redis sets the validity period of the key
Key command
Set the expiration time of the key.
Query all current keys
If the ttl key is executed and the display is -2, it means that the changed value does not exist in the cache.
Display If it is -1, it is persistent (will not expire)
#If the expiration time is set, the remaining expiration time will be displayed
Completely understand the redis validity period issue
EXPIRE key seconds
Starting version: 1.0.0
Time complexity: O(1)
Return value: 1 if the expiration time is successfully set; 0 if the key does not exist or the expiration time cannot be set.
Set the survival time for a given key. When the key expires (the survival time is 0), it will be automatically deleted.
1. The expiration time follows the key and has nothing to do with the value
In Redis, keys with expiration time are called "volatile keys". The expiration time can be removed by deleting the entire key using the DEL command, or overwritten by the SET and GETSET commands. This means that if a command only modifies the value of a key with an expiration time instead of using If you replace it with a new key value, the expiration time will not be changed. For example, executing the INCR command on a key, executing the LPUSH command on a list, or executing the HSET command on a hash table will not modify the expiration time of the key itself.
2. Set the permanent validity period
Use the PERSIST command to clear the timeout and make it a permanent key.
3. The impact of the rename command on the validity period
If the key is modified by the RENAME command, the relevant timeout period will be transferred to the new key.
If the key is modified by the RENAME command, for example, Key_A originally existed, and then the RENAME Key_B Key_A command is called. At this time, regardless of whether the original Key_A is permanent or set to timeout, it will be overwritten by the validity period status of Key_B.
4. Refresh the expiration time
Performing the EXPIRE operation on a key that already has an expiration time will update its expiration time.
5. Expiration accuracy
In Redis 2.4 and previous versions, the expiration time may not be very accurate, with an error of 0-1 second.
Starting from Redis 2.6, the expiration time error is reduced to 0-1 milliseconds.
6. Impact of external factors on expiration time
Since Redis version 2.6, the expiration time of Keys is stored in milliseconds using Unix timestamps. This means that time keeps passing even if the Redis instance is unavailable.
To handle expired work well, the computer must use a stable time. An interesting phenomenon occurs when you synchronize an RDB file between two computers whose times are not synchronized: all keys expire on load.
Even the running instance will check the computer's clock. For example, if you set the validity period of a key to 1000 seconds, and then set your computer time to 2000 seconds in the future, the key will expire immediately, and Not after waiting 1000 seconds.
7. How Redis eliminates expired keys
There are two ways for Redis keys to expire: passive and active.
When some clients try to access it, the key will be discovered and actively expired.
Of course, this is not enough, because some expired keys will never be accessed. In order to ensure that keys expire within a predetermined time, random testing should be performed regularly and the expiration time should be set. All these expired keys will be deleted.
Specifically, this is what Redis does 10 times per second:
1). Test 20 random keys for related expiration detection.
2). Delete all expired keys.
3). If more than 25% of keys expire, repeat step 1.
This is a trivial probability algorithm. The basic assumption is that our sample is this key key control, and we keep repeating expiration detection until the percentage of expired keys falls below 25%, which means that at any given moment, at most 1/4 of the expired keys will be cleared.
8. How to handle expiration when copying AOF files
In order to obtain the correct behavior without sacrificing consistency, when a key expires, DEL will be synthesized along with the AOF text to all attached of slaves. This approach is centralized within the master instance and eliminates the possibility of consistency errors.
However, when slaves are connected to the master, the keys will not expire independently (will wait until the master executes the DEL command), they will still exist in the data set, so when the slave is elected as the master, the keys will be eliminated independently. Execute and then become master.
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