How to restrict a user to a single signed in session - redis

Question

I'm working on a website where a user is only allowed to have a single signed-in session. If a user attempts to authenticate to the website from more than one location the prior session is logged out.
The session information for a given user will be stored in Redis.
My current implementation uses two Redis databases. The first database stores session ID as the key and username as the value. The second database stores the username as the key and some other information including session ID as the value.
When a user logs in, the second Redis database is queried to find an existing session ID. If found, the old session is deleted, this would essentially force the user's old session to log out. After the old session is removed, we create another session in the first database and update the value of the session ID in the second database using the username as the key.
Here is a demonstration of this behavior using the "redis-cli":
redis-cli
select 2
get username
The website gets the session ID of the previous session and then generates a new session ID...
redis-cli
select 1
del old_session_id
set new_session_id username
select 2
set username new_session_id
This works, but I want to optimize it. In this solution, the website would query Redis twice because we have to get the old session id and delete it.
My question is: Can we combine the two queries?
My challenge is how to read the command get username result and automatically run del old_session_id against the Redis database.
Can anybody help me?

Answer 1

According to the SET documentation, you can add the GET option to write a key and get its old value. I would use this on database 2 first. If it returns an old session id, you can use RENAME to change the key name. If it doesn't return an old session id, you can just use another SET.
You should be able to get your setup to work with something like this:
redis-cli
select 2
set username new_session_id get
If that command returns a value, run this:
redis-cli
select 1
rename old_session_id new_session_id
If it does not return a value, run this:
redis-cli
select 1
set new_session_id username