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<!DOCTYPE html>
<html>
<head>
<title>Performance Timeline Level 2</title>
<meta charset='utf-8'>
<script src='https://www.w3.org/Tools/respec/respec-w3c-common' async class=
'remove'></script>
<script class='remove'>
var respecConfig = {
shortName: "performance-timeline-2",
useExperimentalStyles: true,
specStatus: "ED",
edDraftURI: "https://w3c.github.io/performance-timeline/",
// publishDate: "2013-10-23",
editors: [{
name: "Ilya Grigorik",
url: "https://www.igvita.com/",
mailto: "[email protected]",
company: "Google",
companyURL: "https://www.google.com/",
w3cid: "56102"
},{
name: "Jatinder Mann",
mailto: "[email protected]",
company: "Microsoft Corp.",
note: "Until November 2014"
},{
name: "Zhiheng Wang",
company: "Google",
note: "Until July 2013"
}],
wg: "Web Performance Working Group",
wgURI: "https://www.w3.org/webperf/",
license: "w3c-software-doc",
wgPublicList: "public-web-perf",
subjectPrefix: "[Performance Timeline]",
format: "markdown",
otherLinks: [{
key: 'Repository',
data: [{
value: 'We are on GitHub',
href: 'https://github.com/w3c/performance-timeline/'
}, {
value: 'Commit history',
href: 'https://github.com/w3c/performance-timeline/commits/gh-pages/index.html'
}, {
value: 'File a bug',
href: 'https://github.com/w3c/performance-timeline/issues?q=milestone:%22Level%202%22'
}]
},{
key: 'Mailing list',
data: [{
value: '[email protected]',
href: 'https://lists.w3.org/Archives/Public/public-web-perf/'
}]
},{
key: 'Implementation',
data: [{
value: 'Test Suite',
href: 'http://w3c-test.org/performance-timeline/'
}, {
value: 'Test Suite repository',
href: 'https://github.com/web-platform-tests/wpt/tree/master/performance-timeline'
}]
}],
wgPatentURI: "https://www.w3.org/2004/01/pp-impl/45211/status"
};
</script>
</head>
<body>
<section id='abstract'>
<p>This specification extends the High Resolution Time specification
[[!HR-TIME-2]] by providing methods to store and retrieve high resolution
performance metric data.</p>
</section>
<section id='sotd'>
<p>Performance Timeline Level 2 replaces the first version of
[[PERFORMANCE-TIMELINE]] and includes:</p>
<ul>
<li>Extends the base definition of the <a data-cite="!hr-time-2#idl-def-Performance">Performance</a> interface
defined by [[HR-TIME-2]];
</li>
<li>Exposes <a>PerformanceEntry</a> in Web Workers [[WORKERS]];
</li>
<li>Adds support for <a data-lt='PerformanceObserver'>performance
observers</a>.
</li>
</ul>
</section>
<section class='informative'>
<h2>Introduction</h2>
<p>Accurately measuring performance characteristics of web applications is
an important aspect of making web applications faster. This specification
defines the necessary <a>Performance Timeline</a> primitives that enable
web developers to access, instrument, and retrieve various performance
metrics from the full lifecycle of a web application.</p>
<p>[[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 <a>Performance Timeline</a> of
a web application. For example, the following script shows how a developer
can access the <a>Performance Timeline</a> to obtain performance metrics
related to the navigation of the document, resources on the page, and
developer scripts:</p>
<pre class="example html">
<!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>
</pre>
<p>Alternatively, the developer can observe the <a>Performance Timeline</a>
and be notified of new performance metrics and, optionally, previously
buffered performance metrics of specified type, via the
<a>PerformanceObserver</a> interface:</p>
<pre class="example">
<!doctype html>
<html>
<head></head>
<body>
<img id="image0" src="https://www.w3.org/Icons/w3c_main.png" />
<script>
const observer = 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);
// maybe disconnect after processing the events.
