Question

At my current project I actively use redis for various purposes. There are 2 redis databases for current application:

1. The first one contains absolutely temporary data: how many users are online, who are online, various admin’s counters. This db is cleared before the application starts by start-up script.
2. The second database is used for persistent data like user’s ratings, user’s friends, etc.

Everything seems to be correct and everybody is happy.

However, when I’ve started implementing a new functionality in my application, I discover that I need to intersect a set with user’s friends with a set of online users. These sets stored in different redis databases, and I haven’t found any possibility to do this task in redis, except changing application architecture and move all keys into one namespace(database).

Is there actually any way to perform some command in redis using data from multiple databases? Or maybe my use case of redis is wrong and I have to perform a fix of system architecture?

Answer 1

There is not. There is a command that makes it easy to move keys to another DB:

http://redis.io/commands/move

If you move all keys to one DB, make sure you don’t have any key clashes! You could suffix or prefix the keys from the temp DB to make absolutely sure. MOVE will do nothing if the key already exists in the target DB. So make sure you act on a ‘0’ reply

Using multiple DBs is definitely not a good idea:

A Quote from Salvatore Sanfilippo (the creator of redis):

I understand how this can be useful, but unfortunately I consider
Redis multiple database errors my worst decision in Redis design at
all… without any kind of real gain, it makes the internals a lot
more complex. The reality is that databases don’t scale well for a
number of reason, like active expire of keys and VM. If the DB
selection can be performed with a string I can see this feature being
used as a scalable O(1) dictionary layer, that instead it is not.

With DB numbers, with a default of a few DBs, we are communication
better what this feature is and how can be used I think. I hope that
at some point we can drop the multiple DBs support at all, but I think
it is probably too late as there is a number of people relying on this
feature for their work.

https://groups.google.com/forum/#!msg/redis-db/vS5wX8X4Cjg/8ounBXitG4sJ