5.1 Redis Key Expiration Strategy

1. What Does Redis Key Expiration Mean?

It means when a key is actually deleted after an expiration time has been set. Expiration precision is 0–1 ms.

2. Redis Key Expiration Commands

• expire can set a timeout.
  • If the key exists, the reply is 1; if the key does not exist, it returns 0.
• persist can clear the timeout and make the key permanent.
• After a key is renamed with rename, the new key inherits the old key’s expiration.

3. How Redis Key Expiration Works

3.1. Passive Method: Lazy Deletion

• When a key is accessed, Redis checks whether it has expired and deletes it if so.
• Disadvantage: if an expired key is never accessed, it continues occupying memory, which is equivalent to a memory leak.

3.2. Active Method: Periodic Random Deletion

• Because iterating through all keys would create too much CPU load, Redis only randomly selects some keys for deletion.
• Redis performs the following operation 10 times per second:
  1. Randomly select 20 keys that have expiration times.
  2. Delete the expired keys.
  3. If more than 25% of the selected keys were deleted, repeat step 1.
• 

4. References

• EXPIRE – Redis