html5/spec-ja/.workers-spec.en.html


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  <title>Web Workers</title>
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   <p><a class=logo href="http://www.whatwg.org/" rel=home><img alt=WHATWG
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   <h1 id=web-workers>Web Workers</h1>

   <h2 class="no-num no-toc" id=draft>Draft Recommendation &mdash; 17 July
    2008</h2>

   <p>You can take part in this work. <a
    href="http://www.whatwg.org/mailing-list">Join the working group's
    discussion list.</a></p>

   <p><strong>Web designers!</strong> We have a <a
    href="http://blog.whatwg.org/faq/">FAQ</a>, a <a
    href="http://forums.whatwg.org/">forum</a>, and a <a
    href="http://www.whatwg.org/mailing-list#help">help mailing list</a> for
    you!</p>

   <dl>
    <dt>This version:

    <dd><a
     href="http://www.whatwg.org/specs/web-workers/current-work/">http://www.whatwg.org/specs/web-workers/current-work/</a>

    <dt>Version history:

    <dd>Twitter messages (non-editorial changes only): <a
     href="http://twitter.com/WHATWG">http://twitter.com/WHATWG</a>

    <dd>Commit-Watchers mailing list: <a
     href="http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org">http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org</a>

    <dd>Interactive Web interface: <a
     href="http://html5.org/tools/web-workers-tracker">http://html5.org/tools/web-workers-tracker</a>

    <dd>Subversion interface: <a
     href="http://svn.whatwg.org/webworkers/">http://svn.whatwg.org/webworkers/</a>

    <dt>Issues:

    <dd>To send feedback: <a
     href="http://www.whatwg.org/mailing-list">[email protected]</a>

    <dd>To view and vote on feedback: <a
     href="http://www.whatwg.org/issues/">http://www.whatwg.org/issues/</a>

    <dt>Editor:

    <dd>Ian Hickson, Google, [email protected]
   </dl>

   <p class=copyright>&copy; Copyright 2004-2008 Apple Computer, Inc.,
    Mozilla Foundation, and Opera Software ASA.</p>

   <p class=copyright>You are granted a license to use, reproduce and create
    derivative works of this document.</p>
  </div>

  <hr>

  <h2 class="no-num no-toc" id=abstract>Abstract</h2>

  <p>This specification defines an API that allows Web application authors to
   spawn background workers running scripts in parallel to their main page.
   This allows for thread-like operation with message-passing as the
   coordination mechanism.

  <h2 class="no-num no-toc" id=status>Status of this document</h2>

  <p><strong>This is a work in progress!</strong> This document is changing
   on a daily if not hourly basis in response to comments and as a general
   part of its development process. Comments are very welcome, please send
   them to <a href="mailto:[email protected]">[email protected]</a>. Thank
   you.

  <p>The current focus is in developing a first draft proposal.

  <p>Implementors should be aware that this specification is not stable.
   <strong>Implementors who are not taking part in the discussions are likely
   to find the specification changing out from under them in incompatible
   ways.</strong> Vendors interested in implementing this specification
   before it eventually reaches the call for implementations should join the
   <a href="/mailing-list">WHATWG mailing list</a> and take part in the
   discussions.

  <p>This specification is also being produced by the <a
   href="http://www.w3.org/html/wg">W3C HTML WG</a>. The two specifications
   are identical from the table of contents onwards.

  <h2 class="no-num no-toc" id=contents>Table of contents</h2>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
    <ul class=toc>
     <li><a href="#tutorial"><span class=secno>1.1 </span>Tutorial</a>

     <li><a href="#requirements"><span class=secno>1.2
      </span>Requirements</a>

     <li><a href="#conformance"><span class=secno>1.3 </span>Conformance
      requirements</a>
      <ul class=toc>
       <li><a href="#dependencies"><span class=secno>1.3.1
        </span>Dependencies</a>
      </ul>

     <li><a href="#terminology"><span class=secno>1.4 </span>Terminology</a>
    </ul>

   <li><a href="#inside"><span class=secno>2. </span>Inside workers</a>
    <ul class=toc>
     <li><a href="#the-windowworker"><span class=secno>2.1 </span>The
      <code>WindowWorker</code> interface</a>

     <li><a href="#the-queue"><span class=secno>2.2 </span>The queue of
      events</a>

     <li><a href="#the-workers"><span class=secno>2.3 </span>The worker's
      ports</a>

