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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> <i class="fa fa-search"></i> </button> </div> <h1 class="menu-title">Synapse</h1> <div class="right-buttons"> <a href="../../print.html" title="Print this book" aria-label="Print this book"> <i id="print-button" class="fa fa-print"></i> </a> <a href="https://github.com/matrix-org/synapse" title="Git repository" aria-label="Git repository"> <i id="git-repository-button" class="fa fa-github"></i> </a> <a href="https://github.com/matrix-org/synapse/edit/develop/docs/development/synapse_architecture/cancellation.md" title="Suggest an edit" aria-label="Suggest an edit"> <i id="git-edit-button" class="fa fa-edit"></i> </a> </div> </div> <div id="search-wrapper" class="hidden"> <form id="searchbar-outer" class="searchbar-outer"> <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> </form> <div id="searchresults-outer" class="searchresults-outer hidden"> <div id="searchresults-header" class="searchresults-header"></div> <ul id="searchresults"> </ul> </div> </div> <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> <script type="text/javascript"> document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); }); </script> <div id="content" class="content"> <main> <!-- Page table of contents --> <div class="sidetoc"> <nav class="pagetoc"></nav> </div> <h1 id="cancellation"><a class="header" href="#cancellation">Cancellation</a></h1> <p>Sometimes, requests take a long time to service and clients disconnect before Synapse produces a response. To avoid wasting resources, Synapse can cancel request processing for select endpoints marked with the <code>@cancellable</code> decorator.</p> <p>Synapse makes use of Twisted's <code>Deferred.cancel()</code> feature to make cancellation work. The <code>@cancellable</code> decorator does nothing by itself and merely acts as a flag, signalling to developers and other code alike that a method can be cancelled.</p> <h2 id="enabling-cancellation-for-an-endpoint"><a class="header" href="#enabling-cancellation-for-an-endpoint">Enabling cancellation for an endpoint</a></h2> <ol> <li>Check that the endpoint method, and any <code>async</code> functions in its call tree handle cancellation correctly. See <a href="#handling-cancellation-correctly">Handling cancellation correctly</a> for a list of things to look out for.</li> <li>Add the <code>@cancellable</code> decorator to the <code>on_GET/POST/PUT/DELETE</code> method. It's not recommended to make non-<code>GET</code> methods cancellable, since cancellation midway through some database updates is less likely to be handled correctly.</li> </ol> <h2 id="mechanics"><a class="header" href="#mechanics">Mechanics</a></h2> <p>There are two stages to cancellation: downward propagation of a <code>cancel()</code> call, followed by upwards propagation of a <code>CancelledError</code> out of a blocked <code>await</code>. Both Twisted and asyncio have a cancellation mechanism.</p> <table><thead><tr><th></th><th>Method</th><th>Exception</th><th>Exception inherits from</th></tr></thead><tbody> <tr><td>Twisted</td><td><code>Deferred.cancel()</code></td><td><code>twisted.internet.defer.CancelledError</code></td><td><code>Exception</code> (!)</td></tr> <tr><td>asyncio</td><td><code>Task.cancel()</code></td><td><code>asyncio.CancelledError</code></td><td><code>BaseException</code></td></tr> </tbody></table> <h3 id="deferredcancel"><a class="header" href="#deferredcancel">Deferred.cancel()</a></h3> <p>When Synapse starts handling a request, it runs the async method responsible for handling it using <code>defer.ensureDeferred</code>, which returns a <code>Deferred</code>. For example:</p> <pre><code class="language-python">def do_something() -> Deferred[None]: ... @cancellable async def on_GET() -> Tuple[int, JsonDict]: d = make_deferred_yieldable(do_something()) await d return 200, {} request = defer.ensureDeferred(on_GET()) </code></pre> <p>When a client disconnects early, Synapse checks for the presence of the <code>@cancellable</code> decorator on <code>on_GET</code>. Since <code>on_GET</code> is cancellable, <code>Deferred.cancel()</code> is called on the <code>Deferred</code> from <code>defer.ensureDeferred</code>, ie. <code>request</code>. Twisted knows which <code>Deferred</code> <code>request</code> is waiting on and passes the <code>cancel()</code> call on to <code>d</code>.</p> <p>The <code>Deferred</code> being waited on, <code>d</code>, may have its own handling for <code>cancel()</code> and pass the call on to other <code>Deferred</code>s.</p> <p>Eventually, a <code>Deferred</code> handles the <code>cancel()</code> call by resolving itself with a <code>CancelledError</code>.</p> <h3 id="cancellederror"><a class="header" href="#cancellederror">CancelledError</a></h3> <p>The <code>CancelledError</code> gets raised out of the <code>await</code> and bubbles up, as per normal Python exception handling.</p> <h2 id="handling-cancellation-correctly"><a class="header" href="#handling-cancellation-correctly">Handling cancellation correctly</a></h2> <p>In general, when writing code that might be subject to cancellation, two things must be considered:</p> <ul> <li>The effect of <code>CancelledError</code>s raised out of <code>await</code>s.