Synchronous replication

[Daniel Mezzatto]

Redis replication looks to be async. In other words, the master doesnt block the client waiting the reply to propagate the write command to slaves. Its like the "fire-and-forget" write mode of MongoDB.

What would it take to make a synchronous replication so that the master only replies to the client after it is ensured that the command was propagated to all the slaves? Would it be too difficult to implement a config that would allow redis-server to run in either ways?

This would sort of make Redis compliant to the Consistency guarantee of the CAP theorem.

[Brian P O'Rourke]

Hi Daniel,

Redis doesn't do synchronous replication because it's not what typical
users of the DB want (particularly for the use-cases Redis is common
for). If you need to guarantee that a particular transaction has been
processed before continuing a procedure, you can watch for the value
to propagate to your preferred quorum of slaves. If you want fully
synchronous multi-node replication for every operation in a datastore,
you want Zookeeper or something that implements Paxos.

MongoDB's strategies for these topics are something else altogether -
the "fire-and-forget" criticism there came from Mongo not persisting
to disk on a single node before returning an "OK" (by default). See
the fsync options on AOF for how to tune Redis with regard to this
question.

As an aside: the CAP theorem is a tool to guide decision-making, not a
concrete featureset for databases to implement. It's there to help us
point out that pretty much any real-life distributed system will have
tradeoffs between availability and consistency at different levels of
granularity. See for example Redis's configuration setting of
'slave-read-only', which can be used to make consistency/availability
tradeoff decisions during a master/slave transition.

Cheers,
--bpo

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