面试题之redis的过期时间原理

1、消极方法：每次访问key时判断key是否已经过期；

2、积极方法；周期性的从设置了过期时间的key中选择一部分的key进行删除

　　a、随机测试20个带有timeout信息的key

　　b、如果超过25%的key被删除，则重复执行整个流程

以下是redis官网原文

Specifically this is what Redis does 10 times per second:

1. Test 20 random keys from the set of keys with an associated expire.
2. Delete all the keys found expired.
3. If more than 25% of keys were expired, start again from step 1.

官网地址：https://redis.io/commands/expire