takeRecords() method

The takeRecords() method must return a copy of the context object's observer buffer, and also empty context object's observer buffer.

disconnect() method

The disconnect() method must remove the context object from the list of registered performance observer objects of relevant global object, and also empty context object's observer buffer.

supportedEntryTypes attribute

The user agent MUST maintain supported entry types, a list of strings representing the entry types which the user agent supports for the PerformanceObserver interface. This list is populated by specifications that define new entry types via the register a performance entry type algorithm.

When supportedEntryTypes's attribute getter is called, run the following steps:

  1. Let result be a copy of the user agent's supported entry types.
  2. Sort result in alphabetical order.
  3. Return result.

This attribute allows web developers to easily know which entry types are supported by the user agent.

Processing

Queue a PerformanceEntry

To queue a PerformanceEntry (new entry) with an optional add to performance entry buffer flag, which is unset by default, run these steps:

  1. Let interested observers be an initially empty set of PerformanceObserver objects.
  2. Let entryType be new entry’s entryType value.
  3. For each registered performance observer (regObs):
    1. If regObs's options list contains a PerformanceObserverInit item whose entryTypes member includes entryType or whose type member equals to entryType, append regObs's observer to interested observers.
  4. For each observer in interested observers:
    1. Append new entry to observer's observer buffer.
  5. If the add to performance entry buffer flag is set, add new entry to the performance entry buffer.
  6. If the performance observer task queued flag is set, terminate these steps.
  7. Set performance observer task queued flag.
  8. Queue a task that consists of running the following substeps. The task source for the queued task is the performance timeline task source.
    1. Unset performance observer task queued flag for the relevant global object.
    2. Let notify list be a copy of relevant global object's list of registered performance observer objects.
    3. For each PerformanceObserver object po in notify list, run these steps:
      1. Let entries be a copy of po’s observer buffer.
      2. Empty po’s observer buffer.
      3. If entries is non-empty, call po’s callback with entries as first argument and po as the second argument and callback this value. If this throws an exception, report the exception.

The performance timeline task queue is a low priority queue that, if possible, should be processed by the user agent during idle periods to minimize impact of performance monitoring code.

Filter buffer by name and type

Given a buffer and optional name and type string values this algorithm returns a PerformanceEntryList object that contains a list of PerformanceEntry objects sorted in chronological order with respect to startTime; objects with the same startTime have unspecified ordering.

  1. Let the list of entry objects be the empty PerformanceEntryList.
  2. For each PerformanceEntry object (entryObject) in the buffer, in chronological order with respect to startTime:
    1. If name is not null and entryObject's `name` attribute does not match name in a case-sensitive manner, go to next entryObject.
    2. If type is not null and entryObject's `type` attribute does not match type in a case-sensitive manner, go to next entryObject.
    3. Add entryObject to the list of entry objects.
  3. Return the list of entry objects.

Register performance entry type

To register a performance entry type, run the following steps:

  1. Let type be the input string.
  2. Append type to supported entry types.

Privacy and Security

This specification extends the Performance interface defined by [[HR-TIME-2]] and provides methods to queue and retrieve entries from the performance timeline. Please refer to [[HR-TIME-2]] for privacy and security considerations of exposing high-resoluting timing information.

Acknowledgments

Thanks to Arvind Jain, Boris Zbarsky, Jatinder Mann, Nat Duca, Philippe Le Hegaret, Ryosuke Niwa, Shubhie Panicker, Todd Reifsteck, Yoav Weiss, and Zhiheng Wang, for their contributions to this work.