655 lines
39 KiB
HTML
655 lines
39 KiB
HTML
<!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">
|
|
|
|
|
|
|
|
</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 ? "navy" : "light";
|
|
</script>
|
|
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|
<script type="text/javascript">
|
|
try {
|
|
var theme = localStorage.getItem('mdbook-theme');
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|
}
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|
}
|
|
} catch (e) { }
|
|
</script>
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
<script type="text/javascript">
|
|
var theme;
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|
var html = document.querySelector('html');
|
|
html.classList.remove('no-js')
|
|
html.classList.remove('light')
|
|
html.classList.add(theme);
|
|
html.classList.add('js');
|
|
</script>
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
<script type="text/javascript">
|
|
var html = document.querySelector('html');
|
|
var sidebar = 'hidden';
|
|
if (document.body.clientWidth >= 1080) {
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|
sidebar = sidebar || 'visible';
|
|
}
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</script>
|
|
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|
<div class="sidebar-scrollbox">
|
|
<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="turn-howto.html">Configuring a Turn Server</a></li><li class="chapter-item expanded "><a href="delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a href="upgrade.html">Upgrading between Synapse Versions</a></li><li class="chapter-item expanded "><a href="MSC1711_certificates_FAQ.html">Upgrading from pre-Synapse 1.0</a></li><li class="chapter-item expanded affix "><li class="part-title">Usage</li><li class="chapter-item expanded "><a href="federate.html">Federation</a></li><li class="chapter-item expanded "><a href="usage/configuration/index.html">Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="usage/configuration/homeserver_sample_config.html">Homeserver Sample Config File</a></li><li class="chapter-item expanded "><a href="usage/configuration/logging_sample_config.html">Logging Sample Config File</a></li><li class="chapter-item expanded "><a href="structured_logging.html">Structured Logging</a></li><li class="chapter-item expanded "><a href="usage/configuration/user_authentication/index.html">User Authentication</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="openid.html">OpenID Connect</a></li><li class="chapter-item expanded "><div>SAML</div></li><li class="chapter-item expanded "><div>CAS</div></li><li class="chapter-item expanded "><a href="sso_mapping_providers.html">SSO Mapping Providers</a></li></ol></li><li class="chapter-item expanded "><a href="password_auth_providers.html">Password Auth Providers</a></li><li class="chapter-item expanded "><a href="jwt.html">JSON Web Tokens</a></li></ol></li><li class="chapter-item expanded "><a href="CAPTCHA_SETUP.html">Registration Captcha</a></li><li class="chapter-item expanded "><a href="application_services.html">Application Services</a></li><li class="chapter-item expanded "><a href="server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="consent_tracking.html">Consent Tracking</a></li><li class="chapter-item expanded "><a href="url_previews.html">URL Previews</a></li><li class="chapter-item expanded "><a href="user_directory.html">User Directory</a></li><li class="chapter-item expanded "><a href="message_retention_policies.html">Message Retention Policies</a></li><li class="chapter-item expanded "><a href="modules.html">Pluggable Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><div>Third Party Rules</div></li><li class="chapter-item expanded "><a href="spam_checker.html">Spam Checker</a></li><li class="chapter-item expanded "><a href="presence_router_module.html">Presence Router</a></li><li class="chapter-item expanded "><div>Media Storage Providers</div></li></ol></li><li class="chapter-item expanded "><a href="workers.html">Workers</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="synctl_workers.html">Using synctl with Workers</a></li><li class="chapter-item expanded "><a href="systemd-with-workers/index.html">Systemd</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="usage/administration/index.html">Administration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="usage/administration/admin_api/index.html">Admin API</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="admin_api/account_validity.html">Account Validity</a></li><li class="chapter-item expanded "><a href="admin_api/delete_group.html">Delete Group</a></li><li class="chapter-item expanded "><a href="admin_api/event_reports.html">Event Reports</a></li><li class="chapter-item expanded "><a href="admin_api/media_admin_api.html">Media</a></li><li class="chapter-item expanded "><a href="admin_api/purge_history_api.html">Purge History</a></li><li class="chapter-item expanded "><a href="admin_api/purge_room.html">Purge Rooms</a></li><li class="chapter-item expanded "><a href="admin_api/register_api.html">Register Users</a></li><li class="chapter-item expanded "><a href="admin_api/room_membership.html">Manipulate Room Membership</a></li><li class="chapter-item expanded "><a href="admin_api/rooms.html">Rooms</a></li><li class="chapter-item expanded "><a href="admin_api/server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="admin_api/shutdown_room.html">Shutdown Room</a></li><li class="chapter-item expanded "><a href="admin_api/statistics.html">Statistics</a></li><li class="chapter-item expanded "><a href="admin_api/user_admin_api.html">Users</a></li><li class="chapter-item expanded "><a href="admin_api/version_api.html">Server Version</a></li></ol></li><li class="chapter-item expanded "><a href="manhole.html">Manhole</a></li><li class="chapter-item expanded "><a href="metrics-howto.html">Monitoring</a></li><li class="chapter-item expanded "><a href="usage/administration/request_log.html">Request log format</a></li><li class="chapter-item expanded "><div>Scripts</div></li></ol></li><li class="chapter-item expanded "><li class="part-title">Development</li><li class="chapter-item expanded "><a href="development/contributing_guide.html">Contributing Guide</a></li><li class="chapter-item expanded "><a href="code_style.html">Code Style</a></li><li class="chapter-item expanded "><a href="dev/git.html">Git Usage</a></li><li class="chapter-item expanded "><div>Testing</div></li><li class="chapter-item expanded "><a href="opentracing.html">OpenTracing</a></li><li class="chapter-item expanded "><a href="development/database_schema.html">Database Schemas</a></li><li class="chapter-item expanded "><div>Synapse Architecture</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="log_contexts.html" class="active">Log Contexts</a></li><li class="chapter-item expanded "><a href="replication.html">Replication</a></li><li class="chapter-item expanded "><a href="tcp_replication.html">TCP Replication</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="dev/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="dev/cas.html">CAS</a></li></ol></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></ol>
|
|
</div>
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|
</nav>
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
<div class="page">
|
|
|
|
<div id="menu-bar-hover-placeholder"></div>
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
|
<div class="left-buttons">
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|
<i class="fa fa-bars"></i>
|
|
</button>
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|
<i class="fa fa-paint-brush"></i>
|
|
</button>
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|
</ul>
|
|
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (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 a facilities for managing
|
|
the current log context (as well as providing the <code>LoggingContextFilter</code>
|
|
class).</p>
|
|
<p>Deferreds make the whole thing complicated, so this document describes
|
|
how it all works, and how to write code which follows the rules.</p>
|
|
<p>##Logcontexts without Deferreds</p>
|
|
<p>In the absence of any Deferred 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-deferreds"><a class="header" href="#using-logcontexts-with-deferreds">Using logcontexts with Deferreds</a></h2>
|
|
<p>Deferreds --- and in particular, <code>defer.inlineCallbacks</code> --- 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 a deferred:</p>
|
|
<pre><code class="language-python">@defer.inlineCallbacks
|
|
def handle_request(request_id):
|
|
with context.LoggingContext() as request_context:
|
|
request_context.request = request_id
|
|
yield 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 a deferred</li>
|
|
<li><code>handle_request</code> yields the deferred</li>
|
|
<li>The <code>inlineCallbacks</code> wrapper of <code>handle_request</code> returns a deferred</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
|
|
a deferred, you will want to yield on it. To that end, whereever
|
|
functions return a deferred, we adopt the following conventions:</p>
|
|
<p><strong>Rules for functions returning deferreds:</strong></p>
|
|
<blockquote>
|
|
<ul>
|
|
<li>If the deferred is already complete, the function returns with the
|
|
same logcontext it started with.</li>
|
|
<li>If the deferred is incomplete, the function clears the logcontext
|
|
before returning; when the deferred 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 deferred, then the
|
|
logcontext will still be in place. In this case, execution will
|
|
continue immediately after the <code>yield</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 deferred is incomplete, <code>do_request_handling</code> clears
|
|
the logcontext before returning. The logcontext is therefore clear
|
|
when <code>handle_request</code> yields the deferred. At that point, the
|
|
<code>inlineCallbacks</code> wrapper adds a callback to the deferred, and
|
|
returns another (incomplete) deferred to the caller, and it is safe
|
|
to begin processing the next request.</p>
|
|
<p>Once <code>do_request_handling</code>'s deferred completes, it will reinstate
|
|
the logcontext, before running the callback added by the
|
|
<code>inlineCallbacks</code> wrapper. That callback runs 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
|
|
-though that only matters if the caller has its own logcontext which it
|
|
cares about.</p>
|
|
<p>The following sections describe pitfalls and helpful patterns when
|
|
implementing these rules.</p>
|
|
<h2 id="always-yield-your-deferreds"><a class="header" href="#always-yield-your-deferreds">Always yield your deferreds</a></h2>
|
|
<p>Whenever you get a deferred back from a function, you should <code>yield</code> on
|
|
it as soon as possible. (Returning it directly to your caller is ok too,
|
|
if you're not doing <code>inlineCallbacks</code>.) Do not pass go; do not do any
|
|
logging; do not call any other functions.</p>
|
|
<pre><code class="language-python">@defer.inlineCallbacks
|
|
def fun():
|
|
logger.debug("starting")
|
|
yield do_some_stuff() # just like this
|
|
|
|
d = more_stuff()
|
|
result = yield d # also fine, of course
|
|
|
|
return result
|
|
|
|
def nonInlineCallbacksFun():
|
|
logger.debug("just a wrapper really")
