grpc/doc/connection-backoff.md

GRPC Connection Backoff Protocol

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.

We have several parameters:

1. INITIAL_BACKOFF (how long to wait after the first failure before retrying)
2. MULTIPLIER (factor with which to multiply backoff after a failed retry)
3. JITTER (by how much to randomize backoffs).
4. MAX_BACKOFF (upper bound on backoff)
5. MIN_CONNECT_TIMEOUT (minimum time we're willing to give a connection to complete)

Proposed Backoff Algorithm

Exponentially back off the start time of connection attempts up to a limit of MAX_BACKOFF, with jitter.

ConnectWithBackoff()
  current_backoff = INITIAL_BACKOFF
  current_deadline = now() + INITIAL_BACKOFF
  while (TryConnect(Max(current_deadline, now() + MIN_CONNECT_TIMEOUT))
         != SUCCESS)
    SleepUntil(current_deadline)
    current_backoff = Min(current_backoff * MULTIPLIER, MAX_BACKOFF)
    current_deadline = now() + current_backoff +
      UniformRandom(-JITTER * current_backoff, JITTER * current_backoff)

With specific parameters of MIN_CONNECT_TIMEOUT = 20 seconds INITIAL_BACKOFF = 1 second MULTIPLIER = 1.6 MAX_BACKOFF = 120 seconds JITTER = 0.2

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.

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.

Reset Backoff

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.

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.