IoT Platform:MQTT protocol

Supported feature

References

Session expiry

• Configure the Clean Start feature and the session expiry interval when a device is connected.

  MqttConnectionOptions options = new MqttConnectionOptions();
  options.setCleanStart(true);
  options.setSessionExpiryInterval(60L);// Unit: seconds. 
  
  MqttClient mqttClient = new MqttClient(host, clientId, new MemoryPersistence());
  mqttClient.connect(options);

• Configure the Clean Start feature and the session expiry interval when a device is disconnected.

  MqttProperties mqttProperties = new MqttProperties();
  mqttProperties.setSessionExpiryInterval(60L);// Unit: seconds. 
  
  MqttAsyncClient mqttAsyncClient = new MqttAsyncClient(host, clientId, new MemoryPersistence());
  mqttAsyncClient.disconnect(30000, null, null, MqttReturnCode.RETURN_CODE_SUCCESS, mqttProperties);

Message expiry

Specify a message expiration interval when a device sends messages.

IntervalString content = "Hello World";
byte[] payload = content.getBytes();

// Create a message. 
MqttMessage message = new MqttMessage(payload);
// Specify a QoS level for messages. 
message.setQos(1);

MqttProperties mqttProperties = new MqttProperties();

// Specify a message expiration interval. 
mqttProperties.setMessageExpiryInterval(600L);

message.setProperties(mqttProperties);

// Publish messages to a topic. 
MqttClient mqttClient = new MqttClient(host, clientId, new MemoryPersistence());
mqttClient.publish(topic, message);

Subscription options

Supported subscription parameters:

• QoS: The QoS level of MQTT messages. Valid values: 0 (QoS 0 messages) and 1 (QoS 1 messages).

• No Local: specifies whether a client receives the messages it published.

  If you use MQTT 3.1.1 and a client subscribes to a topic to which the client publishes messages, the client receives the messages. If you use MQTT 5.0, you can set this parameter to true when a client subscribes to a topic. In this case, the client does not receive the messages that it published to the topic.

  Valid values:

  • true: The client does not receive the messages that it published.

  • false: The client receives the messages that it published.

• Retain As Publish: specifies whether to retain the RETAIN identifier of a message when the server forwards the message to a client.

  Valid values:

  • true: If the RETAIN identifier exists in a message, the identifier is retained. If the RETAIN identifier does not exist in a message, this value is invalid.

  • false: The RETAIN identifier is not retained regardless of whether the identifier exists in a message.

  Important

  The value of the Retain As Publish parameter does not affect the RETAIN identifier of retained messages.

• Retain Handling: specifies whether the server sends a retained message to a client when a subscription is established between the server and the client.

  Valid values:

  • 0: If a subscription is established between the server and a client, the server sends a retained message.

  • 1: If a new subscription is established between the server and a client, the server sends a retained message.

  • 2: The server does not send a retained message to a client regardless of whether a subscription is established between the server and the client.

MqttSubscription mqttSubscription = new MqttSubscription("aaa/bbb");

// Configure the QoS parameter. 
mqttSubscription.setQos(1);

// Configure the No Local parameter. 
mqttSubscription.setNoLocal(true);

// Configure the Retain As Published parameter. 
mqttSubscription.setRetainAsPublished(true);

// Configure the Retain Handling parameter. 
mqttSubscription.setRetainHandling(1);

MqttClient mqttClient = new MqttClient(host, clientId, new MemoryPersistence());
mqttClient.subscribe(new MqttSubscription[]{mqttSubscription});

Retained messages

// Create a retained message. 
String content = "Hello World";
byte[] payload = content.getBytes();
MqttMessage message = new MqttMessage(payload);
// Specify the message as the retained message. 
message.setRetained(true);

// Publish messages to a topic. 
MqttClient mqttClient = new MqttClient(host, clientId, new MemoryPersistence());
mqttClient.publish(topic, message);

Will messages

// Create a will message. 
String content = "Will Message";
byte[] payload = content.getBytes();
MqttMessage message = new MqttMessage(payload);

MqttConnectionOptions options = new MqttConnectionOptions();
options.setUserName(USERNAME);
options.setPassword(PASSWORD.getBytes());

// Specify the message as a will message. 
options.setWill(topic, message);

// Specify the latency for the will message. 
MqttProperties willMessageProperties = new MqttProperties();
willMessageProperties.setWillDelayInterval(60L);
options.setWillMessageProperties(willMessageProperties);

// Establish a connection. 
MqttClient mqttClient = new MqttClient(host, clientId, new MemoryPersistence());
mqttClient.connect(options);

Feedback on connection establishment

MqttConnectionOptions connOpts = new MqttConnectionOptions();
connOpts.setMaximumPacketSize(1024L);

User properties

MqttProperties properties = new MqttProperties();
List<UserProperty> userPropertys = new ArrayList<>();
userPropertys.add(new UserProperty("key1","value1"));
properties.setUserProperties(userPropertys);

After a device is connected to IoT Platform over MQTT 5.0, you can view the submitted UserProperty parameter in IoT Platform logs.

Request-response pattern

For example, the requester is a device and the responder is your business server. After you use the Advanced Message Queuing Protocol (AMQP) subscription or data forwarding feature, you can parse the ResponseTopic and CorrelationData parameters from the property data in the message. Then, you can call the Pub operation to send a response to the device.

MqttProperties properties = new MqttProperties();
properties.setCorrelationData("requestId12345".getBytes());
properties.setResponseTopic("/" + productKey + "/" + deviceName + "/user/get");

Enhancements in error codes

For more information, see Troubleshooting.

Topic aliases

N/A

Shared subscription

A shared subscription topic is in the $share/${ShareName}/${filter} format.

• $share: a fixed item. The name of each shared subscription topic starts with $share.

• ${ShareName}: a string that can contain only letters, digits, and underscores (_).

  ${ShareName} specifies a group of sessions that share the subscription. Each message that matches the shared subscription is sent to only one session.

• ${filter}: the topic filter for non-shared subscriptions. The value can contain letters, digits, and underscores (_).

MqttConnectionOptions options = new MqttConnectionOptions();
options.setUserName(username);
options.setPassword(password);

MqttClient mqttClient = new MqttClient(host, clientId, new MemoryPersistence());
mqttClient.connect(options);

mqttClient.subscribe("$share/testGroup/user/post", 1);