Abstract

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

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

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

The Working Group expects to demonstrate 2 implementations of the features listed in this specification by the end of the Candidate Recommendation phase.

No changes were done since the previous publication.

This document was published by the Web Performance Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-web-perf@w3.org (subscribe, archives) with [Performance Timeline] at the start of your email's subject. W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. This Candidate Recommendation is expected to advance to Proposed Recommendation no earlier than 10 January 2017. All comments are welcome.

Please see the Working Group's implementation report.

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 September 2015 W3C Process Document.

1. Introduction

This section is non-normative.

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], and [USER-TIMING] 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:

Example 1
<!doctype html>
<html>
  <head>
  </head>
  <body onload="init()">
    <img id="image0" src="https://www.w3.org/Icons/w3c_main.png" />
    <script>
       function init()
       {
            performance.mark("startWork"); // see [USER-TIMING]
            doWork(); // Some developer code
            performance.mark("endWork");
            measurePerf();
       }
       function measurePerf()
       {
           var perfEntries = performance.getEntries();
           for (var i = 0; i < perfEntries.length; i++)
           {
                 if (window.console) {
                   console.log("Name: "        + perfEntries[i].name      +
                               " Entry Type: " + perfEntries[i].entryType +
                               " Start Time: " + perfEntries[i].startTime +
                               " Duration: "   + perfEntries[i].duration  + "\n");
                 }
           }
       }
    </script>
  </body>
</html>

Alternatively, the developer can observe the Performance Timeline and be notified of new performance metrics via the PerformanceObserver interface:

Example 2
<!doctype html>
<html>
  <head>
  </head>
  <body>
    <img id="image0" src="https://www.w3.org/Icons/w3c_main.png" />
    <script>
    var doneObservingEvents = false;
    var observer = new PerformanceObserver(function(list) {
      var perfEntries = list.getEntries();
      for (var i = 0; i < perfEntries.length; i++)
      {
           if (window.console) {
             console.log("Name: "        + perfEntries[i].name      +
                         " Entry Type: " + perfEntries[i].entryType +
                         " Start Time: " + perfEntries[i].startTime +
                         " Duration: "   + perfEntries[i].duration  + "\n");
           }
      }
      // maybe disconnect after processing the events.
      if (doneObservingEvents) {
           observer.disconnect();
      }
    });
    // subscribe to Resource-Timing and User-Timing events
    observer.observe({entryTypes: ['resource', 'mark', 'measure']});
    </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.

2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MUST and MUST NOT are to be interpreted as described in [RFC2119].

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.)

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WebIDL]

3. Performance Timeline

Each ECMAScript global environment has:

To queue a PerformanceEntry (new entry), run these steps:

  1. Let interested observers be an initially empty set of PerformanceObserver objects.
  2. For each registered performance observer (observer):
    1. If observer's PerformanceObserverInit entryTypes includes new entry’s entryType value, append observer to interested observers.
  3. For each observer in interested observers:
    1. Append new entry to observer buffer.
  4. If the performance observer task queued flag is set, terminate these steps.
  5. Set performance observer task queued flag.
  6. 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.
    2. Let notify list be a copy of ECMAScript global environment'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 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.

3.1 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;
    serializer = {attribute};
};

The name attribute MUST return an identifier for this PerformanceEntry object. This identifier does not have to be unique.

The entryType attribute MUST return the type of the interface represented by this PerformanceEntry object.

Note

Valid entryType values are: "mark [USER-TIMING], "measure" [USER-TIMING], "navigation" [NAVIGATION-TIMING-2], "resource" [RESOURCE-TIMING].

The startTime attribute MUST return the time value of the first recorded timestamp of this performance metric.

The duration 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.

3.2 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 getEntries method returns a PerformanceEntryList object returned by 4.1 Filter performance entry buffer by name and type algorithm with name and type set to null.

The getEntriesByType method returns a PerformanceEntryList object returned by 4.1 Filter performance entry buffer by name and type algorithm with name set to null and type set to type.