     <li><a href="#processing"><span class=secno>2.4 </span>Processing
      model</a>
    </ul>

   <li><a href="#creating"><span class=secno>3. </span>Creating workers</a>
    <ul class=toc>
     <li><a href="#the-windowworkercreators"><span class=secno>3.1 </span>The
      <code>WindowWorkerCreators</code> interface</a>
    </ul>

   <li class=no-num><a href="#references">References</a>

   <li class=no-num><a href="#acknowledgements">Acknowledgements</a>
  </ul>
  <!--end-toc-->

  <hr>

  <h2 id=introduction><span class=secno>1. </span>Introduction</h2>

  <h3 id=tutorial><span class=secno>1.1 </span>Tutorial</h3>

  <p><em>This section is non-normative.</em>

  <p class=big-issue>This section is missing.

  <h3 id=requirements><span class=secno>1.2 </span>Requirements</h3>

  <p><em>This section is non-normative.</em>

  <p>This specification aims to address the following use cases and
   requirements:

  <ul>
   <li>Background workers: A Web application needs to keep its data
    synchronised with the server, both sending updates to the server and
    receiving updates from the server, including handling buffering of
    updates for when the application goes offline. The code to do this would
    ideally be independent of the UI code.

   <li>URLs: Workers should be spawned from URLs, not from strings, since
    script rarely has access to its own source.

   <li>Message queuing: Messages sent to a worker before the worker has
    initialised should not be lost.

   <li>Workers should have access to timers.

   <li>Workers should have access to the network.

   <li>Workers should be able to use libraries.

   <li>Implementations should not have to expose <code>Node</code> or
    <code>Document</code> objects to workers.

   <li>Workers should not share anything with the outside world. The objects
    representing the worker in the worker itself and in the context that
    created the worker should be different, for instance.

   <li>Shared workers: Multiple instances of the same Web application would
    want to keep just one connection back to the server.

   <li>Capabilities granting: It should be possible for code running in one
    iframe to negotiate a connection to another iframe, with that connection
    granting certain rights (e.g. adding to an address book but not reading
    from it).

   <li>Delegation: It should be possible for one worker to spawn another
    worker and efficiently delagate a request to that worker, without the
    caller being aware of the delagate and without the original worker having
    to proxy all the messages.

   <li>Workers whose parents are not longer useful should be killed. Workers
    should be able to detect this is about to happen and exit gracefully.
  </ul>

  <h3 id=conformance><span class=secno>1.3 </span>Conformance requirements</h3>

  <p>All diagrams, examples, and notes in this specification are
   non-normative, as are all sections explicitly marked non-normative.
   Everything else in this specification is normative.

  <p>The key words "MUST", "MUST NOT", "REQUIRED", <!--"SHALL", "SHALL
  NOT",-->
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the
   normative parts of this document are to be interpreted as described in
   RFC2119. For readability, these words do not appear in all uppercase
   letters in this specification. <a href="#refsRFC2119">[RFC2119]</a></p>
  <!-- XXX but they should be
  marked up -->

  <p>Requirements phrased in the imperative as part of algorithms (such as
   "strip any leading space characters" or "return false and abort these
   steps") are to be interpreted with the meaning of the key word ("must",
   "should", "may", etc) used in introducing the algorithm.

  <p>Some conformance requirements are phrased as requirements on attributes,
   methods or objects. Such requirements are to be interpreted as
   requirements on user agents.

  <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>The only conformance class defined by this specification is user agents.

  <p>User agents may impose implementation-specific limits on otherwise
   unconstrained inputs, e.g. to prevent denial of service attacks, to guard
   against running out of memory, or to work around platform-specific
   limitations.

  <h4 id=dependencies><span class=secno>1.3.1 </span>Dependencies</h4>

  <p>This specification relies on several other underlying specifications.

  <dl>
   <dt>HTML5

   <dd>
    <p>Many fundamental concepts from HTML5 are used by this specification.
     <a href="#refsHTML5">[HTML5]</a></p>

   <dt>ECMAScript

   <dd>
    <p>This specification is intended to be used with JavaScript as the
     scripting language. <a href="#refsJS">[JS]</a></p>

   <dt>WebIDL

   <dd>
    <p>The IDL blocks in this specification use the semantics of the WebIDL
     specification. <a href="#refsWebIDL">[WebIDL]</a></p>
  </dl>

  <h3 id=terminology><span class=secno>1.4 </span>Terminology</h3>

  <p>For simplicity, terms such as <em>shown</em>, <em>displayed</em>, and
   <em>visible</em> might sometimes be used when referring to the way a
   document is rendered to the user. These terms are not meant to imply a
   visual medium; they must be considered to apply to other media in
   equivalent ways.

