Redis and Optimistic concurrency control: is it possible?

Question

Working in an application that stores entities in redis as a serialized binary blob. I have multiple clients working on the same data set and I wish to use optimistic concurrency.

My requirements are these:

1. Read the serialized entity for a specific key in one roundtrip
2. Write the modifiend entity back to redis. If any other client modified the entity between the read and the write the operation will fail

Is this possible to do in redis? And if so: what redis commands should be executed to do this?

Convoke

I think WATCH-MULTI-EXEC (transcation commands) are helpful – Pule

Answer 1

WATCH key, GET key, MULTI, SET key, then EXEC. The EXEC will fail if the key's value has changed since you executed the WATCH.

http://redis.io/topics/transactions#cas

Nihilism

Answer 2

For anyone using the StackExchange.Redis library, keep in mind that it multiplexes a single connection and the check-and-set pattern described above may not work as expected for 2 requests sharing that connection. See this article in the docs: link

Bathsheb

Answer 3

With a credit to my boss, who invented it and I executed StackExchange.Redis implementation of reliable concurrency. This is an example of checking if adding items to RedisSet has been completed

Scenario: you need to process a batch of items in different processes. When batch is complete, you want only one single thread report this completion. In this scenario I use Set to keep adding the items. Once Set is filled, I need to report this once.

public bool HasComletedRedisSet(string value, long expected)
{
    long actualCount = 0;
    bool added = _db.SetAdd("key1", value);
    if (added)
    {
        actualCount = _db.StringIncrement("key2", 1L);
    }
    return (actualCount == expectedTotalCount);
}

This code is reliably concurrent and only 1 time returns true... without transaction.

When working with Redis concurrency, one might try think about negative scenarios...

Johnnyjohnnycake

This only works on some narow things inc etc. Update/overwrite Set /Get itself is the issue – Ripp