observer.disconnect();
});
// retrieve buffered events and subscribe to new events
// for Resource-Timing and User-Timing
observer.observe({
entryTypes: ["resource", "mark", "measure"],
buffered: true
});
</script>
</body>
</html>
</pre>
<p>The <a>PerformanceObserver</a> 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:</p>
<ul>
<li>Avoid polling the timeline to detect new metrics</li>
<li>Eliminate costly deduplication logic to identify new metrics</li>
<li>Eliminate race conditions with other consumers that may want to
manipulate the buffer</li>
</ul>
<p>The developer is encouraged to use <a>PerformanceObserver</a> where
possible. Further, new performance API's and metrics may only be available
through the <a>PerformanceObserver</a> interface.</p>
</section>
<section id="conformance">
<p>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.)</p>
<p>The IDL fragments in this specification must be interpreted as required
for <a data-cite="!WebIDL#dfn-conforming-idl-fragment">conforming IDL fragments</a>, as described in the Web IDL specification.
[[!WebIDL]]</p>
</section>
<section>
<h2><dfn>Performance Timeline</dfn></h2>
<p>Each <a data-cite="!WebIDL#es-environment">ECMAScript global environment</a> has:</p>
<ul>
<li>a <dfn>performance observer task queued flag</dfn></li>
<li>a <dfn>list of <a>registered performance observer</a> objects</dfn> that is initially empty</li>
<li>a <dfn>performance entry buffer</dfn> to store
<a>PerformanceEntry</a> objects that is initially empty
</li>
</ul>
<p>The <dfn>relevant performance entry buffer</dfn> is the <a>performance entry buffer</a> associated with the
<a data-cite="!WHATWG-HTML#concept-relevant-global">relevant global object</a>.</p>
<section data-dfn-for="Performance" data-link-for="Performance">
<h2>Extensions to the <a data-cite="!hr-time-2#idl-def-Performance">Performance</a> interface</h2>
<p>This extends the <a data-cite="!hr-time-2#idl-def-Performance">Performance</a> interface [[!HR-TIME-2]] and hosts
performance related attributes and methods used to retrieve the
performance metric data from the <a>Performance Timeline</a>.</p>
<pre class="idl">
partial interface Performance {
PerformanceEntryList getEntries ();
PerformanceEntryList getEntriesByType (DOMString type);
PerformanceEntryList getEntriesByName (DOMString name, optional DOMString type);
};
typedef sequence<PerformanceEntry> PerformanceEntryList;
</pre>
<p>The <dfn>PerformanceEntryList</dfn> represents a sequence of
<a>PerformanceEntry</a>, providing developers with all the convenience
methods found on JavaScript arrays.</p>
<section>
<h2><dfn>getEntries()</dfn> method</h2>
<p>Returns a <a>PerformanceEntryList</a> object returned by
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>relevant performance entry buffer</a>, and
<var>name</var> and <var>type</var> set to `null`.</p>
</section>
<section>
<h2><dfn>getEntriesByType()</dfn> method</h2>
<p>Returns a <a>PerformanceEntryList</a> object returned by
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>relevant performance entry buffer</a>,
<var>name</var> set to `null`, and <var>type</var> set
to `type`.</p>
</section>
<section>
<h2><dfn>getEntriesByName()</dfn> method</h2>
<p>Returns a <a>PerformanceEntryList</a> object returned by
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>relevant performance entry buffer</a>,
<var>name</var> set to `name`, and <var>type</var> set
to `null` if optional `entryType` is omitted, and <var>type</var> set
to `type` otherwise.</p>
</section>
</section>
<section data-dfn-for="PerformanceEntry" data-link-for="PerformanceEntry">
<h2>The <dfn>PerformanceEntry</dfn> interface</h2>
<p>The <a>PerformanceEntry</a> interface hosts the performance data of
various metrics.</p>
<pre class='idl'>
[Exposed=(Window,Worker)]
interface PerformanceEntry {
readonly attribute DOMString name;
readonly attribute DOMString entryType;
readonly attribute DOMHighResTimeStamp startTime;
readonly attribute DOMHighResTimeStamp duration;
[Default] object toJSON();
};</pre>
<dl>
<dt><dfn>name</dfn></dt>
<dd>This attribute MUST return an identifier for this
<a>PerformanceEntry</a> object. This identifier does not have to be
unique.</dd>
<dt><dfn>entryType</dfn></dt>
<dd>This attribute MUST return the type of the
interface represented by this <a>PerformanceEntry</a> object.
