Redis Pub/Sub 多机房部署

由于 Master 上 Publish 的消息会自动同步到所有 Slaves，Redis Pub/Sub 很容易扩展

Redis PubSub scales really easily since the Master/Slave replication automatically publishes to all slaves.

引用自 Redis PUBLISH/SUBSCRIBE limits - Stack Overflow

可以在 Master 上 Publish 消息，然后在 Slave 上 Subscribe 消息，反之不行

With replication in-place the publisher can publish in the master host and the subscribers can subscribe to the slave host.

It is important to mention that this relationship is one-way. Master –> Slave relationship are unidirectional. It is impossible to publish to the slave and subscribe to the master.

…

The conclusion is simple: Two ways pub/sub channels across servers require at least 4 Redis hosts.

引用自 Pub Sub Across Servers Using Redis | Manu Cohen-Yashar's Blog

需要注意的是，Slave 虽然不可写（Readonly），但是照样可以 Pub/Sub，只是 Publish 的消息不会同步到 Master，所以不会被 Master 上的 Subscriber 接收到，但是 Slave 自身的 Subscriber 仍工作正常。

多机房部署的情况下，2 个 Redis 实例可实现双向通讯

redis-pub_sub_multi_datacenter-2.png 但是比较低效，Master 所在机房 Publish 的消息会在机房间传输一次（Sync 一次）， Slave 所在机房 Publish 的消息会在机房间传输两次（Send、Sync 各一次），而且 Slave 机房部署的应用需要跨机房直接连接到 Master 才能进行 Publish。

多机房部署的情况下，4 个 Redis 实例可以实现高效的双向通讯

redis-pub_sub_multi_datacenter-4.png 每一次 Publish 的消息会在机房间传输一次，机房间只有 Master/Slave 同步流量。

Pub/Sub 跨机房部署带来的问题

从 Slave Subscribe 消息后，消息 Publish 一方无法获知 Subscriber 数量

 1: # Publish to master, subscribe from master.
 2: $ redis-cli -h master.local subscribe test &
 3: [1] 20590
 4: $ Reading messages... (press Ctrl-C to quit)
 5: 1) "subscribe"
 6: 2) "test"
 7: 3) (integer) 1
 8: $ 
 9: $ redis-cli -h master.local  publish test "hello from master"
10: 1) "message"
11: 2) "test"
12: 3) "hello from master"
13: (integer) 1
14: $ fg
15: redis-cli subscribe test
16:   C-c C-c
17: $ 
18: # Publish to master, subscribe from slave.
19: $ redis-cli -h slave.local -p 6380 subscribe test &
20: [1] 20592
21: $ Reading messages... (press Ctrl-C to quit)
22: 1) "subscribe"
23: 2) "test"
24: 3) (integer) 1
25: $ 
26: $ redis-cli -h master.local publish test "hello from master"
27: (integer) 0
28: 1) "message"
29: 2) "test"
30: 3) "hello from master"
31: $

  • 行 13

    在 Master 上订阅时，发布方得知的订阅者人数为 1

  • 行 27

    在 Slave 上订阅时，发布方得知的订阅者人数为 0


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