  <p>The construction "a <code title="">Foo</code> object", where <code
   title="">Foo</code> is actually an interface, is sometimes used instead of
   the more accurate "an object implementing the interface <code
   title="">Foo</code>".

  <p>The term DOM is used to refer to the API set made available to scripts
   in Web applications, and does not necessarily imply the existence of an
   actual <code>Document</code> object or of any other <code>Node</code>
   objects as defined in the DOM Core specifications. <a
   href="#refsDOM3CORE">[DOM3CORE]</a>

  <p>A DOM attribute is said to be <em>getting</em> when its value is being
   retrieved (e.g. by author script), and is said to be <em>setting</em> when
   a new value is assigned to it.

  <p>If a DOM object is said to be <dfn id=live>live</dfn>, then that means
   that any attributes returning that object must always return the same
   object (not a new object each time), and the attributes and methods on
   that object must operate on the actual underlying data, not a snapshot of
   the data.

  <h2 id=inside><span class=secno>2. </span>Inside workers</h2>

  <h3 id=the-windowworker><span class=secno>2.1 </span>The <code><a
   href="#windowworker">WindowWorker</a></code> interface</h3>

  <pre
   class=idl>[NoInterfaceObject] interface <dfn id=windowworker>WindowWorker</dfn> {
  readonly attribute boolean <a href="#name" title=dom-windowworker-name>name</a>;
  readonly attribute boolean <a href="#closing" title=dom-windowworker-closing>closing</a>;
  void <a href="#close" title=dom-windowworker-close>close</a>();

  // event handler attributes
           attribute <span>EventListener</span> <a href="#onjoin" title=handler-windowworker-onjoin>onjoin</a>;
           attribute <span>EventListener</span> <a href="#onunload" title=handler-windowworker-onunload>onunload</a>;
};</pre>

  <p>Objects that implement the <code><a
   href="#windowworker">WindowWorker</a></code> interface must also implement
   the <code>Window</code> interface (and thus also the
   <code>WindowTimers</code> and <code><a
   href="#windowworkercreators">WindowWorkerCreators</a></code> interfaces)
   and the <code>EventTarget</code> interface.

  <p>The <dfn id=name title=dom-windowworker-name><code>name</code></dfn>
   attribute must return the value it was assigned when the <code><a
   href="#windowworker">WindowWorker</a></code> object was created by the "<a
   href="#run-a">run a worker</a>" algorithm.

  <p>The <dfn id=closing
   title=dom-windowworker-closing><code>closing</code></dfn> attribute must
   return false until the "<a href="#kill-a">kill a worker</a>" processing
   model defined below sets it to false.

  <p>The following are the <span>event handler DOM attributes</span> that
   must be supported by objects implementing the <code><a
   href="#windowworker">WindowWorker</a></code> interface:

  <dl>
   <dt><dfn id=onjoin
    title=handler-windowworker-onjoin><code>onjoin</code></dfn>

   <dd>
    <p>Must be invoked whenever a <code
     title=event-windowworker-join>join</code> event is targeted at or
     bubbles through the <code><a
     href="#windowworker">WindowWorker</a></code> object.

   <dt><dfn id=onunload
    title=handler-windowworker-onunload><code>onunload</code></dfn>

   <dd>
    <p>Must be invoked whenever a <code title=event-unload>unload</code>
     event is targeted at or bubbles through the <code><a
     href="#windowworker">WindowWorker</a></code> object.
  </dl>

  <h3 id=the-queue><span class=secno>2.2 </span>The queue of events</h3>

  <p>Each <code><a href="#windowworker">WindowWorker</a></code> object is
   asssociated with a <dfn id=queue>queue of events</dfn>, which is initially
   empty.

  <p>An event in the queue can be a DOM event or a timeout callback.

  <p>All asynchronous callbacks and events that would be called or dispatched
   in the worker must be added to the worker's queue, with the "<a
   href="#run-a">run a worker</a>" processing model below taking care of
   actually calling the callbacks or dispatching the events.

