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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/log_contexts.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="log-contexts"><a class="header" href="#log-contexts">Log Contexts</a></h1> +<p>To help track the processing of individual requests, synapse uses a +'<code>log context</code>' to track which request it is handling at any given +moment. This is done via a thread-local variable; a <code>logging.Filter</code> is +then used to fish the information back out of the thread-local variable +and add it to each log record.</p> +<p>Logcontexts are also used for CPU and database accounting, so that we +can track which requests were responsible for high CPU use or database +activity.</p> +<p>The <code>synapse.logging.context</code> module provides facilities for managing +the current log context (as well as providing the <code>LoggingContextFilter</code> +class).</p> +<p>Asynchronous functions make the whole thing complicated, so this document describes +how it all works, and how to write code which follows the rules.</p> +<p>In this document, "awaitable" refers to any object which can be <code>await</code>ed. In the context of +Synapse, that normally means either a coroutine or a Twisted +<a href="https://twistedmatrix.com/documents/current/api/twisted.internet.defer.Deferred.html"><code>Deferred</code></a>.</p> +<h2 id="logcontexts-without-asynchronous-code"><a class="header" href="#logcontexts-without-asynchronous-code">Logcontexts without asynchronous code</a></h2> +<p>In the absence of any asynchronous voodoo, things are simple enough. As with +any code of this nature, the rule is that our function should leave +things as it found them:</p> +<pre><code class="language-python">from synapse.logging import context # omitted from future snippets + +def handle_request(request_id): + request_context = context.LoggingContext() + + calling_context = context.set_current_context(request_context) + try: + request_context.request = request_id + do_request_handling() + logger.debug("finished") + finally: + context.set_current_context(calling_context) + +def do_request_handling(): + logger.debug("phew") # this will be logged against request_id +</code></pre> +<p>LoggingContext implements the context management methods, so the above +can be written much more succinctly as:</p> +<pre><code class="language-python">def handle_request(request_id): + with context.LoggingContext() as request_context: + request_context.request = request_id + do_request_handling() + logger.debug("finished") + +def do_request_handling(): + logger.debug("phew") +</code></pre> +<h2 id="using-logcontexts-with-awaitables"><a class="header" href="#using-logcontexts-with-awaitables">Using logcontexts with awaitables</a></h2> +<p>Awaitables break the linear flow of code so that there is no longer a single entry point +where we should set the logcontext and a single exit point where we should remove it.</p> +<p>Consider the example above, where <code>do_request_handling</code> needs to do some +blocking operation, and returns an awaitable:</p> +<pre><code class="language-python">async def handle_request(request_id): + with context.LoggingContext() as request_context: + request_context.request = request_id + await do_request_handling() + logger.debug("finished") +</code></pre> +<p>In the above flow:</p> +<ul> +<li>The logcontext is set</li> +<li><code>do_request_handling</code> is called, and returns an awaitable</li> +<li><code>handle_request</code> awaits the awaitable</li> +<li>Execution of <code>handle_request</code> is suspended</li> +</ul> +<p>So we have stopped processing the request (and will probably go on to +start processing the next), without clearing the logcontext.</p> +<p>To circumvent this problem, synapse code assumes that, wherever you have +an awaitable, you will want to <code>await</code> it. To that end, whereever +functions return awaitables, we adopt the following conventions:</p> +<p><strong>Rules for functions returning awaitables:</strong></p> +<blockquote> +<ul> +<li>If the awaitable is already complete, the function returns with the +same logcontext it started with.</li> +<li>If the awaitable is incomplete, the function clears the logcontext +before returning; when the awaitable completes, it restores the +logcontext before running any callbacks.</li> +</ul> +</blockquote> +<p>That sounds complicated, but actually it means a lot of code (including +the example above) "just works". There are two cases:</p> +<ul> +<li> +<p>If <code>do_request_handling</code> returns a completed awaitable, then the +logcontext will still be in place. In this case, execution will +continue immediately after the <code>await</code>; the "finished" line will +be logged against the right context, and the <code>with</code> block restores +the original context before we return to the caller.</p> +</li> +<li> +<p>If the returned awaitable is incomplete, <code>do_request_handling</code> clears +the logcontext before returning. The logcontext is therefore clear +when <code>handle_request</code> <code>await</code>s the awaitable.</p> +<p>Once <code>do_request_handling</code>'s awaitable completes, it will reinstate +the logcontext, before running the second half of <code>handle_request</code>, +so again the "finished" line will be logged against the right context, +and the <code>with</code> block restores the original context.