WAIT

WAIT numreplicas timeout

Available since: 3.0.0
Time complexity: O(1)
ACL categories: @slow, @blocking, @connection

This command blocks the current client until all the previous write commands are successfully transferred and acknowledged by at least the specified number of replicas. If the timeout, specified in milliseconds, is reached, the command returns even if the specified number of replicas were not yet reached.

The command will always return the number of replicas that acknowledged the write commands sent by the current client before the WAIT command, both in the case where the specified number of replicas are reached, or when the timeout is reached.

A few remarks:

When WAIT returns, all the previous write commands sent in the context of the current connection are guaranteed to be received by the number of replicas returned by WAIT.
If the command is sent as part of a MULTI transaction (since Redict 7.0, any context that does not allow blocking, such as inside scripts), the command does not block but instead just return ASAP the number of replicas that acknowledged the previous write commands.
A timeout of 0 means to block forever.
Since WAIT returns the number of replicas reached both in case of failure and success, the client should check that the returned value is equal or greater to the replication level it demanded.

Consistency and WAIT #

Note that WAIT does not make Redict a strongly consistent store: while synchronous replication is part of a replicated state machine, it is not the only thing needed. However in the context of Sentinel or Redict Cluster failover, WAIT improves the real world data safety.

Specifically if a given write is transferred to one or more replicas, it is more likely (but not guaranteed) that if the master fails, we’ll be able to promote, during a failover, a replica that received the write: both Sentinel and Redict Cluster will do a best-effort attempt to promote the best replica among the set of available replicas.

However this is just a best-effort attempt so it is possible to still lose a write synchronously replicated to multiple replicas.

Implementation details #

Since the introduction of partial resynchronization with replicas (PSYNC feature) Redict replicas asynchronously ping their master with the offset they already processed in the replication stream. This is used in multiple ways:

Detect timed out replicas.
Perform a partial resynchronization after a disconnection.
Implement WAIT.

In the specific case of the implementation of WAIT, Redict remembers, for each client, the replication offset of the produced replication stream when a given write command was executed in the context of a given client. When WAIT is called Redict checks if the specified number of replicas already acknowledged this offset or a greater one.

Examples #

> SET foo bar
OK
> WAIT 1 0
(integer) 1
> WAIT 2 1000
(integer) 1

In the following example the first call to WAIT does not use a timeout and asks for the write to reach 1 replica. It returns with success. In the second attempt instead we put a timeout, and ask for the replication of the write to two replicas. Since there is a single replica available, after one second WAIT unblocks and returns 1, the number of replicas reached.