doc: 阅读Redis keyspace notification文档

中间件/redis/redis.md

@@ -186,6 +186,11 @@
       - [Messages matching both a pattern and a channel subscription](#messages-matching-both-a-pattern-and-a-channel-subscription)
       - [the meaning of the subscription count with pattern matching](#the-meaning-of-the-subscription-count-with-pattern-matching)
       - [Sharded Pub/Sub](#sharded-pubsub)
+      - [Redis Keyspace notifaction](#redis-keyspace-notifaction)
+        - [Type of events](#type-of-events)
+        - [Configuration](#configuration)
+        - [Timing of expired events](#timing-of-expired-events)
+        - [Events in cluster](#events-in-cluster)
 
 
 # redis
@@ -3100,3 +3105,73 @@ Sharded Pub/Sub能够帮助`Pub/Sub`在集群模式下的拓展。其将消息
 
 > 在未引入`Shard Pub/Sub`机制之前，`Pub/Sub`的channel在集群中并不会被hash到slot。此时，`cluster中的每个node独立的维护订阅关系，不同节点之间的订阅并不共享`。并且，发送给某一节点的消息将会广播到整个cluster中所有的nodes。
 
+#### Redis Keyspace notifaction
+Keyspace notification允许客户端针对`Pub/Sub channels`进行订阅，从而接收`events affecting the Redis data set`。
+
+接收事件的示例如下：
+- all the commands affecting a given key
+- all the keys receiving an LPUSH operation
+- all the keys expiring in the database 0
+
+> 在使用`Pub/Sub`时，如果client失去了对redis的连接，并重新连接后，在client丢失连接的时间范围内，所有的事件都会丢失
+
+##### Type of events
+`keyspace notifications are implemented by sending two distinct types of events for every operation affecting the redis data space.`
+
+例如对`database 0`中的`meykey`执行的`DEL`操作，将会传递两个消息，等价于如下publish语句：
+```redis-cli
+PUBLISH __keyspace@0__:mykey del
+PUBLISH __keyevent@0__:del mykey
+```
+其中`__keyspace@0__:mykey`代表所有针对`mykey`的事件；`__keyevent@0__:del`只针对`mykey`的`del` operation。
+
+第一种类型的事件，其channel名称包含keyspace prefix，被称为`key-space notification`。
+
+第二种类型的事件，channel名称中包含keyevent prfix, 被称为`Key-event notification`。
+
+在前面示例中，对mykey的del event生成了两条消息：
+- `The key-space channel receives as message the name of the event`
+- `The key-event channel receives as message the name of the key`
+
+##### Configuration
+默认情况下，keyspace event notification处于disabled状态，因为其并非必须特性且会消耗部分CPU资源。可以通过`redis.conf`中的`notify-keyspace-events`配置或通过`CONFIG SET`来对其进行启用。
+
+将该参数设置为空字符串会禁用notifications，为了启用该特性，需要将指定为非空，字符串由如下字符组成：
+- 字符串中至少应该存在`K或E`，否则事件将不会被传递
+- 如果要对lists启用`key-space events`，那么可以配置为`Kl`
+- 可以将参数设置为`KEA`为大部分data types启用events
+
+字符的含义如下表所示：
+```
+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
+t     Stream commands
+d     Module key type events
+x     Expired events (events generated every time a key expires)
+e     Evicted events (events generated when a key is evicted for maxmemory)
+m     Key miss events generated when a key that doesn't exist is accessed (Note: not included in the 'A' class)
+n     New key events generated whenever a new key is created (Note: not included in the 'A' class)
+o     Overwritten events generated every time a key is overwritten (Note: not included in the 'A' class)
+c     Type-changed events generated every time a key's type changes (Note: not included in the 'A' class)
+A     Alias for "g$lshztdxe", so that the "AKE" string means all the events except "m", "n", "o" and "c".
+```
+
+##### Timing of expired events
+如果key存在`ttl`，那么redis将会在如下两种时机下让key过期：
+- 当key被command访问，并发现key过期
+- 当backgroud system查找到expired keys
+
+当key通过上述两种机制之一被发现过期时，会生成expired events。redis`并不保证`在key的ttl到期后`立马`会产生expired event。
+
+如果该key未持续被command访问，且同时存在多个key关联ttl，那么`the key time to live drops to zero`的时间和`the exipred event`生成的时间可能存在较大的延迟。
+
+`Expired events`将会在redis server删除key时被生成，并非`ttl`减为0的时间。
+
+##### Events in cluster
+redis cluster中的每个node都会生成其自己的keyspace相关事件。但是，和集群环境下`Pub/Sub`机制不同的是，`events notifications`并不会被广播到cluster中的其他节点。keyspace events是`node-specific`的，从cluster接收到所有的keyspace events时，clients需要为每个node都进行订阅。