The getEntriesByName method returns a PerformanceEntryList object returned by 4.1 Filter performance entry buffer by name and type algorithm with name set to name and type set to null if optional entryType is omitted, and type set to type otherwise.

3.3 The PerformanceObserver interface

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

Each PerformanceObserver has these associated concepts:

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

A registered performance observer consists of an observer (a PerformanceObserver object) and options (a PerformanceObserverInit dictionary).

dictionary PerformanceObserverInit {
    required sequence<DOMString> entryTypes;
};

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

callback PerformanceObserverCallback = void (PerformanceObserverEntryList entries,
                                             PerformanceObserver observer);

[Constructor(PerformanceObserverCallback callback),
 Exposed=(Window,Worker)]
interface PerformanceObserver {
    void observe(PerformanceObserverInit options);
    void disconnect();
};

PerformanceObserverInit.entryTypes is a list of valid entryType names to be observed. The list MUST NOT be empty and types not recognized by the user agent MUST be ignored.

Note

To keep the performance overhead to minimum the application should 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.

The PerformanceObserverEntryList interface provides the same getEntries, getEntriesByType, getEntriesByName methods as the Performance interface, except that PerformanceObserverEntryList operates on the observed emitted list of events instead of the global timeline.

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

  1. If options' entryTypes attribute is not present, throw a JavaScript TypeError.
  2. Filter unsupported entryType entryType names within the entryTypes sequence, and replace the entryTypes sequence with the new filtered sequence.
  3. If the options' entryTypes attribute is an empty sequence, throw a JavaScript TypeError.
  4. If the list of registered performance observer objects associated with the ECMAScript global environment of the interface object's contains a registered performance observer that is the context object, replace the context object's options with options.
  5. Otherwise, append a new registered performance observer object to the list of registered performance observer objects associated with the ECMAScript global environment of the interface object, with the context object as observer and options as the options.

The disconnect method must remove the context object from the list of PerformanceObserver objects associated with the ECMAScript global environment of the interface object, and also empty context object's observer buffer.

4. Processing

4.1 Filter performance entry buffer by name and type

Given 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.

  1. Let the list of entry objects be the empty PerformanceEntryList.
  2. For each PerformanceEntry object (entryObject) in the performance entry 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.

5. 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.

A. 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.

B. References

B.1 Normative references

[HR-TIME-2]
Ilya Grigorik; James Simonsen; Jatinder Mann. W3C. High Resolution Time Level 2. 1 November 2016. W3C Candidate Recommendation. URL: https://www.w3.org/TR/hr-time-2/
[RFC2119]
S. Bradner. IETF. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. W3C. Web IDL. 15 September 2016. W3C Working Draft. URL: https://www.w3.org/TR/WebIDL-1/

B.2 Informative references

[NAVIGATION-TIMING-2]
Tobin Titus; Jatinder Mann; Arvind Jain. W3C. Navigation Timing Level 2. 22 April 2016. W3C Working Draft. URL: https://www.w3.org/TR/navigation-timing-2/
[PERFORMANCE-TIMELINE]
Jatinder Mann; Zhiheng Wang. W3C. Performance Timeline. 12 December 2013. W3C Recommendation. URL: https://www.w3.org/TR/performance-timeline/
[RESOURCE-TIMING]
Arvind Jain; Todd Reifsteck; Jatinder Mann; Zhiheng Wang; Anderson Quach. W3C. Resource Timing Level 1. 21 July 2016. W3C Candidate Recommendation. URL: https://www.w3.org/TR/resource-timing-1/
[USER-TIMING]
Jatinder Mann; Zhiheng Wang; Anderson Quach. W3C. User Timing. 12 December 2013. W3C Recommendation. URL: https://www.w3.org/TR/user-timing/
[WORKERS]
Ian Hickson. W3C. Web Workers. 24 September 2015. W3C Working Draft. URL: https://www.w3.org/TR/workers/