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99 lines
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<div id="projectname">GRPC C++
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 <span id="projectnumber">1.22.0</span>
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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>
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<p>We have several parameters:</p><ol type="1">
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<li>INITIAL_BACKOFF (how long to wait after the first failure before retrying)</li>
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</ol>
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<ol type="1">
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<li>MULTIPLIER (factor with which to multiply backoff after a failed retry)</li>
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</ol>
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<ol type="1">
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<li>JITTER (by how much to randomize backoffs).</li>
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</ol>
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<ol type="1">
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<li>MAX_BACKOFF (upper bound on backoff)</li>
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</ol>
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<ol type="1">
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<li>MIN_CONNECT_TIMEOUT (minimum time we're willing to give a connection to complete)</li>
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</ol>
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<h2>Proposed Backoff Algorithm</h2>
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<p>Exponentially back off the start time of connection attempts up to a limit of MAX_BACKOFF, with jitter.</p>
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<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>
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<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>
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<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>
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<h2>Reset Backoff</h2>
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<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>
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<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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