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<title>GRPC C++: GRPC Connection Backoff Protocol</title>
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<div class="title">GRPC Connection Backoff Protocol </div>  </div>
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<div class="textblock"><p>When we do a connection to a backend which fails, it is typically desirable to not retry immediately (to avoid flooding the network or the server with requests) and instead do some form of exponential backoff.</p>
<p>We have several parameters:</p><ol type="1">
<li>INITIAL_BACKOFF (how long to wait after the first failure before retrying)</li>
</ol>
<ol type="1">
<li>MULTIPLIER (factor with which to multiply backoff after a failed retry)</li>
</ol>
<ol type="1">
<li>JITTER (by how much to randomize backoffs).</li>
</ol>
<ol type="1">
<li>MAX_BACKOFF (upper bound on backoff)</li>
</ol>
<ol type="1">
<li>MIN_CONNECT_TIMEOUT (minimum time we're willing to give a connection to complete)</li>
</ol>
<h1><a class="anchor" id="autotoc_md77"></a>
Proposed Backoff Algorithm</h1>
<p>Exponentially back off the start time of connection attempts up to a limit of MAX_BACKOFF, with jitter.</p>
<div class="fragment"><div class="line">ConnectWithBackoff()</div>
<div class="line">  current_backoff = INITIAL_BACKOFF</div>
<div class="line">  current_deadline = now() + INITIAL_BACKOFF</div>
<div class="line">  while (TryConnect(Max(current_deadline, now() + MIN_CONNECT_TIMEOUT))</div>
<div class="line">         != SUCCESS)</div>
<div class="line">    SleepUntil(current_deadline)</div>
<div class="line">    current_backoff = Min(current_backoff * MULTIPLIER, MAX_BACKOFF)</div>
<div class="line">    current_deadline = now() + current_backoff +</div>
<div class="line">      UniformRandom(-JITTER * current_backoff, JITTER * current_backoff)</div>
</div><!-- fragment --><p>With specific parameters of MIN_CONNECT_TIMEOUT = 20 seconds INITIAL_BACKOFF = 1 second MULTIPLIER = 1.6 MAX_BACKOFF = 120 seconds JITTER = 0.2</p>
<p>Implementations with pressing concerns (such as minimizing the number of wakeups on a mobile phone) may wish to use a different algorithm, and in particular different jitter logic.</p>
<p>Alternate implementations must ensure that connection backoffs started at the same time disperse, and must not attempt connections substantially more often than the above algorithm.</p>
<h1><a class="anchor" id="autotoc_md78"></a>
Reset Backoff</h1>
<p>The back off should be reset to INITIAL_BACKOFF at some time point, so that the reconnecting behavior is consistent no matter the connection is a newly started one or a previously disconnected one.</p>
<p>We choose to reset the Backoff when the SETTINGS frame is received, at that time point, we know for sure that this connection was accepted by the server. </p>
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