How Redis implements the automatic cleanup function of data expiration

Redis is an open source memory data structure storage system that can be used to store and read key-value pairs. It supports a variety of data structures, such as strings, lists, hash tables, Collection etc. Since Redis is a memory-based storage system, if the data is not automatically cleaned when it expires, it can easily lead to memory overflow. Therefore, this article will introduce how Redis implements the automatic cleanup function of data expiration and provide specific code examples.

1. Overview of Redis data expiration

Redis supports setting the expiration time of data. The expiration time can be set to a fixed time, such as 1 hour, 1 day, etc., or it can also be set to a time Stamp indicates expiration at that point in time. The expiration time can be set through EXPIRE, EXPIREAT, PEXPIRE, PEXPIREAT and other commands. Its functions are:

1. Automatically clean up expired data.
2. Improve memory utilization.
3. Prevent data from being retained for a long time.

2. Redis data expiration implementation

Redis data expiration is implemented through two scheduled tasks, they are:

1. Periodically scan expired keys (expired keys): The role of this task is to check expired keys and delete them. This task checks every second and deletes all expired keys, if the key has expired then the key will be deleted.
2. Lazy (safe) deletion: The role of this task is to check whether keys have expired when they are used, and delete them. This task only runs when called on an expired key. Once an expired key is called, the key is immediately deleted.

The implementation of Redis data expiration relies on the above two scheduled tasks. Therefore, to enable data expiration, you need to configure it through the following two parameters:

maxmemory-policy: volatile-lru
maxmemory-policy: allkeys-lru

Among them, volatile-lru Indicates that only the LRU (Least Recently Used) elimination operation will be performed on the keys with an expiration time set. allkeys-lru means that the LRU elimination operation will be performed on all keys. The main difference between these two parameters is: volatile-lru will only evict expired keys when memory is full, while allkeys-lru will evict all keys.

3. Redis data expiration code implementation

The following is an example of data expiration automatic cleaning code using the Python Redis module:

import redis
import time

redis_client = redis.Redis(host='localhost', port=6379, db=0)

# 设置键值对和过期时间
redis_client.set('key1', 'value1', ex=5)

# 检查键值对是否存在以及剩余过期时间
if redis_client.exists('key1'):
    ttl = redis_client.ttl('key1')
    print('key1 exists with remaining ttl: ', ttl)

# 等待5秒，过期自动删除
time.sleep(5)

# 检查键值对是否存在以及剩余过期时间
if redis_client.exists('key1'):
    ttl = redis_client.ttl('key1')
    print('key1 exists with remaining ttl: ', ttl)
else:
    print('key1 does not exist.')

In the above code example, we use the Redis module Set key-value pairs and expiration time. We use the exists function to check if the key exists and the ttl function to get the expiration time. Finally, we wait 5 seconds, check again whether the key exists, and output the corresponding results.

4. Summary

Redis data expiration is a very important function. It can effectively reduce memory usage, prevent data from being retained for a long time and improve memory utilization. Redis provides two scheduled tasks to clean up expired keys. Data expiration can be enabled by configuring the maxmemory-policy parameter. In terms of code implementation, we can use the Python Redis module to set the key-value pair and expiration time, and use the exists and ttl functions to check whether the key exists and get the expiration time.

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