How can I read from Redis inside a MULTI block in Ruby?

Question

I\'m encapsulating a complicated set of Redis commands in a MULTI transaction, but the logic in the transaction depends on values already in Redis. But all reads within a transa

Answer 1

As Sergio states in his comment, you can't optionally execute a MULTI block like that in Redis. See the documentation on transactions:

Either all of the commands or none are processed.

You can, however, use WATCH to implement optimistic locking using check-and-set (pseudo code):

SET foo bar
WATCH foo
$foo = GET foo
MULTI
if $foo == 'bar'
  SET foo baz
EXEC
GET foo

Using WATCH, the transaction will only be executed if the watched key(s) has not been changed. If the watch key is changed, the EXEC will fail, and you can try again.

Another possibility is using the scripting functionality, but that's only available in the 2.6 release candidate.