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MQTT的一些关键特点和概念#xff1a;
发布/订阅模型#xff1a;
MQTT采用发布/订阅#xff08;Publ…MQTTMessage Queuing Telemetry Transport是一种轻量级、开放标准的消息传输协议设计用于受限设备和低带宽、不稳定网络的通信。
MQTT的一些关键特点和概念
发布/订阅模型
MQTT采用发布/订阅Publish/Subscribe模型。发布者Publisher发布消息到特定的主题Topic而订阅者Subscriber则订阅感兴趣的主题。这种模型提供了一种松耦合的通信方式。MQTT支持基于TLS/SSL的加密通信提供一定的安全性。
主题Topic
主题是消息的标签或类别。发布者将消息发布到特定的主题而订阅者则通过订阅特定的主题来接收相关消息。
消息队列
MQTT消息以队列的方式传输。当发布者发布消息时消息被发送到相应主题的队列订阅者可以从队列中接收消息。
QoS级别Quality of Service
MQTT支持不同的QoS级别用于确保消息的可靠性和传递顺序。包括 QoS 0最多一次消息发出后不进行确认。 QoS 1至少一次确保消息至少被传递一次。 QoS 2只有一次确保消息仅被传递一次。
保持连接
MQTT客户端与服务器之间保持持久连接。这允许客户端随时接收实时消息而不需要频繁地建立和断开连接。
清理会话Clean Session
客户端可以选择创建一个清理会话或一个持久会话。清理会话意味着服务器不会保留客户端的订阅信息而持久会话则会保存订阅信息。
遗嘱消息Will Message
客户端可以指定遗嘱消息以便在客户端异常断开连接时服务器可以将遗嘱消息发布到预定的主题。
中继器Broker
MQTT系统通常包含一个中继器或代理负责接收、路由和分发消息。这个中继器被称为MQTT Broker。
MQTT服务端
首先安装MQTT库例如 Eclipse Paho MQTT C库。
// mqtt_server.c#include stdio.h
#include stdlib.h
#include string.h
#include unistd.h
#include MQTTClient.h#define SERVER_ADDRESS tcp://localhost:1883
#define CLIENT_ID MQTT_Server
#define TOPIC test_topicvolatile MQTTClient_deliveryToken deliveredtoken;void messageArrived(void *context, char *topicName, int topicLen, MQTTClient_message *message) {printf(Received message on topic: %s\n, topicName);printf(Message: %.*s\n, message-payloadlen, (char *)message-payload);MQTTClient_freeMessage(message);MQTTClient_free(topicName);
}int main() {MQTTClient client;MQTTClient_connectOptions conn_opts MQTTClient_connectOptions_initializer;int rc;// Create MQTT clientMQTTClient_create(client, SERVER_ADDRESS, CLIENT_ID, MQTTCLIENT_PERSISTENCE_NONE, NULL);// Set callback for incoming messagesMQTTClient_setCallbacks(client, NULL, NULL, messageArrived, NULL);// Connect to the brokerconn_opts.keepAliveInterval 20;conn_opts.cleansession 1;if ((rc MQTTClient_connect(client, conn_opts)) ! MQTTCLIENT_SUCCESS) {printf(Failed to connect to the broker, return code %d\n, rc);exit(EXIT_FAILURE);}// Subscribe to a topicif ((rc MQTTClient_subscribe(client, TOPIC, 1)) ! MQTTCLIENT_SUCCESS) {printf(Failed to subscribe to topic, return code %d\n, rc);exit(EXIT_FAILURE);}printf(MQTT server is running...\n);// Keep the program runningwhile (1) {sleep(1);}// Disconnect from the brokerMQTTClient_disconnect(client, 10000);MQTTClient_destroy(client);return 0;
}
MQTT客户端
// mqtt_client.c#include stdio.h
#include stdlib.h
#include string.h
#include MQTTClient.h#define SERVER_ADDRESS tcp://localhost:1883
#define CLIENT_ID MQTT_Client
#define TOPIC test_topic
#define QOS 1int main() {MQTTClient client;MQTTClient_connectOptions conn_opts MQTTClient_connectOptions_initializer;int rc;// Create MQTT clientMQTTClient_create(client, SERVER_ADDRESS, CLIENT_ID, MQTTCLIENT_PERSISTENCE_NONE, NULL);// Connect to the brokerconn_opts.keepAliveInterval 20;conn_opts.cleansession 1;if ((rc MQTTClient_connect(client, conn_opts)) ! MQTTCLIENT_SUCCESS) {printf(Failed to connect to the broker, return code %d\n, rc);exit(EXIT_FAILURE);}// Publish a messageconst char *message Hello, MQTT!;MQTTClient_message pubmsg MQTTClient_message_initializer;pubmsg.payload (void *)message;pubmsg.payloadlen (int)strlen(message);pubmsg.qos QOS;pubmsg.retained 0;MQTTClient_deliveryToken token;if ((rc MQTTClient_publishMessage(client, TOPIC, pubmsg, token)) ! MQTTCLIENT_SUCCESS) {printf(Failed to publish message, return code %d\n, rc);exit(EXIT_FAILURE);}printf(Message published to topic: %s\n, TOPIC);// Disconnect from the brokerMQTTClient_disconnect(client, 10000);MQTTClient_destroy(client);return 0;
}
