redis key expiration settings

EXPIRE key seconds

is used to set an expiration time for a key. The second parameter indicates how many seconds will pass before the key expires. After a key expires, the key will be automatically deleted. In Redis terminology, keys with expiration times are often called volatile.

When using delete or overwrite operations on this key, the expiration time will be cleared. These operations include DEL, SET, GETSET and all *STORE commands. Those commands that modify the key value will not modify the expiration time, such as IINCR to modify the value, LPUSH to add a new value to the queue, and HSET to modify the members in the hash table.

If you just want to clear the expiration time, you can call the PERSIST command so that the key will not expire. The expiration time is an attribute of the key and will not change due to the key name modification (RENAME). When RENAME is used to overwrite other keys, only an overwriting effect has passed, and the renamed key will still retain its expiration time.

Note: calling the EXPIRE/PEXPIRE command with a non-positive number on a key, or calling EXPIREAT/PEXPIREAT with a past time will directly remove the key.

Update expiration time

Calling EXPIRE on a key with an expiration time can update the expiration time of the key.

Return value

Calling EXPIRE key seconds will return 0 or 1. 0 means that the key does not exist, and 1 means that the key has been set to timeout.

How Redis handles keys with expiration time

Keys with expiration time

Under normal circumstances, redis The key exists until the key is explicitly deleted (via the DEL command) or cleared due to memory constraints. Setting a timeout for a key requires additional memory to record relevant information. Redis will ensure that the key will be removed when it expires.

Expiration time precision

Starting from redis2.6, the deviation of expiration time is between 0 and 1 millisecond.

Expiration time logic

The expiration time of the key uses the Unix absolute timestamp (with milliseconds as the precision). Even if the redis instance is closed, by that Unix time If you poke it, the key will also expire (it just won't be cleared immediately).

If you want to move data in one redis to redis on another computer by moving RDB files, you need to ensure that the unix absolute timestamps of the two computers are consistent.

If you set an expiration time for a key, do not modify your computer time casually, because redis will often check the system time. If you adjust the time forward, the keys that should expire will not expire. If you adjust the time backward, keys will expire that shouldn't expire.

How redis cleans up expired keys:

Redis cleans up expired keys in two ways: passive cleaning and active cleaning.

(1) Passive cleaning: When accessing a key with an expiration time, if it is found that the key has expired, the key will be cleared.

(2) redis will perform 10 checks per second, each check includes:

• Randomly select 20 keys with expiration time, and then delete them All expired keys.

• If more than 25% of the keys among these 20 have expired, then perform step 1) again.

This is a simple probability algorithm, we assume that the selected key can be used as a sample of all keys with expiration time. Through the above operations, we can basically determine that the ratio of timed-out keys to keys with timeout periods is less than 25%. Then the excess memory occupied by expired keys that have not been cleaned up should be less than 25%.

How to handle expiration cleanup in replication and AOF files

In order not to destroy consistency, when a key expires and is cleaned up, the DEL operation will be synchronized with the AOF file and all replica nodes. In this way, the master node is responsible for the expiration cleanup operation. The slave node will not process the expiration cleanup of keys, but will only wait for the DEL command from the master node, so that the key spaces of the master node and the slave node will remain consistent.

For more redis knowledge, please pay attention to the redis introductory tutorial column.

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