</p> +</li> +</ul> +<p>As an aside, it's worth noting that <code>handle_request</code> follows our rules</p> +<ul> +<li>though that only matters if the caller has its own logcontext which it +cares about.</li> +</ul> +<p>The following sections describe pitfalls and helpful patterns when +implementing these rules.</p> +<h2 id="always-await-your-awaitables"><a class="header" href="#always-await-your-awaitables">Always await your awaitables</a></h2> +<p>Whenever you get an awaitable back from a function, you should <code>await</code> on +it as soon as possible. Do not pass go; do not do any logging; do not +call any other functions.</p> +<pre><code class="language-python">async def fun(): + logger.debug("starting") + await do_some_stuff() # just like this + + coro = more_stuff() + result = await coro # also fine, of course + + return result +</code></pre> +<p>Provided this pattern is followed all the way back up to the callchain +to where the logcontext was set, this will make things work out ok: +provided <code>do_some_stuff</code> and <code>more_stuff</code> follow the rules above, then +so will <code>fun</code>.</p> +<p>It's all too easy to forget to <code>await</code>: for instance if we forgot that +<code>do_some_stuff</code> returned an awaitable, we might plough on regardless. This +leads to a mess; it will probably work itself out eventually, but not +before a load of stuff has been logged against the wrong context. +(Normally, other things will break, more obviously, if you forget to +<code>await</code>, so this tends not to be a major problem in practice.)</p> +<p>Of course sometimes you need to do something a bit fancier with your +awaitable - not all code follows the linear A-then-B-then-C pattern. +Notes on implementing more complex patterns are in later sections.</p> +<h2 id="where-you-create-a-new-awaitable-make-it-follow-the-rules"><a class="header" href="#where-you-create-a-new-awaitable-make-it-follow-the-rules">Where you create a new awaitable, make it follow the rules</a></h2> +<p>Most of the time, an awaitable comes from another synapse function. +Sometimes, though, we need to make up a new awaitable, or we get an awaitable +back from external code. We need to make it follow our rules.</p> +<p>The easy way to do it is by using <code>context.make_deferred_yieldable</code>. Suppose we want to implement +<code>sleep</code>, which returns a deferred which will run its callbacks after a +given number of seconds. That might look like:</p> +<pre><code class="language-python"># not a logcontext-rules-compliant function +def get_sleep_deferred(seconds): + d = defer.Deferred() + reactor.callLater(seconds, d.callback, None) + return d +</code></pre> +<p>That doesn't follow the rules, but we can fix it by calling it through +<code>context.make_deferred_yieldable</code>:</p> +<pre><code class="language-python">async def sleep(seconds): + return await context.make_deferred_yieldable(get_sleep_deferred(seconds)) +</code></pre> +<h2 id="fire-and-forget"><a class="header" href="#fire-and-forget">Fire-and-forget</a></h2> +<p>Sometimes you want to fire off a chain of execution, but not wait for +its result. That might look a bit like this:</p> +<pre><code class="language-python">async def do_request_handling(): + await foreground_operation() + + # *don't* do this + background_operation() + + logger.debug("Request handling complete") + +async def background_operation(): + await first_background_step() + logger.debug("Completed first step") + await second_background_step() + logger.debug("Completed second step") +</code></pre> +<p>The above code does a couple of steps in the background after +<code>do_request_handling</code> has finished. The log lines are still logged +against the <code>request_context</code> logcontext, which may or may not be +desirable. There are two big problems with the above, however. The first +problem is that, if <code>background_operation</code> returns an incomplete +awaitable, it will expect its caller to <code>await</code> immediately, so will have +cleared the logcontext. In this example, that means that 'Request +handling complete' will be logged without any context.</p> +<p>The second problem, which is potentially even worse, is that when the +awaitable returned by <code>background_operation</code> completes, it will restore +the original logcontext. There is nothing waiting on that awaitable, so +the logcontext will leak into the reactor and possibly get attached to +some arbitrary future operation.</p> +<p>There are two potential solutions to this.</p> +<p>One option is to surround the call to <code>background_operation</code> with a +<code>PreserveLoggingContext</code> call. That will reset the logcontext before +starting <code>background_operation</code> (so the context restored when the +deferred completes will be the empty logcontext), and will restore the +current logcontext before continuing the foreground process:</p> +<pre><code class="language-python">async def do_request_handling(): + await foreground_operation() + + # start background_operation off in the empty logcontext, to + # avoid leaking the current context into the reactor. + with PreserveLoggingContext(): + background_operation() + + # this will now be logged against the request context + logger.debug("Request handling complete") +</code></pre> +<p>Obviously that option means that the operations done in +<code>background_operation</code> would be not be logged against a logcontext +(though that might be fixed by setting a different logcontext via a +<code>with LoggingContext(...)