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ZeroMQ的订阅发布模式是一种单向的数据发布,当客户端向服务端订阅消息之后,服务端便会将产生的消息源源不断的推送给订阅者,本文的示例代码来源于文献[1]示例代码的修改。
发布-订阅图示
发布者使用PUB套接字将消息发送到队列中,订阅者使用SUB套接字从队列中源源不断的接收消息。新的订阅者可以随时加入,但之前的消息是无法接收到的;已有的订阅者可以随时退出;订阅者还可以添加“过滤器”用来有选择性的接收消息。使用方法简介
首先要创建一个上下文环境,然后使用它创建套接字:void *context = zmq_ctx_new ();
对于服务端来说,使用”ZMQ_PUB”创建socket,并且绑定到一个周知的端口,然后便可以不断的广播消息了:
void *publisher = zmq_socket (context, ZMQ_PUB);int rc = zmq_bind (publisher, "tcp://*:5556");
如果使用TCP连接并且订阅者是慢速的,那么消息将在发布方排队;可以使用高水位标记(High-Water Marks,HWM)来定义缓冲区的大小,在ZeroMQ v2.x版本中HWM默认是无限制的,而在v3.x中默认情况下它是1000。对于PUB套接字,当到达HWM时,将丢弃数据。设置HWM参数:
zmq_setsockopt(publisher, ZMQ_SNDHWM, &nMaxNum, sizeof(nMaxNum));
对于客户端来说,要使用”ZMQ_SUB”创建socket,并且链接(zmq_connect)到待订阅的服务端;此外,要想接收到服务推送的消息,还必须使用zmq_setsockopt和ZMQ_SUBSCRIBE来配置该订阅。zmq_setsockopt的ZMQ_SUBSCRIBE选项可以带有一个”过滤器“,用以选择性的接收来自服务端的消息。该”过滤器”为空,则接收全部的消息;”过滤器”还可以有多个,它们之间是”or”的关系,即接收满足任一条件的消息。当然也可以使用zmq_setsockopt配置选项ZMQ_UNSUBSCRIBE来取消订阅,示例如下:
void *context = zmq_ctx_new ();void *subscriber = zmq_socket (context, ZMQ_SUB);int rc = zmq_connect (subscriber, "tcp://localhost:5556");char filter1[] = "10001 "; char filter2[] = "20002 ";rc = zmq_setsockopt (subscriber, ZMQ_SUBSCRIBE,filter1, strlen (filter1)); //接收消息的前缀为filter1的消息rc = zmq_setsockopt (subscriber, ZMQ_SUBSCRIBE,filter2, strlen (filter2)); //接收消息的前缀为filter2的消息
接收和发送消息:此处使用的方法是zmq_recv()和zmq_send(),相对于zmq_msg_send()和zmq_msg_recv(),它们会自己调用消息发送和接收的初始化方法等。
int zmq_recv (void *s, void *buf, size_t len, int flags);int zmq_send (void *s, const void *buf, size_t len, int flags);
示例代码
//服务端:#include#include #include #include #include #include #include #include #include using namespace std;int main (){ void *context = zmq_ctx_new (); void *publisher = zmq_socket (context, ZMQ_PUB); int rc = zmq_bind (publisher, "tcp://*:5556"); assert (rc == 0); // Initialize random number generator srand(time(0)); while (1) { int zipcode, temperature, relhumidity; zipcode = rand() % 100000; temperature = rand() % 215 - 80; relhumidity = rand() % 50 + 10; ostringstream os; os << setw(5) << setfill('0')<< zipcode <<" " << temperature <<" "<< relhumidity << "\n"; zmq_send(publisher, os.str().c_str(), strlen(os.str().c_str()), 0); } zmq_close (publisher); zmq_ctx_destroy (context); return 0;}
//客户端:#include#include #include #include #include using namespace std;int main (int argc, char *argv []){ // Socket to talk to server printf ("Collecting updates from weather server...\n"); void *context = zmq_ctx_new (); void *subscriber = zmq_socket (context, ZMQ_SUB); int rc = zmq_connect (subscriber, "tcp://localhost:5556"); assert (rc == 0); char filter1[] = "10001 "; char filter2[] = "20002 "; rc = zmq_setsockopt (subscriber, ZMQ_SUBSCRIBE,filter1, strlen (filter1)); assert (rc == 0); rc = zmq_setsockopt (subscriber, ZMQ_SUBSCRIBE,filter2, strlen (filter2)); assert (rc == 0); // Process 100 updates int size; char buffer [256]; for (int update_nbr = 0; update_nbr < 100; update_nbr++) { memset(buffer, 0, 256*sizeof(char)); size = zmq_recv (subscriber, buffer, 255, 0); if (size == -1){ cout<< "receiver error , skip this message"<
NOTE:
在文献[1]中指出:[1].《ZeroMQ云时代极速消息通信库》.电子工业出版社,2015.