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<!DOCTYPE html>
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<div class="annotation">
<h1>lookout.rb</h1>
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<li id="section-1">
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<div class="content"><div class='highlight'><pre><span class="class"><span class="keyword">module</span> <span class="title">Spyglass</span></span>
<span class="class"><span class="keyword">class</span> <span class="title">Lookout</span></span>
<span class="keyword">include</span> <span class="constant">Singleton</span>, <span class="constant">Logging</span></pre></div></div>
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<p>This method is the main entry point for the Lookout class. It takes
a socket object.</p>
</div>
<div class="content"><div class='highlight'><pre> <span class="function"><span class="keyword">def</span> <span class="title">start</span><span class="params">(socket)</span></span>
trap_signals</pre></div></div>
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<p>The Lookout doesn't know anything about the app itself, so there's
no app related setup to do here.</p>
</div>
<div class="content"><div class='highlight'><pre> loop <span class="keyword">do</span></pre></div></div>
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<p>Accepts a new connection on our socket. This class won't actually
do anything interesting with this connection, it will pass it down
to the <code>Master</code> class created below to do the actual request handling.</p>
</div>
<div class="content"><div class='highlight'><pre> conn = socket.accept
out <span class="string">"Received incoming connection"</span></pre></div></div>
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<p>In this block the Lookout forks a new process and invokes a Master,
passing along the socket it received and the connection it accepted
above.</p>
</div>
<div class="content"><div class='highlight'><pre> <span class="variable">@master_pid</span> = fork <span class="keyword">do</span>
master = <span class="constant">Master</span>.new(conn, socket)
master.start
<span class="keyword">end</span></pre></div></div>
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<p>The Lookout can now close its handle on the client socket. This doesn't
translate to the socket being closed on the clients end because the
forked Master process also has a handle on the same socket. Since this
handle is now cleaned up it's up to the Master process to ensure that
its handle gets cleaned up.</p>
</div>
<div class="content"><div class='highlight'><pre> conn.close</pre></div></div>
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<p>Now this process blocks until the Master process exits. The Master process
will only exit once traffic is slow enough that it has reached its timeout
without receiving any new connections.</p>
</div>
<div class="content"><div class='highlight'><pre> <span class="constant">Process</span>.waitpid(<span class="variable">@master_pid</span>)</pre></div></div>
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<p>The interaction of fork(2)/waitpid(2) above deserve some explanation.</p>
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<h3>Why fork(2)? Why not just spin up the Master?</h3>
<p>The whole point of the Lookout process is to be very lean. The only resource
that it initializes is the listening socket for the server. It doesn't load
any of your application into memory, so its resource footprint is very small.</p>
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<p>The reason that it does a fork(2) before invoking the Master is because once
the Master times out we want the Lookout process to remain lean when accepting
the next connection. </p>
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<a class="pilcrow" href="#section-11">¶</a>
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<p>If it were to load the application code without forking
then there would be no (simple) way for it to later unload the application code.</p>
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<p>By doing a fork(2), then waiting for the Master process to exit, that guarantees
that all resources (notably memory usage) that were in use by the Master process
will be reclaimed by the kernel. </p>
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<h3>Who knows what your app will demand!</h3>
<p>While handling requests your app may require lots of memory. Containing this in a
child process, and exiting that process, is the easiest way to ensure that memory
bloat isn't shared with our simple parent process.</p>
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<a class="pilcrow" href="#section-14">¶</a>
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<p>This allows our Lookout process will to go back around
the loop with nothing more than it started with, just a listening socket.</p>
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<li id="section-15">
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<a class="pilcrow" href="#section-15">¶</a>
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<p>The fork(2)/waitpid(2) approach requires little code to implement, and pushes
responsibility down to the kernel to track resource usage and nicely clean up
the Master process when it's finished.</p>
</div>
<div class="content"><div class='highlight'><pre> <span class="keyword">end</span>
<span class="keyword">end</span>
<span class="function"><span class="keyword">def</span> <span class="title">trap_signals</span></span>
[<span class="symbol">:INT</span>, <span class="symbol">:QUIT</span>].each <span class="keyword">do</span> |sig|
trap(sig) {
<span class="keyword">begin</span>
<span class="constant">Process</span>.kill(sig, <span class="variable">@master_pid</span>) <span class="keyword">if</span> <span class="variable">@master_pid</span>
<span class="keyword">rescue</span> <span class="constant">Errno::ESRCH</span>
<span class="keyword">end</span>
exit
}
<span class="keyword">end</span>
<span class="keyword">end</span>
<span class="keyword">end</span>
<span class="keyword">end</span></pre></div></div>
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