<!DOCTYPE HTML> <html lang="en" class="sidebar-visible no-js light"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>Log Contexts - 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"> <link rel="stylesheet" href="docs/website_files/version-picker.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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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. 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