  <p>Once the <code><a href="#windowworker">WindowWorker</a></code>'s <code
   title=dom-windowworker-closing><a href="#closing">closing</a></code>
   attribute is set to true, the queue must discard anything else that would
   be added to it. Effectively, once the <code
   title=dom-windowworker-closing><a href="#closing">closing</a></code>
   attribute is true, timers stop firing, notifications for all pending
   asynchronous operations are dropped, etc.

  <h3 id=the-workers><span class=secno>2.3 </span>The worker's ports</h3>

  <p>Workers communicate with other workers and with <span title="browsing
   context">browsing contexts</span> through <span title="channel
   messaging">message channels</span> and their <code>MessagePort</code>
   objects.

  <p>Each <code><a href="#windowworker">WindowWorker</a></code> <var
   title="">worker</var> has a list of <dfn id=the-workers0>the worker's
   ports</dfn>, which consists of all the <code>MessagePort</code> objects
   that are entangled with another port and that have one (but only one) port
   whose <code title=dom-MessagePort-ownerWindow>ownerWindow</code> is <var
   title="">worker</var>. This list includes all the <code>MessagePort</code>
   objects that are in events pending in the <a href="#queue">queue of
   events</a>.

  <h3 id=processing><span class=secno>2.4 </span>Processing model</h3>

  <p>When a user agent is to <dfn id=run-a>run a worker</dfn> named <var
   title="">name</var> for a script with <span>URL</span> <var
   title="">url</var>, a browsing context <var title="">owner browsing
   context</var> and a <code>Document</code> <var title="">owner
   document</var>, it must run the following steps in a completely separate
   and parallel execution environment:

  <ol>
   <li>
    <p>Attempt to <span>fetch</span><!-- XXX --> the resource identified by
     <var title="">url</var>.</p>

    <p>If the attempt fails, then abort these steps and invoke the <a
     href="#worker0" title="worker creation failed">error handling steps</a>
     defined by the algorithm that called this one.</p>

    <p>If the attempt succeeds, then let <var title="">script</var> be the
     resource that was obtained.</p>

    <p class=note>As with <code>script</code> elements, the MIME type of the
     script is ignored. Unlike with <code>script</code> elements, there is no
     way to override the type. It's always assumed to be JavaScript.</p>
    <!-- XXX people will complain about
    this. I guess we might want to examine the MIME type... -->
    

   <li>
    <p>Create a new <code><a href="#windowworker">WindowWorker</a></code>
     object, <var title="">window</var>.</p>

   <li>
    <p>Set the <code title=dom-windowworker-name><a
     href="#name">name</a></code> attribute of <var title="">window</var> to
     the value of <var title="">name</var>.</p>

   <li>
    <p>Let <var title="">script</var>'s <span>script execution context</span>
     (and thus also <span>global object</span>) be <var
     title="">window</var>.</p>

   <li>
    <p>Let <var title="">script</var>'s <span>script browsing context</span>
     be <var title="">owner browsing context</var>.</p>

   <li>
    <p>Let <var title="">script</var>'s <span>script document context</span>
     be <var title="">owner document</var>.</p>

   <li>
    <p>Invoke the <a href="#worker" title="worker creation succeeded">success
     steps</a> defined by the algorithm that called this one. (This will add
     an event to the <a href="#queue">queue of events</a>.)</p>

   <li>
    <p>Start monitoring <var title="">worker</var>, such that whenever the
     <var title="">window</var> object's <code
     title=dom-windowworker-closing><a href="#closing">closing</a></code>
     attribute is false and all <a href="#the-workers0">the worker's
     ports</a> have their <code title=dom-MessagePort-active>active</code>
     attributes set to false, the user agent suspends execution of script in
     that worker until such time as either the <code
     title=dom-windowworker-closing><a href="#closing">closing</a></code>
     attribute switches to true or one of the <code>MessagePort</code>
     objects in the list of <a href="#the-workers0">the worker's ports</a>
     has an <code title=dom-MessagePort-active>active</code> attribute with
     the value true.</p>

   <li>
    <p>Start monitoring <var title="">worker</var>, such that as soon as the
     list of <a href="#the-workers0">the worker's ports</a> becomes empty,
     the <var title="">window</var> object's <code
     title=dom-windowworker-closing><a href="#closing">closing</a></code>
     attribute is set to true.</p>

