Redis - how to understand publishing and subscribing

brief introduction

Redis Publish subscription （publish/subscribe） It's a message communication mode , sender （publish） Send a message , subscriber （subscribe） Receiving and eliminating ,Redis Clients can subscribe to any number of channels .
subscribe / Release message map ：

The picture below shows the channel channel1 , And three clients that subscribe to this channel —— client2 、 client5 and client1 The relationship between , When new news passes PUBLISH Command to channel channel1 when , This message will be sent to the three clients who subscribe to it ：

Subscribe command

These are commands used to realize data communication , The real scene can be an Internet chat room , Radio, etc ：

Subscription examples

Subscribers ：

The sender ：

Subscription principle

Redis yes C Language writing , When publishing and subscribing messages ,Redis Encapsulate it in a suffix named dot c In the file of ,pubsub.c
Every Redis The server processes all maintain a... That represents the state of the server redis.h/redisServer structure , Structural pubsub_channels Property is a dictionary , This dictionary is used to save subscription channel information , among , The key of the dictionary is the channel being subscribed , The value of a dictionary is a linked list , All clients subscribing to this channel are saved in the linked list . Client subscription , It is linked to the end of the linked list of the corresponding channel , Unsubscribe is to remove the client node from the list .

shortcoming :
1、 If a client subscribes to a channel , But I can't read messages fast enough , So the constant backlog of information will make redis The volume of the output buffer becomes larger and larger , This may make redis It's slowing down , Even the immediate collapse .
2、 It has to do with the reliability of data transmission , If the subscriber is disconnected , Then he'll lose all the news that the publisher posted in the short term .

Implementation scenarios for publishing and subscribing

1、 Real time communication message system
2、 WeChat official account （ Click to follow , Send a blog in the background , Subscribers can listen ） etc.
There are also some more complex scenes , Message oriented middleware can be used to ,RabbitMQ,RocketMQ,kafka…