ZeroMQ之模式二 发布者订阅者（publish-subscribe)

一、前言

zeromq pub-sub模型，发布者将一系列数据发送到订阅者。例如Rss 和微博关注

二、code

               #-------------#               |  Publisher  |               +-------------+               |     PUB     |               '-------------'                    bind                      |                      |                   updates                      |      .---------------+---------------.      |               |               |   updates         updates         updates      |               |               |      |               |               |      v               v               v   connect         connect         connect.------------.  .------------.  .------------.|    SUB     |  |    SUB     |  |    SUB     |+------------+  +------------+  +------------+| Subscriber |  | Subscriber |  | Subscriber |#------------#  #------------#  #------------#

例子如下：发布者发布一系列有地点，温度和相对湿度的气候数据，订阅者获取感兴趣的数据例如1001站点

publish

package com.bitch.mq.pub_sub;import org.zeromq.ZMQ;import java.util.Random;////  Weather update server in Java//  Binds PUB socket to tcp://*:5556//  Publishes random weather updates//public class wuserver {    public static void main (String[] args) throws Exception {        //  Prepare our context and publisher        ZMQ.Context context = ZMQ.context(1);        ZMQ.Socket publisher = context.socket(ZMQ.PUB);        publisher.bind("tcp://*:5556");        //publisher.bind("ipc://weather");        //  Initialize random number generator        Random srandom = new Random(System.currentTimeMillis());        while (!Thread.currentThread().isInterrupted ()) {            //  Get values that will fool the boss            int zipcode, temperature, relhumidity;            zipcode = 10000 + srandom.nextInt(10000) ;            temperature = srandom.nextInt(215) - 80 + 1;            relhumidity = srandom.nextInt(50) + 10 + 1;            //  Send message to all subscribers            String update = String.format("%05d %d %d", zipcode, temperature, relhumidity);//            System.out.println("send data****************"+update);            publisher.send(update, 0);        }        publisher.close ();        context.term ();    }}

subscribe

package com.bitch.mq.pub_sub;import org.zeromq.ZMQ;import java.util.StringTokenizer;////  Weather update client in Java//  Connects SUB socket to tcp://localhost:5556//  Collects weather updates and finds avg temp in zipcode//public class wuclient {    public static void main (String[] args) {        ZMQ.Context context = ZMQ.context(1);        //  Socket to talk to server        System.out.println("Collecting updates from weather server");        ZMQ.Socket subscriber = context.socket(ZMQ.SUB);        subscriber.connect("tcp://localhost:5556");        //  Subscribe to zipcode, default is NYC, 10001        String filter = (args.length > 0) ? args[0] : "10001 ";        subscriber.subscribe(filter.getBytes());        //  Process 100 updates        int update_nbr;        long total_temp = 0;        for (update_nbr = 0; update_nbr < 100; update_nbr++) {            //  Use trim to remove the tailing '0' character            String string = subscriber.recvStr(0).trim();            System.out.println(string);            StringTokenizer sscanf = new StringTokenizer(string, " ");            int zipcode = Integer.valueOf(sscanf.nextToken());            int temperature = Integer.valueOf(sscanf.nextToken());            int relhumidity = Integer.valueOf(sscanf.nextToken());            total_temp += temperature;        }        System.out.println("Average temperature for zipcode '"                + filter + "' was " + (int) (total_temp / update_nbr));                subscriber.close();        context.term();    }}

三、模式存在问题

1、一个订阅方实际上可以链接到不止一个发布方，每次使用一条连接命令。数据会交叉到达，
因此没有哪个单一的发布方能够淹没其它发布方。
2、 如果一个发布方没有与它连接的订阅方，它会简单的扔掉所有消息。
3、  如果你使用TCP，并且订阅方很慢，消息将会在发布方排队。我们将会看到随后如何使用“高水准”的方法来避免这种情况的出现。