Redis, session expiration, and reverse lookup

Question

I\'m currently bulding a web app and would like to use Redis to store sessions. At login, the session is inserted into Redis with a corresponding user id, and expiration set at

Answer 1

On the current release branch of Redis (2.6), you cannot have notifications when items are expired. It will probably change with the next versions.

In the meantime, to support your requirement, you need to manually implement expiration notification support. So you have:

session: -> a hash storing your session data - one of the field is 
user: -> a set of 

You need to remove sessionid from the user set when the session expires. So you can maintain a additional sorted set whose score is a timestamp.

When you create session 10 for user 100:

MULTI
HMSET session:10 userid:100 ... other session data ...
SADD user:100 10
ZADD to_be_expired  10
EXEC

Then, you need to build a daemon which will poll the zset to identify the session to expire (ZRANGEBYSCORE). For each expired session, it has to maintain the data structure:

• pop the session out of the zset (ZREMRANGEBYRANK)
• retrieve session userid (HMGET)
• delete session (DEL)
• remove session from userid set (SREM)

The main difficulty is to ensure there is no race conditions when the daemon polls and processes the items. See my answer to this question to see how it can be implemented:

how to handle session expire basing redis?