    <p class=note>This can never happen at this point in the algorithm: as
     soon as the worker ran the <a href="#worker" title="worker creation
     succeeded">success steps</a>, the <a href="#queue">queue of events</a>
     received a <code title=event-join>join</code> event with a
     <code>MessagePort</code>.</p>

   <li>
    <p>Run <var title="">script</var> until it either returns, fails to catch
     an exception, or gets prematurely aborted by the "<a href="#kill-a">kill
     a worker</a>" algorithm below.</p>

    <p class=note>If the script gets aborted by the "<a href="#kill-a">kill a
     worker</a>" algorithm, then that same algorithm will cause there to only
     be a single event in the <a href="#queue">queue of events</a> at the
     next step, namely the <code title=message-unload>unload</code> event.
     However, if the event is ignored then it will become true as soon as
     that port is garbage collected.</p>

   <li>
    <p><i>Event loop</i>: Wait until either there is an event in the <a
     href="#queue">queue of events</a> associated with <var
     title="">window</var> or the <var title="">window</var> object's <code
     title=dom-windowworker-closing><a href="#closing">closing</a></code>
     attribute is set to true.</p>

   <li>
    <p>Dispatch the oldest event or callback in the <a href="#queue">queue of
     events</a>, if any. The handling of this event or the execution of this
     callback might get prematurely aborted by the "<a href="#kill-a">kill a
     worker</a>" algorithm below.</p>

   <li>
    <p>If there are any more events in the <a href="#queue">queue of
     events</a> or if the <var title="">window</var> object's <code
     title=dom-windowworker-closing><a href="#closing">closing</a></code>
     attribute is set to false, then jump back to the step above labeled
     <i>event loop</i>.</p>

   <li>
    <p class=big-issue>timers, intervals, XMLHttpRequests, database
     transactions, etc, must be killed; ports must be deactivated and
     unentangled (do not send unload)</p>
  </ol>

  <hr>

  <p>When a user agent is to <dfn id=kill-a>kill a worker</dfn>, it must run
   the following steps in parallel with the worker's main loop (the "<a
   href="#run-a">run a worker</a>" processing model defined above):

  <ol>
   <li>
    <p>Create an <code>Event</code> object with the event name <code
     title=event-unload>unload</code>, which does not bubble and is not
     cancelable, and add it to the worker's <code><a
     href="#windowworker">WindowWorker</a></code> object's <a
     href="#queue">queue of events</a>, targetted at the <code><a
     href="#windowworker">WindowWorker</a></code> object itself.

   <li>
    <p>Set the worker's <code><a href="#windowworker">WindowWorker</a></code>
     object's <code title=dom-windowworker-closing><a
     href="#closing">closing</a></code> attribute to true.

   <li>
    <p>Wait a user-agent-defined amount of time. If the "<a href="#run-a">run
     a worker</a>" processing model defined above immediately starts running
     event listeners registered for <code title=event-unload>unload</code>
     event, this time should not be zero &mdash; the idea is that the <code
     title=event-unload>unload</code> event can be used to clean up when
     shutting down unexpectedly.

   <li>
    <p>If there are any events in the <a href="#queue">queue of events</a>
     other than the <code title=event-unload>unload</code> event that this
     algorithm just added, discard them without dispatching them.

   <li>
    <p>If the <code title=event-unload>unload</code> event that this
     algorithm just added hasn't yet been dispatched, then abort the script
     currently running in the worker.

   <li>
    <p>Wait a user-agent-defined amount of time.

   <li>
    <p>Abort the script currently running in the worker (if any script is
     running, then it will be a handler for the <code
     title=event-unload>unload</code> event).
  </ol>

  <hr>

  <p>When a script invokes the <dfn id=close
   title=dom-windowworker-close><code>close()</code></dfn> method on a
   <code><a href="#windowworker">WindowWorker</a></code> object, the user
   agent must run the following steps:

  <ol>
   <li>
    <p>Create an <code>Event</code> object with the event name <code
     title=event-unload>unload</code>, which does not bubble and is not
     cancelable, and add it to the <code><a
     href="#windowworker">WindowWorker</a></code> object's <a
     href="#queue">queue of events</a>, targetted at the <code><a
     href="#windowworker">WindowWorker</a></code> object itself.