</code> in <code>background_operation</code>).</p> +<p>The second option is to use <code>context.run_in_background</code>, which wraps a +function so that it doesn't reset the logcontext even when it returns +an incomplete awaitable, and adds a callback to the returned awaitable to +reset the logcontext. In other words, it turns a function that follows +the Synapse rules about logcontexts and awaitables into one which behaves +more like an external function --- the opposite operation to that +described in the previous section. It can be used like this:</p> +<pre><code class="language-python">async def do_request_handling(): + await foreground_operation() + + context.run_in_background(background_operation) + + # this will now be logged against the request context + logger.debug("Request handling complete") +</code></pre> +<h2 id="passing-synapse-deferreds-into-third-party-functions"><a class="header" href="#passing-synapse-deferreds-into-third-party-functions">Passing synapse deferreds into third-party functions</a></h2> +<p>A typical example of this is where we want to collect together two or +more awaitables via <code>defer.gatherResults</code>:</p> +<pre><code class="language-python">a1 = operation1() +a2 = operation2() +a3 = defer.gatherResults([a1, a2]) +</code></pre> +<p>This is really a variation of the fire-and-forget problem above, in that +we are firing off <code>a1</code> and <code>a2</code> without awaiting on them. The difference +is that we now have third-party code attached to their callbacks. Anyway +either technique given in the <a href="#fire-and-forget">Fire-and-forget</a> +section will work.</p> +<p>Of course, the new awaitable returned by <code>gather</code> needs to be +wrapped in order to make it follow the logcontext rules before we can +yield it, as described in <a href="#where-you-create-a-new-awaitable-make-it-follow-the-rules">Where you create a new awaitable, make it +follow the +rules</a>.</p> +<p>So, option one: reset the logcontext before starting the operations to +be gathered:</p> +<pre><code class="language-python">async def do_request_handling(): + with PreserveLoggingContext(): + a1 = operation1() + a2 = operation2() + result = await defer.gatherResults([a1, a2]) +</code></pre> +<p>In this case particularly, though, option two, of using +<code>context.run_in_background</code> almost certainly makes more sense, so that +<code>operation1</code> and <code>operation2</code> are both logged against the original +logcontext. This looks like:</p> +<pre><code class="language-python">async def do_request_handling(): + a1 = context.run_in_background(operation1) + a2 = context.run_in_background(operation2) + + result = await make_deferred_yieldable(defer.gatherResults([a1, a2])) +</code></pre> +<h2 id="a-note-on-garbage-collection-of-awaitable-chains"><a class="header" href="#a-note-on-garbage-collection-of-awaitable-chains">A note on garbage-collection of awaitable chains</a></h2> +<p>It turns out that our logcontext rules do not play nicely with awaitable +chains which get orphaned and garbage-collected.</p> +<p>Imagine we have some code that looks like this:</p> +<pre><code class="language-python">listener_queue = [] + +def on_something_interesting(): + for d in listener_queue: + d.callback("foo") + +async def await_something_interesting(): + new_awaitable = defer.Deferred() + listener_queue.append(new_awaitable) + + with PreserveLoggingContext(): + await new_awaitable +</code></pre> +<p>Obviously, the idea here is that we have a bunch of things which are +waiting for an event. (It's just an example of the problem here, but a +relatively common one.)</p> +<p>Now let's imagine two further things happen. First of all, whatever was +waiting for the interesting thing goes away. (Perhaps the request times +out, or something <em>even more</em> interesting happens.)</p> +<p>Secondly, let's suppose that we decide that the interesting thing is +never going to happen, and we reset the listener queue:</p> +<pre><code class="language-python">def reset_listener_queue(): + listener_queue.clear() +</code></pre> +<p>So, both ends of the awaitable chain have now dropped their references, +and the awaitable chain is now orphaned, and will be garbage-collected at +some point. Note that <code>await_something_interesting</code> is a coroutine, +which Python implements as a generator function. When Python +garbage-collects generator functions, it gives them a chance to +clean up by making the <code>await</code> (or <code>yield</code>) raise a <code>GeneratorExit</code> +exception. In our case, that means that the <code>__exit__</code> handler of +<code>PreserveLoggingContext</code> will carefully restore the request context, but +there is now nothing waiting for its return, so the request context is +never cleared.</p> +<p>To reiterate, this problem only arises when <em>both</em> ends of a awaitable +chain are dropped. Dropping the the reference to an awaitable you're +supposed to be awaiting is bad practice, so this doesn't +actually happen too much. Unfortunately, when it does happen, it will +lead to leaked logcontexts which are incredibly hard to track down.</p> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + + <a rel="prev" href="development/experimental_features.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="replication.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/experimental_features.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="replication.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 |