<p class="note">Example `entryType` values defined by other
specifications include: <code>"mark"</code> and <code>"measure"</code>
[[USER-TIMING-2]], <code>"navigation"</code> [[NAVIGATION-TIMING-2]],
<code>"resource"</code> [[RESOURCE-TIMING-2]],
<!-- TODO: add long task spec reference -->
and <code>"longtask"</code>.</p></dd>
<dt><dfn>startTime</dfn></dt>
<dd>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 <code>0</code>.</dd>
<dt><dfn>duration</dfn></dt>
<dd>This attribute MUST return the time value of the
duration of the entire event being recorded by this
<a>PerformanceEntry</a>. Typically, this would be the time difference
between the last recorded timestamp and the first recorded timestamp of
this <a>PerformanceEntry</a>. If the duration concept doesn't apply, a
performance metric may choose to return a `duration` of
<code>0</code>.</dd>
</dl>
<p>When <dfn>toJSON</dfn> is called, run [[!WebIDL]]'s
<a data-cite="WEBIDL#default-tojson-operation">default toJSON
operation</a>.</p>
</section>
<section data-link-for="PerformanceObserver" data-dfn-for=
"PerformanceObserver">
<h2>The <dfn>PerformanceObserver</dfn> interface</h2>
<p>The <a>PerformanceObserver</a> interface can be used to observe the
<a>Performance Timeline</a> to be notified of new performance metrics as
they are recorded, and optionally buffered performance metrics.
<p>Each <a>PerformanceObserver</a> has these associated concepts:</p>
<ul>
<li>A <dfn>PerformanceObserverCallback</dfn> set on creation.</li>
<li> A <a>PerformanceEntryList</a> object called the <dfn>observer
buffer</dfn> that is initially empty.
</li>
</ul>
<p>The `PerformanceObserver(callback)` constructor must create a new
<a>PerformanceObserver</a> object with <a>PerformanceObserverCallback</a>
set to <var>callback</var> and then return it.</p>
<p>A <dfn>registered performance observer</dfn> is a <a data-cite="!WHATWG-INFRA#struct">struct</a>
consisting of an <var>observer</var> (a <a>PerformanceObserver</a> object)
and <var>options</var> (a <a>PerformanceObserverInit</a> dictionary).</p>
<pre class="idl">
callback PerformanceObserverCallback = void (PerformanceObserverEntryList entries,
PerformanceObserver observer);
[Constructor(PerformanceObserverCallback callback), Exposed=(Window,Worker)]
interface PerformanceObserver {
void observe (PerformanceObserverInit options);
void disconnect ();
PerformanceEntryList takeRecords();
};
</pre>
<p class="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.</p>
<section>
<h2><dfn>observe()</dfn> method</h2>
<p>The <a>observe()</a> method instructs the user agent to
<dfn>register the observer</dfn> and must run these steps:</p>
<ol data-link-for="PerformanceObserverInit">
<li>Let <var>entry types</var> be <var>options</var> <a>entryTypes</a>
sequence.</li>
<li>Remove all unsupported types from <var>entry types</var>. The user
agent SHOULD notify developers if <var>entry types</var> is modified.
For example, a console warning listing removed types might be appropriate.</li>
<li>If the resulting <var>entry types</var> 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.</li>
<li>If the <a>list of registered performance observer objects</a> of
<a data-cite="!WHATWG-HTML#concept-relevant-global">relevant global object</a> contains a <a>registered performance
observer</a> whose `observer` is the <a data-cite="!WHATWG-DOM#context-object">context object</a>,
replace its `options`, with <var>options</var>.
</li>
<li>Otherwise, append a new <a>registered performance observer</a>
object to the <a>list of registered performance observer objects</a> of
<a data-cite="!WHATWG-HTML#concept-relevant-global">relevant global object</a>, with the
<a data-cite="!WHATWG-DOM#context-object">context object</a> as `observer` and <var>options</var> as the `options`.
</li>
<li>If <var>options</var>' <a>buffered</a> flag is set, for each
<var>entry type</var> in <var>entry types</var> sequence:
<ol>
<li>Let <var>entries</var> be the <a>PerformanceEntryList</a>
object returned by the
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>relevant performance entry buffer</a>,
<var>name</var> set to `null` and <var>type</var> set to
<var>entry type</var>.
