Introduction to MQTT Development Basics

Using the roles of a TV station, reporter, and audience makes it easier to understand:

TV Station: In MQTT, it is referred to as the server (broker).

It has the following functions:

① Accepts network connections from clients; // reporters/audience connect to the TV station

② Accepts application information published by clients; // accepts messages published by reporters

③ Handles subscription and unsubscription requests from clients; // processes audience subscription and unsubscription requests

④ Forwards application messages to subscribed clients; // forwards messages reported by reporters to the audience

Both reporters and audiences are clients, reporters can also be audiences, and audiences can also be reporters.

They have the following functions:

① Publish information; // publish, reporters

② Subscribe to messages; // Subscribe, audience

③ Unsubscribe or delete messages;

④ Disconnect from the server

1. Install Software

On the PC, you need to run the MQTT server software emqx and the MQTT client software MQTTX.

For emqx, unzip “emqx-5.3.0-windows-amd64.zip” to a directory without Chinese characters. Assume it is unzipped to “d:\abc”.

For MQTTX, double-click to install “MQTTX-Setup-1.7.2-x64.exe”.

2. Start and Stop MQTT Service

Assuming the emqx installation directory is “D:\abc\emqx-5.3.0-windows-amd64”, execute the following command in the command line to start the MQTT service:

D:\abc\emqx-5.3.0-windows-amd64\bin\emqx start

Then you can access “http://localhost:18083” through the browser to access the EMQX Dashboard management console for device connection and related metrics monitoring management. The default username is “admin” and the password is “public”.

If you want to stop the MQTT service, execute the following command in the command line:

D:\abc\emqx-5.3.0-windows-amd64\bin\emqx stop

3. Using the MQTT Client

After running MQTTX, establish a connection as shown in the figure below:

After establishing a connection, subscribe to a topic (for example, “/topic/humiture”) as shown in the figure below:

Then, you can publish messages to yourself as shown in the figure below:

paho.mqtt.c.tar.bz2 is the library and example code.

mqtt_test is the source code corresponding to this section.

1. Define some parameters

22 #define ADDRESS "tcp://192.168.5.10:1883"23 #define CLIENTID "ExampleClientSub"24 #define TOPIC_SUBSCRIBE "/iot/down"25 #define TOPIC_PUBLISH "/iot/up"26 #define QOS 127 #define TIMEOUT 10000L2829 #define USER_NAME "100ask"30 #define PASSWORD "100ask"

2. Connect to the broker

The code is as follows:

57 int main(int argc, char* argv[])58 {59 MQTTClient client;60 MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;61 int rc;6263 if ((rc = MQTTClient_create(&client, ADDRESS, CLIENTID,64 MQTTCLIENT_PERSISTENCE_NONE, NULL)) != MQTTCLIENT_SUCCESS)65 {66 printf("Failed to create client, return code %d\n", rc);67 rc = EXIT_FAILURE;68 goto exit;69 }7071 if ((rc = MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered)) != MQTTCLIENT_SUCCESS)72 {73 printf("Failed to set callbacks, return code %d\n", rc);74 rc = EXIT_FAILURE;75 goto destroy_exit;76 }7778 conn_opts.keepAliveInterval = 20;79 conn_opts.cleansession = 1;80 conn_opts.username = USER_NAME;81 conn_opts.password = PASSWORD;82 if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)83 {84 printf("Failed to connect, return code %d\n", rc);85 rc = EXIT_FAILURE;86 goto destroy_exit;87 }

3. Subscribe to messages

The code is as follows:

91 if ((rc = MQTTClient_subscribe(client, TOPIC_SUBSCRIBE, QOS)) != MQTTCLIENT_SUCCESS)92 {93 printf("Failed to subscribe, return code %d\n", rc);94 rc = EXIT_FAILURE;95 }

When a subscribed message is received, the “message received callback function” set in line 71 is called, and the code is as follows:

40 int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message)41 {42 printf("Message arrived\n");43 printf(" topic: %s\n", topicName);44 printf(" message: %.*s\n", message->payloadlen, (char*)message->payload);45 MQTTClient_freeMessage(&message);46 MQTTClient_free(topicName);47 return 1;48 }

This function simply prints the message.

4. Publish messages

The code is as follows:

98 int ch;99 int cnt = 0;100 MQTTClient_message pubmsg = MQTTClient_message_initializer;101 char buf[100];102 MQTTClient_deliveryToken token;103104 while (1)105 {106 ch = getchar();107108 if (ch =='Q' || ch == 'q')109 {110 break;111 }112113 sprintf(buf, "publish %d", cnt++);114 pubmsg.payload = buf;115 pubmsg.payloadlen = (int)strlen(buf);116 pubmsg.qos = QOS;117 pubmsg.retained = 0;118119 if ((rc = MQTTClient_publishMessage(client, TOPIC_PUBLISH, &pubmsg, &token)) != MQTTCLIENT_SUCCESS)120 {121 printf("Failed to publish message, return code %d\n", rc);122 continue;123 }124125 rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);126 printf("Message with delivery token %d delivered\n", token); 127 }