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<li class="toc-h1"><a href="#the-pantsdaemon-class">The PantsDaemon Class</a></li>
<li class="toc-h1"><a href="#pailgun">Pailgun</a></li>
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<h1 id="pants-daemon">Pants Daemon</h1>
<p>The Pants Daemon (pantsd) is a system introduced to enable Pants to keep information about the build warm in memory between runs.
It consists of a process running in the background (currently, one for each buildroot), which listens to filesystem events and keeps a build graph warm. It then passes that graph to subsequent runs that request it.</p>
<p>This document outlines all the moving pieces of pantsd, and explains how it works through an end-to-end run.</p>
<h2 id="processmanager">ProcessManager</h2>
<p><a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/process_manager.py#223"><code>ProcessManager</code></a> is a class designed to keep track of processes. Besides changing their state (paused, terminating...), it allows a process to fork in different ways. Classes that extend <code>ProcessManager</code> can (and often do) inject code to be run before and after forking by overriding the functions <code>pre_fork</code>, <code>post_fork_child</code>, <code>post_fork_parent</code>. An example of one of these functions is in <a href="https://github.com/pantsbuild/pants/blob/aa70d5a3911f78ac9e1f64d2a85ff9dbec1b9dd0/src/python/pants/java/nailgun_executor.py#L296"><code>NailgunExecutor</code></a>, where we start a Nailgun server that we can connect to, after forking the main Pants process.</p>
<h2 id="the-pantsdaemon-class">The <code>PantsDaemon</code> Class</h2>
<p>The <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/pants_daemon.py#81"><code>PantsDaemon</code></a> class is responsible for managing the lifetime of the pantsd process and the services associated with it. A <code>PantsDaemon</code> is created with the inner class <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/pants_daemon.py#100"><code>Factory</code></a>, and it has two modes of initialization:</p>
<ul>
<li>Stub initialization, which parses options, launches watchman, and not much else. This mode is used to determine whether it needs to fully initialize the daemon or not.</li>
<li>Full initialization, which spawns the daemon process, initializes all the services, the legacy engine and the native code.</li>
</ul>
<p>Initialization is encapsulated in the <code>PantsDaemon.Factory.create()</code> method.
<code>PantsDaemon</code> is a <code>ProcessManager</code>, which means one can know if it's alive, or if it needs to restart.</p>
<p><code>PantsDaemon</code>s can be <code>launch()</code>ed, which will terminate the process it was running, and call <code>daemon_spawn()</code> to fork a new process. This new process will run the code in <code>PantsDaemon.post_fork_child()</code>, which in short means it will run <code>os.spawnve</code> to execute the <code>pants_daemon.py:launch()</code> function, which will call <code>PantsDaemon.run_sync()</code>. <code>run_sync()</code> does a lot of things, but the vital things are calling <code>_setup_services()</code> to spin up services, and <code>_run_services()</code> to start an infinite loop polling them. More on services later.</p>
<h2 id="pailgun">Pailgun</h2>
<p>The <a href="http://www.martiansoftware.com/nailgun/protocol.html">Nailgun Protocol</a> is a protocol designed to allow clients to make command-line requests to a <a href="http://www.martiansoftware.com/nailgun/index.html">Nailgun Server</a>. It supports an interface similar to <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/engine/isolated_process.py#22"><code>Process</code></a>, except for it having no concern about making operations hermetic, and the Nailgun Protocol supports streaming access to stdin/stdout. Pailgun is an extension of the Nailgun Protocol, which is clients use to ask the Pants Daemon to spawn pants invocations.</p>
<p>The protocol is subject to change slightly in <a href="https://github.com/pantsbuild/pants/pull/6579">#6579</a>.</p>
<p>In Pantsd, <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/pailgun_server.py#91"><code>PailgunServer</code></a> is the class responsible for reading Pailgun requests and handling them, by spawning <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/pailgun_server.py#52"><code>PailgunHandler</code></a>s in new threads.</p>
<h2 id="services">Services</h2>
<p>A Pants Daemon process has several services that it polls in order. Every service runs in a separate thread, and can be paused, resumed or terminated. Services can communicate with each other.</p>
<p>Examples of services are <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/service/scheduler_service.py#21"><code>SchedulerService</code></a>, which takes care of listening responding to those events to keep a warm <code>Graph</code>, and <a href="https://github.com/pantsbuild/pants/blob/master/src/python/pants/pantsd/service/pailgun_service.py#15"><code>PailgunService</code></a>, which listens to <code>SchedulerService</code> and manages the lifetime of a <code>PailgunServer</code> responsible for spawning pants runs when requested by clients, it takes a <code>DaemonPantsRunner</code> as one of its arguments, to use as a template to spawn pants runs when requested by the clients.</p>
<h3 id="pailgunservice">PailgunService</h3>
<p>A <code>PailgunService</code> is a <code>PantsService</code> which spins up and polls a <code>PailgunServer</code>.
