doc: 阅读redis cluster replcia文档

中间件/redis/redis.md

@@ -225,6 +225,7 @@
       - [Client connections and redirection handling](#client-connections-and-redirection-handling)
         - [CLUSTER SLOTS](#cluster-slots)
       - [Multi-keys operations](#multi-keys-operations)
+      - [Scaling reads using replica nodes](#scaling-reads-using-replica-nodes)
 
 
 # redis
@@ -3586,3 +3587,14 @@ MSET {user:1000}.name Angela {user:1000}.surname White
 
 当指定hash slots的迁移完成后，关于该hash slot所有的`multi-key operations`都允许被执行。
 
+#### Scaling reads using replica nodes
+通常来讲，replica nodes在接收到命令时，都会将其重定向给负责该hash slot的master node。但是，对于readonly命令，client可以使用replicas来对读操作进行扩展。
+
+`READONLY`会告知redis cluster replica node如下信息：当前client`接收读取可能过时的数据`，并且`不会发送写请求`。
+
+当connection处于readonly模式时，cluster只会在`the operation involves keys not served by the replica's master node`时，才会向client发送重定向。造成该问题的原因可能如下：
+- the client sent a command about hash slots never served by the master of this replica
+- the cluster was reconfigured and the replica is no longer able to serve commands for a given hash slot
+
+当发生该场景时，replica会向client发送重定向，而client则应该更新其`hash slot map`。
+