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WAIT

Syntax​

WAIT numreplicas timeout

Time complexity: O(1)

ACL categories: @slow, @connection

This command blocks the current client until all previous write commands are acknowledged by at least numreplicas replicas, or until timeout (in milliseconds) is reached.

It returns the number of replicas that acknowledged the writes. If the requested number of replicas acknowledged before the timeout, the command returns as soon as that happens; otherwise, it returns when the timeout expires. In either case, the returned count may be lower than numreplicas.

WAIT can only be issued on a master instance. When called on a replica, it returns an error.

Note that WAIT does not make Dragonfly a strongly consistent store: acknowledged writes can still be lost during a failover, depending on which replica is promoted. However, it greatly reduces the window of data loss.

Dragonfly-specific behavior​

Dragonfly's WAIT differs from Redis in a few ways:

  • Stronger acknowledgment guarantee. Redis only waits for the writes issued on the calling connection. Dragonfly waits for all writes performed on the instance up to the moment WAIT is called, from any connection. This is a strictly stronger guarantee.
  • A timeout of 0 is bounded. In Redis, timeout = 0 blocks forever. In Dragonfly, it is capped at 10 minutes, after which the command returns the current acknowledgment count.
  • Early return on role or state changes. WAIT returns the current count early if the instance starts shutting down or a takeover begins, and returns an error if the instance becomes a replica while waiting (for example, due to a concurrent REPLICAOF).
  • Fixed replica set. The set of replicas is snapshotted when WAIT is invoked. If fewer connected replicas remain than numreplicas and all of them have already acknowledged, the command returns immediately instead of waiting out the timeout.

As in Redis, when WAIT is called inside a MULTI/EXEC transaction it does not block, and instead returns the number of replicas that have already acknowledged all prior writes.

Return​

Integer reply: the number of replicas that acknowledged all write commands issued before the WAIT call.

Examples​

dragonfly> SET foo bar
OK
dragonfly> WAIT 1 100
(integer) 1
dragonfly> WAIT 5 100
(integer) 1

In the example above, the instance has a single replica. The first WAIT returns as soon as that replica acknowledges the write. The second call asks for five replicas; since only one exists, it returns 1 immediately.