Count Keys matching pattern in redis

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Anjna Bhati

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Jun 26, 2016, 4:45:04 AM6/26/16

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Hi,

I am trying to keep around 4 million keys in redis with a specific initial pattern and then want to get those keys' count from redis. Each key has an expiry so the count returns only those keys that have not expired. Using Keys and scan is too expensive an operation and slows down the application. Is there an alternative in redis for doing this without affecting the application's performance or any collection that I can use that will serve my purpose. Please guide.

Regards,

Anjna Bhati

Itamar Haber

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Jun 26, 2016, 9:26:37 AM6/26/16

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Hi Anjna,

There are two alternatives to online ad-hoc scanning (with `SCAN` and never with `KEYS`) for key patterns.

Alternative 1 - keyspace notifications: read the doc at http://redis.io/topics/notifications and then set up a process to listen to expiration events. This will have minimal implication on your application and you can maintain any counters needed with that listener. The caveat of this approach is that events (PubSub) are delivered at most once, so the listener could potentially miss some, which would lead to counters going out of sync.

Alternative 2 - "manual" expiration: instead of using Redis' expiration, keep a Sorted Set with your keys' names and their expiration timestamp as score. You'll have to change the app slightly to stop setting the TTL and instead ZADD to that zset. Then, either with every read request or periodically (or both), check that zset to find any expired keys and deal with them as needed (i.e. update the relevant pattern counter) before removing them from the zset and DELeting the actual key.

Cheers,

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