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diff --git a/v1.90/development/synapse_architecture/cancellation.html b/v1.90/development/synapse_architecture/cancellation.html new file mode 100644 index 0000000000..8f7d25bb88 --- /dev/null +++ b/v1.90/development/synapse_architecture/cancellation.html @@ -0,0 +1,536 @@ +<!DOCTYPE HTML> +<html lang="en" class="sidebar-visible no-js light"> + <head> + <!-- Book generated using mdBook --> + <meta charset="UTF-8"> + <title>Cancellation - Synapse</title> + <!-- Custom HTML head --> + <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> + <meta name="description" content=""> + <meta name="viewport" content="width=device-width, initial-scale=1"> + <meta name="theme-color" content="#ffffff" /> + + <link rel="icon" href="../../favicon.svg"> + <link rel="shortcut icon" href="../../favicon.png"> + <link rel="stylesheet" href="../../css/variables.css"> + <link rel="stylesheet" href="../../css/general.css"> + <link rel="stylesheet" href="../../css/chrome.css"> + <link rel="stylesheet" href="../../css/print.css" media="print"> + <!-- Fonts --> + <link rel="stylesheet" href="../../FontAwesome/css/font-awesome.css"> + <link rel="stylesheet" href="../../fonts/fonts.css"> + <!-- Highlight.js Stylesheets --> + <link rel="stylesheet" href="../../highlight.css"> + <link rel="stylesheet" href="../../tomorrow-night.css"> + <link rel="stylesheet" href="../../ayu-highlight.css"> + + <!-- Custom theme stylesheets --> + <link rel="stylesheet" href="../../docs/website_files/table-of-contents.css"> + <link rel="stylesheet" href="../../docs/website_files/remove-nav-buttons.css"> + <link rel="stylesheet" href="../../docs/website_files/indent-section-headers.css"> + </head> + <body> + <!-- Provide site root to javascript --> + <script type="text/javascript"> + var path_to_root = "../../"; + var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? 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Contexts</a></li><li class="chapter-item expanded "><a href="../../replication.html">Replication</a></li><li class="chapter-item expanded "><a href="../../development/synapse_architecture/streams.html">Streams</a></li><li class="chapter-item expanded "><a href="../../tcp_replication.html">TCP Replication</a></li><li class="chapter-item expanded "><a href="../../development/synapse_architecture/faster_joins.html">Faster remote joins</a></li></ol></li><li class="chapter-item expanded "><a href="../../development/internal_documentation/index.html">Internal Documentation</a></li><li><ol class="section"><li class="chapter-item expanded "><div>Single Sign-On</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../../development/cas.html">CAS</a></li></ol></li><li class="chapter-item expanded "><a href="../../development/room-dag-concepts.html">Room DAG concepts</a></li><li class="chapter-item expanded "><div>State Resolution</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../auth_chain_difference_algorithm.html">The Auth Chain Difference Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../../media_repository.html">Media Repository</a></li><li class="chapter-item expanded "><a href="../../room_and_user_statistics.html">Room and User Statistics</a></li></ol></li><li class="chapter-item expanded "><div>Scripts</div></li><li class="chapter-item expanded affix "><li class="part-title">Other</li><li class="chapter-item expanded "><a href="../../deprecation_policy.html">Dependency Deprecation Policy</a></li><li class="chapter-item expanded "><a href="../../other/running_synapse_on_single_board_computers.html">Running Synapse on a Single-Board Computer</a></li></ol> + </div> + <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div> + </nav> + + <div id="page-wrapper" class="page-wrapper"> + + 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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> \ No newline at end of file |