Interviewer: How is the expired key in Redis deleted?

Introduction

We can set the expiration time for the key in Redis, then when the key expires, when will it be deleted?

If we write the Redis expiration policy, we will think of the following three options

1. Delete regularly, create a timer while setting the expiration time of the key.When the expiration time of the key comes, delete the key immediately
2. Lazy delete, every time you get a key, determine whether the key expires, if it expires, delete the key, if not, return the key
3. Delete regularly, check the keys every once in a while, and delete the expired keys in it

The timing deletion strategy is not friendly to the CPU. When there are many expired keys, the Redis thread is used to delete the expired keys, which will affect the response of normal requests

Lazy delete read CPU is better, but it will waste a lot of memory.If a key is set to expire in memory, but it is not accessed, it will always exist in memory

Remove policy periodically is more CPU and memory friendly

Key expiration policy in Redis

The following two strategies are selected for the deletion of redis expired keys

1. Lazy delete
2. Remove periodically

Lazy delete

When the client accesses the key, it checks the expiration time of the key, and deletes it immediately if it expires

Remove periodically

Redis will put keys with expiration time set in a separate dictionary, and traverse this dictionary regularly to delete expired keys. The traversal strategy is as follows

1. Expiration scan is performed 10 times per second, and 20 keys are randomly selected from the expired dictionary each time
2. Delete expired keys among the 20 keys
3. If the ratio of expired keys exceeds 1/4, go to step one
4. The upper limit of each scan time does not exceed 25ms by default to avoid thread stuck

Because the expired key in Redis is deleted by the main thread, in order not to block the user's request, the expired key is deleted a small number of times.The source code can refer to the activeExpireCycle method in expire.c

Why should I understand the deletion policy of redis expired keys?

There is only one purpose, let you know Set the expiration time of the key to a random range, not all at the same time, otherwise frequent scanning of expired dictionaries will causeCauses the client's request to be stuck

Reference Blog

"Redis Deep Adventure" old money