TTL for a set member

Question

Is it possible in Redis to set TTL (time to live) not for a specific key, but for a member for a set?

I am using a structure for tags proposed by Redis documentation - the data are simple key-value pairs, and the tags are sets containing keys corresponding to each tag, e.g.

> SETEX id:id_1 100 'Lorem ipsum' OK > SADD tag:tag_1 id:id_1 (integer) 1 

The key id:id_1 will expire as expected but i don't see an efficient way to remove the corresponding member from the tag:tag_1 set.

One way I came up is using a cron job containing a script which would remove expired keys from sets periodically - by adding all the tag names to another set and then iterating through all the tags, then all the ids corresponding to each tag and checking if corresponding key exists - if not, calling SREM.

I don't think it will be an efficient way and I would possibly like to keep the tags as clean as possible, because the size of the sets will probably affect performance of searching by multiple tags (SINTER). Is there a more "internal" way?

Answer 1

No, this isn’t possible (and not planned either). The recommended approach is to use an ordered set with score set to timestamp and then manually removing expired keys. To query for non-expired keys, you can use ZRANGEBYSCORE $now +inf, to delete expired keys, ZREMRANGEBYSCORE -inf $now will do the trick.

In my application, I simply issue both commands every time I query the set. I also combine this with (long) expiration time on the set itself to eventually purge unused sets.

This article walks through it in more detail.

Answer 2

It is not possible to directly expire items in list, sets, or zsets.

You need to implement a mechanism to be notified when the master item expires, so that you can maintain the corresponding sets accordingly.

See the answer to this question, I think it applies to your use case (replace session by id, and user by tag):

Redis, session expiration, and reverse lookup