A <code>PailgunServer</code> is a <code>TCPServer</code> with <code>ThreadingMixIn</code>, which listens to Pailgun requests in a socket and spins up instances of <code>PailgunHandler</code>s to handle them. It overrides <code>ThreadingMixIn.process_request_thread()</code> to spin up one thread and one handler per request. A <code>PailgunServer</code> holds a reference to the class <code>DaemonPantsRunner</code>, which can be used to run pants from the server.
A <code>PailgunHandler</code> is a class that parses the requests sent to the server, and uses <code>DaemonPantsRunner</code> to invoke pants with the environment and arguments specified by the request.
A <code>DaemonPantsRunner</code> implements a <code>run()</code> method that creates an instance of <code>LocalPantsRunner</code>, which will be used to run the requested pants command.</p>
<h2 id="an-end-to-end-run-with-pantsd">An end-to-end run with Pantsd</h2>
<p>To understand what the process is for spinning up and closing down pantsd, here is what happens when we run pantsd for the first time:</p>
<p>If we run the command <code>./pants --enable-pantsd list src/scala::</code>, the following happens:</p>
<ol>
<li><code>PantsRunner::run()</code> is called, which will prompt parsing of bootstrap options. <code>--enable-pantsd</code> is a bootstrap option.</li>
<li>
<p>In that function, we determine whether we need to run in pantsd mode or local mode.
If we choose local mode, an instance of <code>LocalPantsRunner</code> is created and the run will continue as if pantsd didn't exist.</p>
</li>
<li>
<p>Since <code>--enable-pantsd</code> was toggled on, we will create an instance of <code>RemotePantsRunner</code>, and run it. <code>RemotePantsRunner</code> will:</p>
</li>
<li>Maybe launch pantsd, by calling <code>PantsDaemon.Factory.maybe_launch()</code>. This method will:<ul>
<li>Create a stub instance of <code>PantsDaemon</code>.</li>
<li>Check if any of the fingerprinted options have changed by calling <code>ProcessManager.needs_restart()</code>. If they have (or there is no pantsd running at the moment), it will fully initialize an instance of <code>PantsDaemon</code> by calling <code>PantsDaemon.Factory.create()</code>, and launch it with <code>PantsDaemon.launch()</code>. In either case, it will return a <code>PantsDaemon.Handle</code> to a process running <code>PantsDaemon.run_sync()</code>, which will poll all the services.</li>
</ul>
</li>
<li>With a handle to the pantsd process, <code>RemotePantsRunner</code> will now call <code>_run_pants_with_retry()</code>, which will try to <code>_connect_and_execute()</code> to the port supplied by the handle, probably more than once.
To do that, it will create a <code>NailgunClient</code> instance with will use the pailgun protocol described above to tell the pantsd process to invoke pants, with a call along the lines of:
<code>result = client.execute('./pants', *self._args, **modified_env)</code></li>
<li>
<p>After that request is finished, it will record the result and the local process will exit. But before that happens, this is what happens from the pantsd side:</p>
</li>
<li>
<p>The <code>PailgunService</code> will receive that request, and it will handle it as follows:</p>
</li>
<li><code>PailgunService</code> is endlessly polling for requests via calling <code>PailgunServer.handle_request()</code>.</li>
<li>When that method receives a request, it will call <code>SocketServer._handle_request_noblock()</code>, which will call <code>ThreadingMixIn.process_request()</code> (it would usually call <code>SocketServer.process_request</code>, but <code>PailgunServer</code> also extends <code>ThreadingMixIn</code>, which intentionally overrides this function).</li>
<li><code>ThreadingMixIn.process_request()</code> will spin up a new <code>Thread</code> and call <code>PailgunServer.process_request_thread()</code> (it would usually call <code>ThreadingMixIn.process_request_thread()</code>, but we override it).</li>
<li><em>(note: we are now in a separate thread)</em> <code>PailgunServer.process_request_thread()</code> will create an instance of <code>PailgunHandler</code>, and call <code>PailgunHandler.handle_request()</code>.</li>
<li><code>PailgunHandler.handle_request()</code> will create an instance of <code>DaemonPantsRunner</code>, and it will <code>run()</code> it. Creating an instance of <code>DaemonPantsRunner</code> with <code>DaemonPantsRunner.create()</code> means that it will call the <code>SchedulerService</code> to get a warm graph.</li>
<li>
<p><code>DaemonPantsRunner</code> will create an instance of <code>LocalPantsRunner</code> with the graph (and options, and such) it got from the <code>SchedulerService</code>, and run as if the daemon didn't exist.</p>
</li>
<li>
<p>The <code>PailgunServer</code> will wait until <code>LocalPantsRunner</code> is finished in <code>handle_request</code>.</p>
</li>
</ol>
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