Redis get or create?

Question

I want to use Redis as a random seed cache. When I want the value for a key, if nothing's there yet, I'll produce a random string and store it for later reuse.

How do I perform an atomic GET EXISTING OR SET AND RETURN THIS VALUE?

Answer 1

You could use SETNX to try and set the value first. Then the GET would give you the existing value or the new one you tried to set.

SETNX key value

This may return 0 or 1 if you care to know if this is a new value

Answer 2

It seems there is no single command that can do this. Using MULTI and WATCH:

First:

GET key

If null, then:

WATCH key
MULTI
    SET key value
EXEC

If [null] (indicating the transaction aborted), the key was created in the meantime and must exist by now:

GET key