</li>
<li>Append <var>entries</var> to the <a data-cite="!WHATWG-DOM#context-object">context object</a>'s
<a>observer buffer</a>.
</li>
</ol>
</li>
</ol>
<section data-dfn-for="PerformanceObserverInit" data-link-for=
"PerformanceObserverInit">
<h2><dfn>PerformanceObserverInit</dfn> dictionary</h2>
<pre class="idl">
dictionary PerformanceObserverInit {
required sequence<DOMString> entryTypes;
boolean buffered = false;
};
</pre>
<dl>
<dt><dfn>entryTypes</dfn></dt>
<dd>A list of entry types to be observed. The list MUST NOT be empty
and types not recognized by the user agent MUST be ignored.</dd>
</dl>
<dl>
<dt><dfn>buffered</dfn></dt>
<dd>A flag to indicate whether buffered entries should be queued into
observer's buffer.</dd>
</dl>
</section>
<section data-dfn-for="PerformanceObserverEntryList" data-link-for=
"PerformanceObserverEntryList">
<h2><dfn>PerformanceObserverEntryList</dfn> interface</h2>
<pre class="idl">
[Exposed=(Window,Worker)]
interface PerformanceObserverEntryList {
PerformanceEntryList getEntries();
PerformanceEntryList getEntriesByType (DOMString type);
PerformanceEntryList getEntriesByName (DOMString name, optional DOMString type);
};
</pre>
<section>
<h2><dfn>getEntries()</dfn> method</h2>
<p>Returns a <a>PerformanceEntryList</a> object returned by
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>observer buffer</a>, and
<var>name</var> and <var>type</var> set to `null`.</p>
</section>
<section>
<h2><dfn>getEntriesByType()</dfn> method</h2>
<p>Returns a <a>PerformanceEntryList</a> object returned by
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>observer buffer</a>,
<var>name</var> set to `null`, and <var>type</var> set
to `type`.</p>
</section>
<section>
<h2><dfn>getEntriesByName()</dfn> method</h2>
<p>Returns a <a>PerformanceEntryList</a> object returned by
<a href="#filter-buffer-by-name-and-type"></a> algorithm with
<var>buffer</var> set to <a>observer buffer</a>,
<var>name</var> set to `name`, and <var>type</var> set
to `null` if optional `entryType` is omitted, and <var>type</var> set
to `type` otherwise.</p>
</section>
</section>
</section>
<section>
<h2><dfn>takeRecords()</dfn> method</h2>
<p>The <a>takeRecords()</a> method must return a copy of the
<a data-cite="!WHATWG-DOM#context-object">context object</a>'s <a>observer buffer</a>, and also empty
<a data-cite="!WHATWG-DOM#context-object">context object</a>'s <a>observer buffer</a>.</p>
</section>
<section>
<h2><dfn>disconnect()</dfn> method</h2>
<p>The <a>disconnect()</a> method must remove the <a data-cite="!WHATWG-DOM#context-object">context object</a>
from the <a>list of registered performance observer objects</a> of
<a data-cite="!WHATWG-HTML#concept-relevant-global">relevant global object</a>, and also empty
<a data-cite="!WHATWG-DOM#context-object">context object</a>'s
<a>observer buffer</a>.</p>
</section>
</section>
</section>
<section>
<h2>Processing</h2>
<section data-link-for="PerformanceObserver">
<h2>Queue a <code>PerformanceEntry</code></h2>
<p>To <dfn>queue a PerformanceEntry</dfn> (<var>new entry</var>) with an
optional <var>add to performance entry buffer</var> flag, which is unset
by default, run these steps:</p>
<ol>
<li>Let <i>interested observers</i> be an initially empty set of
<a>PerformanceObserver</a> objects.
</li>
<li>For each <a>registered performance observer</a> (<i>observer</i>):
<ol>
<li>If <i>observer</i>'s <a>PerformanceObserverInit</a> <a data-lt=
"PerformanceObserverInit.entryTypes">entryTypes</a> includes <i>new
entry</i>’s <a data-lt="PerformanceEntry.entryType">entryType</a>
value, append <i>observer</i> to <i>interested observers</i>.
