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<div class="title">Polling Engine Usage on gRPC client and Server </div>  </div>
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<div class="textblock"><p><em>Author: Sree Kuchibhotla () - Sep 2018</em></p>
<p>This document talks about how polling engine is used in gRPC core (both on client and server code paths).</p>
<h2>gRPC client</h2>
<h3>Relation between Call, Channel (sub-channels), Completion queue, <code>grpc_pollset</code></h3>
<ul>
<li>A gRPC Call is tied to a channel (more specifically a sub-channel) and a completion queue for the lifetime of the call.</li>
<li>Once a <em>sub-channel</em> is picked for the call, the file-descriptor (socket fd in case of TCP channels) is added to the pollset corresponding to call's completion queue. (Recall that as per <a class="el" href="grpc-cq_8md.html">grpc-cq</a>, a completion queue has a pollset by default)</li>
</ul>
<div class="image">
<img src="../images/grpc-call-channel-cq.png" alt="image"/>
</div>
<h3>Making progress on Async <code>connect()</code> on sub-channels (<code>grpc_pollset_set</code> usecase)</h3>
<ul>
<li>A gRPC channel is created between a client and a 'target'. The 'target' may resolve in to one or more backend servers.</li>
<li>A sub-channel is the 'connection' from a client to the backend server</li>
<li>While establishing sub-channels (i.e connections) to the backends, gRPC issues async <a href="https://github.com/grpc/grpc/blob/v1.15.1/src/core/lib/iomgr/tcp_client_posix.cc#L296"><code>connect()</code></a> calls which may not complete right away. When the <code>connect()</code> eventually succeeds, the socket fd is make 'writable'<ul>
<li>This means that the polling engine must be monitoring all these sub-channel <code>fd</code>s for writable events and we need to make sure there is a polling thread that monitors all these fds</li>
<li>To accomplish this, the <code>grpc_pollset_set</code> is used the following way (see picture below)</li>
</ul>
</li>
</ul>
<div class="image">
<img src="../images/grpc-client-lb-pss.png" alt="image"/>
</div>
<h2>gRPC server</h2>
<ul>
<li>The listening fd (i.e., the socket fd corresponding to the server listening port) is added to each of the server completion queues. Note that in gRPC we use SO_REUSEPORT option and create multiple listening fds but all of them map to the same listening port</li>
<li>A new incoming channel is assigned to some server completion queue picked randomly (note that we currently <a href="https://github.com/grpc/grpc/blob/v1.15.1/src/core/lib/iomgr/tcp_server_posix.cc#L231">round-robin</a> over the server completion queues)</li>
</ul>
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