</li> <li>The effect of <code>Deferred</code>s being <code>cancel()</code>ed.</li> </ul> <p>Examples of code that handles cancellation incorrectly include:</p> <ul> <li><code>try-except</code> blocks which swallow <code>CancelledError</code>s.</li> <li>Code that shares the same <code>Deferred</code>, which may be cancelled, between multiple requests.</li> <li>Code that starts some processing that's exempt from cancellation, but uses a logging context from cancellable code. The logging context will be finished upon cancellation, while the uncancelled processing is still using it.</li> </ul> <p>Some common patterns are listed below in more detail.</p> <h3 id="async-function-calls"><a class="header" href="#async-function-calls"><code>async</code> function calls</a></h3> <p>Most functions in Synapse are relatively straightforward from a cancellation standpoint: they don't do anything with <code>Deferred</code>s and purely call and <code>await</code> other <code>async</code> functions.</p> <p>An <code>async</code> function handles cancellation correctly if its own code handles cancellation correctly and all the async function it calls handle cancellation correctly. For example:</p> <pre><code class="language-python">async def do_two_things() -> None: check_something() await do_something() await do_something_else() </code></pre> <p><code>do_two_things</code> handles cancellation correctly if <code>do_something</code> and <code>do_something_else</code> handle cancellation correctly.</p> <p>That is, when checking whether a function handles cancellation correctly, its implementation and all its <code>async</code> function calls need to be checked, recursively.</p> <p>As <code>check_something</code> is not <code>async</code>, it does not need to be checked.</p> <h3 id="cancellederrors"><a class="header" href="#cancellederrors">CancelledErrors</a></h3> <p>Because Twisted's <code>CancelledError</code>s are <code>Exception</code>s, it's easy to accidentally catch and suppress them. Care must be taken to ensure that <code>CancelledError</code>s are allowed to propagate upwards.</p> <table width="100%"> <tr> <td width="50%" valign="top"> <p><strong>Bad</strong>:</p> <pre><code class="language-python">try: await do_something() except Exception: # `CancelledError` gets swallowed here. logger.info(...) </code></pre> </td> <td width="50%" valign="top"> <p><strong>Good</strong>:</p> <pre><code class="language-python">try: await do_something() except CancelledError: raise except Exception: logger.info(...) </code></pre> </td> </tr> <tr> <td width="50%" valign="top"> <p><strong>OK</strong>:</p> <pre><code class="language-python">try: check_something() # A `CancelledError` won't ever be raised here. except Exception: logger.info(...) </code></pre> </td> <td width="50%" valign="top"> <p><strong>Good</strong>:</p> <pre><code class="language-python">try: await do_something() except ValueError: logger.info(...) </code></pre> </td> </tr> </table> <h4 id="defergatherresults"><a class="header" href="#defergatherresults">defer.gatherResults</a></h4> <p><code>defer.gatherResults</code> produces a <code>Deferred</code> which:</p> <ul> <li>broadcasts <code>cancel()</code> calls to every <code>Deferred</code> being waited on.</li> <li>wraps the first exception it sees in a <code>FirstError</code>.</li> </ul> <p>Together, this means that <code>CancelledError</code>s will be wrapped in a <code>FirstError</code> unless unwrapped. Such <code>FirstError</code>s are liable to be swallowed, so they must be unwrapped.</p> <table width="100%"> <tr> <td width="50%" valign="top"> <p><strong>Bad</strong>:</p> <pre><code class="language-python">async def do_something() -> None: await make_deferred_yieldable( defer.gatherResults([...], consumeErrors=True) ) try: await do_something() except CancelledError: raise except Exception: # `FirstError(CancelledError)` gets swallowed here. logger.info(...) </code></pre> </td> <td width="50%" valign="top"> <p><strong>Good</strong>:</p> <pre><code class="language-python">async def do_something() -> None: await make_deferred_yieldable( defer.gatherResults([...], consumeErrors=True) ).addErrback(unwrapFirstError) try: await do_something() except CancelledError: raise except Exception: logger.info(...) </code></pre> </td> </tr> </table> <h3 id="creation-of-deferreds"><a class="header" href="#creation-of-deferreds">Creation of <code>Deferred</code>s</a></h3> <p>If a function creates a <code>Deferred</code>, the effect of cancelling it must be considered. <code>Deferred</code>s that get shared are likely to have unintended behaviour when cancelled.