</li>
</ol>
</li>
<li>For each <i>observer</i> in <i>interested observers</i>:
<ol>
<li>Append <i>new entry</i> to <a>observer buffer</a>.
</li>
</ol>
</li>
<li>If the <var>add to performance entry buffer</var> flag is set, add
<i>new entry</i> to the <a>performance entry buffer</a>.
</li>
<li>If the <a>performance observer task queued flag</a> is set, terminate
these steps.
</li>
<li>Set <a>performance observer task queued flag</a>.
</li>
<li>
<a>Queue a task</a> that consists of running the following substeps.
The <a data-cite="!WHATWG-HTML#task-source">task source</a> for the queued task is the <i>performance
timeline</i> task source.
<ol>
<li>Unset <a>performance observer task queued flag</a> for the
<a data-cite="!WHATWG-HTML#concept-relevant-global">relevant global object</a>.</li>
<li>Let <i>notify list</i> be a copy of <a data-cite="!WHATWG-HTML#concept-relevant-global">relevant global object</a>'s <a>list of registered performance observer objects</a>.
</li>
<li>For each <a>PerformanceObserver</a> object <i>po</i> in <i>notify
list</i>, run these steps:
<ol>
<li>Let <i>entries</i> be a copy of <i>po</i>’s <a>observer
buffer</a>.
</li>
<li>Empty <i>po</i>’s <a>observer buffer</a>.
</li>
<li>If <i>entries</i> is non-empty, call <i>po</i>’s callback
with <i>entries</i> as first argument and <i>po</i> as the second
argument and <a data-cite="!WebIDL#dfn-callback-this-value">callback this value</a>. If this <a data-cite="!WebIDL#dfn-throw">throw</a>s an
exception, <a data-cite="!WHATWG-HTML#report-the-exception">report the exception</a>.
</li>
</ol>
</li>
</ol>
</li>
</ol>
<p>The <i>performance timeline</i> <a
data-cite="!WHATWG-HTML#task-queue">task queue</a> 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.</p>
</section>
<section data-link-for="PerformanceEntry">
<h2>Filter <var>buffer</var> by <var>name</var> and <var>type</var></h2>
<p>Given a <var>buffer</var> and optional <var>name</var> and
<var>type</var> string values this algorithm returns a
<a>PerformanceEntryList</a> object that contains a list of
<a>PerformanceEntry</a> objects sorted in chronological order with respect
to <a>startTime</a>; objects with the same <a>startTime</a> have
unspecified ordering.</p>
<ol>
<li>Let the <dfn>list of entry objects</dfn> be the empty
<a>PerformanceEntryList</a>.
</li>
<li>For each <a>PerformanceEntry</a> object (<dfn>entryObject</dfn>) in
the <var>buffer</var>, in chronological order with
respect to <a>startTime</a>:
<ol>
<li>If <var>name</var> is not `null` and <a>entryObject</a>'s
`name` attribute does not match <var>name</var> in a
<a data-cite="!WHATWG-HTML#case-sensitive">case-sensitive</a> manner, go to next <a>entryObject</a>.
</li>
<li>If <var>type</var> is not `null` and <a>entryObject</a>'s
`type` attribute does not match <var>type</var> in a
<a data-cite="!WHATWG-HTML#case-sensitive">case-sensitive</a> manner, go to next <a>entryObject</a>.
</li>
<li>Add <a>entryObject</a> to the <a>list of entry objects</a>.
</li>
</ol>
</li>
<li>Return the <a>list of entry objects</a>.
</li>
</ol>
</section>
</section>
<section id="privacy-security">
<h3>Privacy and Security</h3>
<p>This specification extends the <a data-cite="!hr-time-2#idl-def-Performance">Performance</a> interface defined by
[[HR-TIME-2]] and provides methods to queue and retrieve entries from the
<a>performance timeline</a>. Please refer to [[HR-TIME-2]] for privacy and
security considerations of exposing high-resoluting timing information.</p>
</section>
<section class="appendix" id="idl-index"></section>
<section class="appendix">
<h2>Acknowledgments</h2>
<p>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.</p>
</section>
</body>
</html>