Publish and subscribe (pub/sub) is a message communication pattern whose main purpose is to decouple the coupling between message publishers and message subscribers, which is similar to the observer pattern in the design pattern. Pub/sub not only solves the direct code-level coupling between publishers and subscribers, but also solves the coupling between the two in physical deployment. As a pub/sub server , redis functions as message routing between subscribers and publishers. Subscribers can subscribe to the redis server for message types they are interested in through the subscribe and psubscribe commands . Redis calls the message type a channel . When a publisher sends a specific type of message to the redis server through the publish command . All clients subscribed to this message type will receive this message. The message passing here is many-to-many. A client can subscribe to multiple channels, and can also send messages to multiple channels .
Do an experiment below. Two different clients are used here, one is the redis -cli that comes with redis, and the other is written with the java version client jedis . The java code is as follows:
package com.jd.redis.client;
import redis.clients.jedis.Jedis; import redis.clients.jedis.JedisPubSub;
public class TestPubSub extends JedisPubSub {
@Override publicvoid onMessage(String channel, String message) { System.out.println("onMessage: channel["+channel+"], message["+message+"]"); }
@Override publicvoid onPMessage(String pattern, String channel, String message) { System.out.println("onPMessage: channel["+channel+"], message["+message+"]"); }
@Override publicvoid onSubscribe(String channel,int subscribedChannels) { System.out.println("onSubscribe: channel["+channel+"],"+ "subscribedChannels["+subscribedChannels+"]"); }
@Override publicvoid onUnsubscribe(String channel,int subscribedChannels) { System.out.println("onUnsubscribe: channel["+channel+"], "+ "subscribedChannels["+subscribedChannels+"]"); }
@Override publicvoid onPUnsubscribe(String pattern,int subscribedChannels) { System.out.println("onPUnsubscribe: pattern["+pattern+"],"+ "subscribedChannels["+subscribedChannels+"]"); }
@Override publicvoid onPSubscribe(String pattern,int subscribedChannels) { System.out.println("onPSubscribe: pattern["+pattern+"], "+ "subscribedChannels["+subscribedChannels+"]"); }
publicstaticvoid main(String[] args) { Jedis jr = null; try { jr = new Jedis("192.168.157.128", 6379, 0);//redis服务地址和端口号 TestPubSub sp = new TestPubSub(); sp.proceed(jr.getClient(),"news.share", "news.blog"); //sp.proceedWithPatterns(jr.getClient(), "news.*"); } catch (Exception e) { e.printStackTrace(); } finally{ if(jr!=null){ jr.disconnect(); } } } } |
代码就是用TestPubSub对象来订阅，对象中的那此onXXX方法监听到相应事件
1 首先运行此java程序；
onSubscribe: channel[news.share], subscribedChannels[1] onSubscribe: channel[news.blog], subscribedChannels[2] |
2 启动redis-cli
redis 127.0.0.1:6379> psubscribe news.* Reading messages... (press Ctrl-C to quit) 1) "psubscribe" 2) "news.*" 3) (integer) 1 |
3 再启动一个redis-cli用来发布两条消息：
redis 127.0.0.1:6379> publish news.share "share a link http://www.google.com" (integer) 2 redis 127.0.0.1:6379> publish news.blog "I post a blog" (integer) 2 |
4.查看两个订阅client的输出
此时java client打印如下内容：
onMessage: channel[news.share], message[share a link http://www.google.com] onMessage: channel[news.blog], message[I post a blog] |
另一个redis-cli输出如下：
1) "pmessage" 2) "news.*" 3) "news.share" 4) "share a link http://www.google.com" 1) "pmessage" 2) "news.*" 3) "news.blog" 4) "I post a blog" |
redis client使用psubscribe订阅了一个使用通配符的通道(*表示任意字符串)，此订阅会收到所有与news.*匹配的通道消息。redis- cli打印到控制台的订阅成功消息表示使用psubscribe命令订阅news.*成功后，连接订阅通道总数为1。
当我们在一个client使用publish向news.share和news.blog通道发出两个消息后。redis返回的(integer) 2表示有两个连接收到了此消息。
看完一个小例子后应该对pub/sub功能有了一个感性的认识。需要注意的是当一个连接通过subscribe或者psubscribe订阅通道后就进入订阅模式。在这种模式除了再订阅额外的通道或者用unsubscribe或者punsubscribe命令退出订阅模式，就不能再发送其他命令。另外使用 psubscribe命令订阅多个通配符通道，如果一个消息匹配上了多个通道模式的话，会多次收到同一个消息。
redis的pub/sub还是有点太单薄（实现才用150行代码）。在安全，认证，可靠性这方便都没有太多支持