</p> <table width="100%"> <tr> <td width="50%" valign="top"> <p><strong>Bad</strong>:</p> <pre><code class="language-python">cache: Dict[str, Deferred[None]] = {} def wait_for_room(room_id: str) -> Deferred[None]: deferred = cache.get(room_id) if deferred is None: deferred = Deferred() cache[room_id] = deferred # `deferred` can have multiple waiters. # All of them will observe a `CancelledError` # if any one of them is cancelled. return make_deferred_yieldable(deferred) # Request 1 await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org") # Request 2 await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org") </code></pre> </td> <td width="50%" valign="top"> <p><strong>Good</strong>:</p> <pre><code class="language-python">cache: Dict[str, Deferred[None]] = {} def wait_for_room(room_id: str) -> Deferred[None]: deferred = cache.get(room_id) if deferred is None: deferred = Deferred() cache[room_id] = deferred # `deferred` will never be cancelled now. # A `CancelledError` will still come out of # the `await`. # `delay_cancellation` may also be used. return make_deferred_yieldable(stop_cancellation(deferred)) # Request 1 await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org") # Request 2 await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org") </code></pre> </td> </tr> <tr> <td width="50%" valign="top"> </td> <td width="50%" valign="top"> <p><strong>Good</strong>:</p> <pre><code class="language-python">cache: Dict[str, List[Deferred[None]]] = {} def wait_for_room(room_id: str) -> Deferred[None]: if room_id not in cache: cache[room_id] = [] # Each request gets its own `Deferred` to wait on. deferred = Deferred() cache[room_id]].append(deferred) return make_deferred_yieldable(deferred) # Request 1 await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org") # Request 2 await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org") </code></pre> </td> </table> <h3 id="uncancelled-processing"><a class="header" href="#uncancelled-processing">Uncancelled processing</a></h3> <p>Some <code>async</code> functions may kick off some <code>async</code> processing which is intentionally protected from cancellation, by <code>stop_cancellation</code> or other means. If the <code>async</code> processing inherits the logcontext of the request which initiated it, care must be taken to ensure that the logcontext is not finished before the <code>async</code> processing completes.</p> <table width="100%"> <tr> <td width="50%" valign="top"> <p><strong>Bad</strong>:</p> <pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None async def do_something_else( to_resolve: Deferred[None] ) -> None: await ... logger.info("done!") to_resolve.callback(None) async def do_something() -> None: if not cache: to_resolve = Deferred() cache = ObservableDeferred(to_resolve) # `do_something_else` will never be cancelled and # can outlive the `request-1` logging context. run_in_background(do_something_else, to_resolve) await make_deferred_yieldable(cache.observe()) with LoggingContext("request-1"): await do_something() </code></pre> </td> <td width="50%" valign="top"> <p><strong>Good</strong>:</p> <pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None async def do_something_else( to_resolve: Deferred[None] ) -> None: await ... logger.info("done!") to_resolve.callback(None) async def do_something() -> None: if not cache: to_resolve = Deferred() cache = ObservableDeferred(to_resolve) run_in_background(do_something_else, to_resolve) # We'll wait until `do_something_else` is # done before raising a `CancelledError`. await make_deferred_yieldable( delay_cancellation(cache.observe()) ) else: await make_deferred_yieldable(cache.observe()) with LoggingContext("request-1"): await do_something() </code></pre> </td> </tr> <tr> <td width="50%"> <p><strong>OK</strong>:</p> <pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None async def do_something_else( to_resolve: Deferred[None] ) -> None: await ... logger.info("done!") to_resolve.callback(None) async def do_something() -> None: if not cache: to_resolve = Deferred() cache = ObservableDeferred(to_resolve) # `do_something_else` will get its own independent # logging context. `request-1` will not count any # metrics from `do_something_else`. run_as_background_process( "do_something_else", do_something_else, to_resolve, ) await make_deferred_yieldable(cache.observe()) with LoggingContext("request-1"): await do_something() </code></pre> </td> <td width="50%"> </td> </tr> </table> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="../../development/dependencies.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="../../log_contexts.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> <div style="clear: both"></div> </nav> </div> </div> <nav class="nav-wide-wrapper" aria-label="Page navigation"> <a rel="prev" href="../../development/dependencies.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="../../log_contexts.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> </nav> </div> <script type="text/javascript"> window.playground_copyable = true; </script> <script src="../../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> <script src="../../mark.min.js" type="text/javascript" charset="utf-8"></script> <script src="../../searcher.js" type="text/javascript" charset="utf-8"></script> <script src="../../clipboard.min.js" type="text/javascript" charset="utf-8"></script> <script src="../../highlight.js" type="text/javascript" charset="utf-8"></script> <script src="../../book.js" type="text/javascript" charset="utf-8"></script> <!-- Custom JS scripts --> <script type="text/javascript" src="../../docs/website_files/table-of-contents.js"></script> </body> </html>