通过订阅 redis 事件实现“时间截止后删除”功能

思考

情景
有一个这样的场景，某系统里为用户开辟了一个空间，这个空间在有效期里可以随意使用。但是到期后要回收。我们可以通过定时任务对数据库表中存在的空间信息进行检查，如果截止时间到了，就进行对应的操作。也可以把这个定时的工程扔给系统以外。

另一种思路
我们可以尝试另一种方案，例如创建空间的时候，将空间 id 作为 key 的一部分存放在 redis 中，而 ex 设置为有效时间。在 redis 将这个超时的 key 删除的时候，通知我们系统，从而完成清除空间的操作。

实施

具体实现如下：

1,打开事件

1.1,修改配置文件

默认的 redis 并没有开启这个功能，需要修改配置文件中的 notify-keyspace-events 配置

############################# Event notification ##############################

# Redis can notify Pub/Sub clients about events happening in the key space.
# This feature is documented at http://redis.io/topics/notifications
# 
# For instance if keyspace events notification is enabled, and a client
# performs a DEL operation on key "foo" stored in the Database 0, two
# messages will be published via Pub/Sub:
#
# PUBLISH __keyspace@0__:foo del
# PUBLISH __keyevent@0__:del foo
#
# It is possible to select the events that Redis will notify among a set
# of classes. Every class is identified by a single character:
#
#  K     Keyspace events, published with __keyspace@<db>__ prefix.
#  E     Keyevent events, published with __keyevent@<db>__ prefix.
#  g     Generic commands (non-type specific) like DEL, EXPIRE, RENAME, ...
#  $     String commands
#  l     List commands
#  s     Set commands
#  h     Hash commands
#  z     Sorted set commands
#  x     Expired events (events generated every time a key expires)
#  e     Evicted events (events generated when a key is evicted for maxmemory)
#  A     Alias for g$lshzxe, so that the "AKE" string means all the events.
#
#  The "notify-keyspace-events" takes as argument a string that is composed
#  by zero or multiple characters. The empty string means that notifications
#  are disabled at all.
#
#  Example: to enable list and generic events, from the point of view of the
#           event name, use:
#
#  notify-keyspace-events Elg
#
#  Example 2: to get the stream of the expired keys subscribing to channel
#             name __keyevent@0__:expired use:
#
#  notify-keyspace-events Ex
#
#  By default all notifications are disabled because most users don't need
#  this feature and the feature has some overhead. Note that if you don't
#  specify at least one of K or E, no events will be delivered.
notify-keyspace-events Ex

是的，重点就是：

notify-keyspace-events Ex

Ex 代表：

# 以  “__keyevent@<db>__“  为前缀发布Keyevent事件，<db>为所使用的redis库编号
1,E: Keyevent events, published with __keyevent@<db>__ prefix.
# 过期事件（每次键过期时生成的事件）
2,x: Expired events (events generated every time a key expires)

1.2,运行时通过参数设置

#设置：
redis-cli: CONFIG SET notify-keyspace-events "Ex"

#查询
redis-cli:CONFIG GET notify-keyspace-event

#redis: 指的是进入redis-cli后，并且执行成功的情况

但是这种重启 redis 服务就没了（恢复为配置文件中的设置）。

2,程序方面

2.1,增加依赖包

pom.xml:

<!-- https://mvnrepository.com/artifact/org.springframework.data/spring-data-redis -->
<dependency>
	<groupId>org.springframework.data</groupId>
	<artifactId>spring-data-redis</artifactId>
	<version>1.8.22.RELEASE</version>
</dependency>

<!-- https://mvnrepository.com/artifact/redis.clients/jedis -->
<dependency>
	<groupId>redis.clients</groupId>
	<artifactId>jedis</artifactId>
	<version>2.10.2</version>
</dependency>

# spring用的是4.3.7.RELEASE。需要注意匹配。

代码：

import org.springframework.data.redis.connection.Message;
import org.springframework.data.redis.connection.MessageListener;


public class RedisKeyExpiredMessageDelegate implements MessageListener {
    private final String EVENT_CHANNEL = "__keyevent@0__:expired";
  
    @Override
    public void onMessage(Message message, byte[] bytes) {
        if (EVENT_CHANNEL.equalsIgnoreCase(new String(message.getChannel()))) {
            String msg = new String(message.getBody());

            System.out.println(msg + "过期");

        }
    }
}

spring 配置文件:

<!-- redis 连接工厂 -->
<bean id="redisConnectionFactory" class="org.springframework.data.redis.connection.jedis.JedisConnectionFactory"
	  p:host-name="localhost"
	  p:port="6379"/>

<!-- redis消息监听 -->
<bean id="redisMessageListener"
	  class="org.springframework.data.redis.listener.adapter.MessageListenerAdapter">
	<constructor-arg>
		<bean class="com.imadiaos.redis.RedisKeyExpiredMessageDelegate" />
	</constructor-arg>
</bean>
<bean id="redisContainer"
	  class="org.springframework.data.redis.listener.RedisMessageListenerContainer">
	<property name="connectionFactory" ref="redisConnectionFactory" />
	<property name="messageListeners">
		<map>
			<entry key-ref="redisMessageListener">
				<list>
					<!--  <bean class="org.springframework.data.redis.listener.ChannelTopic">
						<constructor-arg value="__keyevent@1__:expired" /> </bean>  -->
					<!-- <bean class="org.springframework.data.redis.listener.PatternTopic">
						<constructor-arg value="*" /> </bean> -->
					<bean class="org.springframework.data.redis.listener.PatternTopic">
						<constructor-arg value="__key*__:expired" />
					</bean>
				</list>
			</entry>
		</map>
	</property>
</bean>

代码示例： github.com/imadiaos