How Does Redis Delete Expired Keys? Lazy vs Periodic Strategies Explained

Deletion Strategies

Lazy Deletion : Redis checks a key's expiration only when the key is accessed. This approach is CPU‑friendly because it limits work to the specific key being used, but it can waste memory if many keys expire without further access.

Periodic Deletion : Redis runs a background task at intervals that scans a random subset of keys in each database's expiration dictionary, deleting those that have expired. This method reduces memory waste while keeping CPU impact moderate.

Persistence Handling of Expired Keys

RDB : When generating an RDB snapshot, Redis skips expired keys, so they are not saved to the dump file.

AOF : Every command execution checks for key expiration; if a key is expired, Redis issues a delete command, which is then recorded in the AOF log.

AOF Rewrite : Similar to RDB, the rewrite process filters out expired keys before writing the new AOF file.

Replication Handling of Expired Keys

Master : Executes the standard deletion strategies (lazy or periodic) on its own data.

Slave : Deletion of expired keys on a replica is driven by the master. After the master deletes an expired key, it sends an explicit delete command to the slave; the slave will not delete the key on its own until it receives this command, though it will still serve client requests for the key until then.