   <li>
    <p>Set the worker's <code><a href="#windowworker">WindowWorker</a></code>
     object's <code title=dom-windowworker-closing><a
     href="#closing">closing</a></code> attribute to true.

   <li>
    <p>For each <code>MessagePort</code> object that is entangled with
     another port and that has one (but only one) port whose <code
     title=dom-MessagePort-ownerWindow>ownerWindow</code> is the <code><a
     href="#windowworker">WindowWorker</a></code> object on which the method
     was invoked, run the following substeps:</p>

    <ol>
     <li>
      <p>Unentangle the two ports.

     <li>
      <p>Set both ports' <code title=dom-MessagePort-active>active</code>
       attribute to false.

     <li>
      <p>At the next available opportunity, after any scripts have finished
       executing<!-- XXX queue -->, <span>fire a simple event</span> called
       <code title=event-unload>unload</code> at the other port (the one
       whose <code title=dom-MessagePort-ownerWindow>ownerWindow</code> is
       not the <code><a href="#windowworker">WindowWorker</a></code> object
       on which the <code title=dom-windowworker-close><a
       href="#close">close()</a></code> method was called).
    </ol>
  </ol>

  <h2 id=creating><span class=secno>3. </span>Creating workers</h2>

  <h3 id=the-windowworkercreators><span class=secno>3.1 </span>The <code><a
   href="#windowworkercreators">WindowWorkerCreators</a></code> interface</h3>

  <pre
   class=idl>[NoInterfaceObject] interface <dfn id=windowworkercreators>WindowWorkerCreators</dfn> {
  <span>MessagePort</span> <span title=dom-WindowWorkerCreators-createWorker>createWorker</span>(in DOMString scriptURI);
  <span>MessagePort</span> <span title=dom-WindowWorkerCreators-createNamedWorker>createNamedWorker</span>(in DOMString name, in DOMString scriptURI);
};</pre>

  <p>Objects that implement the <code>Window</code> interface must also
   implement the <code><a
   href="#windowworkercreators">WindowWorkerCreators</a></code> interface.

  <hr>

  <p>When the <code
   title=dom-WindowWorkerCreators-createWorker>createWorker()</code> method
   is invoked, the user agent must run the following steps:

  <ol>
   <li class=big-issue>... <!--
resolve url
if that fails, throw
if it's not same-orgin, throw
run create worker steps
-->
    
  </ol>

  <hr>

  <p>When the <code
   title=dom-WindowWorkerCreators-createNamedWorker>createNamedWorker()</code>
   method is invoked, the user agent must run the following steps:

  <ol>
   <li class=big-issue>... <!--
if /name/ = "", jump to createWorker() steps
resolve url
if that fails, throw
if it's not same-orgin, throw
look for named worker whose name is /name/ and that has closing=false and whose document's origin is the same as the calling scripts'
if you don't find one, run 'create worker' steps with given name and resulting absolute url; return returned port
check that resulting absolute url matches the one in the worker; if not, throw
create two ports
return port 1
invoke the "join a worker" steps with the two ports
-->
    
  </ol>

  <hr>

  <p>The steps to <dfn id=create>create a worker</dfn> are as follows:

  <ol>
   <li class=big-issue>... <!--
create two ports
in a separate thread, 'run a worker' with name "" and resulting absolute url
return port 1
success steps: invoke the "join a worker" steps with the two ports
failure steps: fire 'error' on port1
-->
    
  </ol>

  <p class=big-issue><dfn id=worker>worker creation succeeded</dfn>, <dfn
   id=worker0>worker creation failed</dfn>

  <hr>

  <p>The steps to <dfn id=join-a>join a worker</dfn> are as follows:

  <ol>
   <li class=big-issue>... <!--
given two ports port1, port2, and a worker
entangle the ports
fire 'join' event on the worker, passing port2
fire 'load' event on port1.
-->
    
  </ol>

  <p class=big-issue>...

  <h2 class=no-num id=references>References</h2>

  <p class=big-issue>This section will be written in a future
   draft.<!--XXX-->

  <h2 class=no-num id=acknowledgements>Acknowledgements</h2>
  <!-- ACKS -->

  <p>Thanks to Maciej Stachowiak and Mike Smith for their useful and
   substantial comments.

  <p>Huge thanks to the whole Gears team, who pioneered this technology and
   whose experience has been a huge influence on this specification.