|
|
return do_some_stuff() # this is ok too - the caller will yield on
|
|
# it anyway.
|
|
</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> (as wrapped by <code>inlineCallbacks</code>) and
|
|
<code>nonInlineCallbacksFun</code>.</p>
|
|
<p>It's all too easy to forget to <code>yield</code>: for instance if we forgot that
|
|
<code>do_some_stuff</code> returned a deferred, 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>yield</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
|
|
Deferreds - 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-deferred-make-it-follow-the-rules"><a class="header" href="#where-you-create-a-new-deferred-make-it-follow-the-rules">Where you create a new Deferred, make it follow the rules</a></h2>
|
|
<p>Most of the time, a Deferred comes from another synapse function.
|
|
Sometimes, though, we need to make up a new Deferred, or we get a
|
|
Deferred back from external code. We need to make it follow our rules.</p>
|
|
<p>The easy way to do it is with a combination of <code>defer.inlineCallbacks</code>,
|
|
and <code>context.PreserveLoggingContext</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 wrapping it with
|
|
<code>PreserveLoggingContext</code> and <code>yield</code> ing on it:</p>
|
|
<pre><code class="language-python">@defer.inlineCallbacks
|
|
def sleep(seconds):
|
|
with PreserveLoggingContext():
|
|
yield get_sleep_deferred(seconds)
|
|
</code></pre>
|
|
<p>This technique works equally for external functions which return
|
|
deferreds, or deferreds we have made ourselves.</p>
|
|
<p>You can also use <code>context.make_deferred_yieldable</code>, which just does the
|
|
boilerplate for you, so the above could be written:</p>
|
|
<pre><code class="language-python">def sleep(seconds):
|
|
return 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield foreground_operation()
|
|
|
|
# *don't* do this
|
|
background_operation()
|
|
|
|
logger.debug("Request handling complete")
|
|
|
|
@defer.inlineCallbacks
|
|
def background_operation():
|
|
yield first_background_step()
|
|
logger.debug("Completed first step")
|
|
yield 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
|
|
Deferred, it will expect its caller to <code>yield</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
|
|
Deferred returned by <code>background_operation</code> completes, it will restore
|
|
the original logcontext. There is nothing waiting on that Deferred, 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield 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 deferred, and adds a callback to the returned deferred to
|
|
reset the logcontext. In other words, it turns a function that follows
|
|
the Synapse rules about logcontexts and Deferreds 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
yield 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 deferred via <code>defer.gatherResults</code>:</p>
|
|
<pre><code class="language-python">d1 = operation1()
|
|
d2 = operation2()
|
|
d3 = defer.gatherResults([d1, d2])
|
|
</code></pre>
|
|
<p>This is really a variation of the fire-and-forget problem above, in that
|
|
we are firing off <code>d1</code> and <code>d2</code> without yielding 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 Deferred returned by <code>gatherResults</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-deferred-make-it-follow-the-rules">Where you create a new Deferred, 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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
with PreserveLoggingContext():
|
|
d1 = operation1()
|
|
d2 = operation2()
|
|
result = yield defer.gatherResults([d1, d2])
|
|
</code></pre>
|
|
<p>In this case particularly, though, option two, of using
|
|
<code>context.preserve_fn</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">@defer.inlineCallbacks
|
|
def do_request_handling():
|
|
d1 = context.preserve_fn(operation1)()
|
|
d2 = context.preserve_fn(operation2)()
|
|
|
|
with PreserveLoggingContext():
|
|
result = yield defer.gatherResults([d1, d2])
|
|
</code></pre>
|
|
<h2 id="was-all-this-really-necessary"><a class="header" href="#was-all-this-really-necessary">Was all this really necessary?</a></h2>
|
|
<p>The conventions used work fine for a linear flow where everything
|
|
happens in series via <code>defer.inlineCallbacks</code> and <code>yield</code>, but are
|
|
certainly tricky to follow for any more exotic flows. It's hard not to
|
|
wonder if we could have done something else.</p>
|
|
<p>We're not going to rewrite Synapse now, so the following is entirely of
|
|
academic interest, but I'd like to record some thoughts on an
|
|
alternative approach.</p>
|
|
<p>I briefly prototyped some code following an alternative set of rules. I
