17_ Redis_ Redis publish subscription

Redis Publish subscribe (pub/sub) It's a kind of Message communication mode ∶ sender (pub) Send a message , subscriber (sub) receive messages .

Redis Clients can subscribe to any number of channels . for example ： A user can pay attention to as many as he wants up Lord

subscribe / Release message map ∶

first ： Message sender
the second ： channel
Third ： Message subscribers

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 :

command ： These commands are widely used to build instant messaging applications , Such as Internet chat room (chatroom) And real-time broadcasting 、 Real time reminder, etc .

test

Subscribers ：

127.0.0.1:6379> SUBSCRIBE xsy666 // Subscribe to a channel ：xsy666
Reading messages… (press Ctrl-C to quit)
1 ) “subscribe”
2 ) “xsy666”
3 ) (integer) 1
// Waiting to read the push message
1 ) “message” // news
2 ) “xsy666” // Which channel
3 ) “hello,xsy” // The details of the message
1 ) “message”
2 ) “xsy666”
3 ) “hello,zhangsan”
1 ) “message”
2 ) “xsy666”
3 ) “byebye”

The sender ：

127.0.0.1:6379> PUBLISH xsy666 hello,xsy // Publishers post messages to channels !
(integer) 1
127.0.0.1:6379> PUBLISH xsy666 hello,zhangsan
(integer) 1
127.0.0.1:6379> PUBLISH xsy666 byebye
(integer) 1

principle

Redis It's using C Realized , Through analysis Redis Source code pubsub.c file , Understand the underlying implementation of publish and subscribe mechanisms , In order to deepen the understanding of Redis The understanding of the .

Redis adopt PUBLISH、SUBSCRIBE and PSUBSCRIBE And so on to realize the function of publish and subscribe .

adopt SUBSCRIBE After subscribing to a channel ,redis-server It maintains a dictionary , The key of the dictionary is one by one channel, The value of a dictionary is a linked list , The linked list holds all the subscriptions channel The client of .SUBSCRIBE The key to command , Is to add the client to the given channel In the subscription list of .

adopt PUBLISH Command to send messages to subscribers ,redis-server The given channel is used as the key , In what it maintains channel Look up the list of all clients who subscribe to this channel in the dictionary , Traverse the list , Publish the message to all subscribers .

Pub/Sub Literally, it's publishing ( Publish ) With subscription ( Subscribe ), stay Redis in , You can set to a certain key Value for message publishing and message subscription , When one key After the news is released on the value , All clients that subscribe to it will receive the corresponding message . The most obvious use of this feature is as a real-time messaging system , For example, ordinary instant chat , Group chat and other functions .

Use scenarios ：
1. Real time message system
2. Live chat
3. subscribe , Focus on