|
|
think it would work, but I certainly didn't get as far as thinking how
|
|
it would interact with concepts as complicated as the cache descriptors.</p>
|
|
<p>My alternative rules were:</p>
|
|
<ul>
|
|
<li>functions always preserve the logcontext of their caller, whether or
|
|
not they are returning a Deferred.</li>
|
|
<li>Deferreds returned by synapse functions run their callbacks in the
|
|
same context as the function was orignally called in.</li>
|
|
</ul>
|
|
<p>The main point of this scheme is that everywhere that sets the
|
|
logcontext is responsible for clearing it before returning control to
|
|
the reactor.</p>
|
|
<p>So, for example, if you were the function which started a
|
|
<code>with LoggingContext</code> block, you wouldn't <code>yield</code> within it --- instead
|
|
you'd start off the background process, and then leave the <code>with</code> block
|
|
to wait for it:</p>
|
|
<pre><code class="language-python">def handle_request(request_id):
|
|
with context.LoggingContext() as request_context:
|
|
request_context.request = request_id
|
|
d = do_request_handling()
|
|
|
|
def cb(r):
|
|
logger.debug("finished")
|
|
|
|
d.addCallback(cb)
|
|
return d
|
|
</code></pre>
|
|
<p>(in general, mixing <code>with LoggingContext</code> blocks and
|
|
<code>defer.inlineCallbacks</code> in the same function leads to slighly
|
|
counter-intuitive code, under this scheme).</p>
|
|
<p>Because we leave the original <code>with</code> block as soon as the Deferred is
|
|
returned (as opposed to waiting for it to be resolved, as we do today),
|
|
the logcontext is cleared before control passes back to the reactor; so
|
|
if there is some code within <code>do_request_handling</code> which needs to wait
|
|
for a Deferred to complete, there is no need for it to worry about
|
|
clearing the logcontext before doing so:</p>
|
|
<pre><code class="language-python">def handle_request():
|
|
r = do_some_stuff()
|
|
r.addCallback(do_some_more_stuff)
|
|
return r
|
|
</code></pre>
|
|
<p>--- and provided <code>do_some_stuff</code> follows the rules of returning a
|
|
Deferred which runs its callbacks in the original logcontext, all is
|
|
happy.</p>
|
|
<p>The business of a Deferred which runs its callbacks in the original
|
|
logcontext isn't hard to achieve --- we have it today, in the shape of
|
|
<code>context._PreservingContextDeferred</code>:</p>
|
|
<pre><code class="language-python">def do_some_stuff():
|
|
deferred = do_some_io()
|
|
pcd = _PreservingContextDeferred(LoggingContext.current_context())
|
|
deferred.chainDeferred(pcd)
|
|
return pcd
|
|
</code></pre>
|
|
<p>It turns out that, thanks to the way that Deferreds chain together, we
|
|
automatically get the property of a context-preserving deferred with
|
|
<code>defer.inlineCallbacks</code>, provided the final Defered the function
|
|
<code>yields</code> on has that property. So we can just write:</p>
|
|
<pre><code class="language-python">@defer.inlineCallbacks
|
|
def handle_request():
|
|
yield do_some_stuff()
|
|
yield do_some_more_stuff()
|
|
</code></pre>
|
|
<p>To conclude: I think this scheme would have worked equally well, with
|
|
less danger of messing it up, and probably made some more esoteric code
|
|
easier to write. But again --- changing the conventions of the entire
|
|
Synapse codebase is not a sensible option for the marginal improvement
|
|
offered.</p>
|
|
<h2 id="a-note-on-garbage-collection-of-deferred-chains"><a class="header" href="#a-note-on-garbage-collection-of-deferred-chains">A note on garbage-collection of Deferred chains</a></h2>
|
|
<p>It turns out that our logcontext rules do not play nicely with Deferred
|
|
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")
|
|
|
|
@defer.inlineCallbacks
|
|
def await_something_interesting():
|
|
new_deferred = defer.Deferred()
|
|
listener_queue.append(new_deferred)
|
|
|
|
with PreserveLoggingContext():
|
|
yield new_deferred
|
|
</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 deferred chain have now dropped their references,
|
|
and the deferred chain is now orphaned, and will be garbage-collected at
|
|
some point. Note that <code>await_something_interesting</code> is a generator
|
|
function, and when Python garbage-collects generator functions, it gives
|
|
them a chance to clean up by making the <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 deferred
|
|
chain are dropped. Dropping the the reference to a deferred you're
|
|
supposed to be calling is probably 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/database_schema.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/database_schema.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> |