Send a message to an MQTT client

WebSphere® MQ applications can send MQTT v3 clients messages by publishing to subscriptions created by clients, or by sending messages directly. MQTT clients can send messages to one another by publishing to topics subscribed to by other clients.

An MQTT client subscribes to a publication, which it receives from WebSphere MQ

Do the task, Publishing a message to the MQTT client utility from IBM WebSphere MQ Explorer to send a publication from WebSphere MQ to an MQTT client.

The standard way for an MQTT v3 client to receive messages is for it to create a subscription to a topic, or set of topics. In the example code snippet, Figure 1, the MQTT client subscribes using the topic string "MQTT Examples". A WebSphere MQ C application, Figure 2, publishes to the topic using the topic string "MQTT Examples". In the code snippet Figure 3, the MQTT client receives the publication in the callback method, messageArrived.

For further information about how to configure WebSphere MQ to send publications in response to subscriptions from MQTT clients, see Publishing a message in response to an MQTT client subscription .

A WebSphere MQ application sends a message directly to an MQTT client

Do the task, Sending a message to an MQTT client using IBM WebSphere MQ Explorer to send a message directly from WebSphere MQ to an MQTT client.

A message sent in this way to an MQTT client is called an unsolicited message. MQTT v3 clients receive unsolicited messages as publications with a topic name set. The telemetry (MQXR) service sets the topic name to the remote queue name.

You cannot send unsolicited messages to the WebSphere MQ daemon for devices: the daemon might shut down if it receives an unsolicited message. An MQTT v3 client cannot send an unsolicited message to another MQTT v3 client, nor to a WebSphere MQ queue.

For further information about how to configure WebSphere MQ to send messages directly to MQTT clients, see Sending a message to a client directly.

An MQTT client publishes a message

An MQTT v3 client can publish a message that is received by another MQTT v3 client, but it cannot send an unsolicited message. The code snippet, Figure 4 shows how an MQTT v3 client, written in Java, publishes a message.

The typical pattern for sending a message to one specific MQTT v3 client, is for each client to create a subscription to its own ClientIdentifier. Do the task, Publish a message to a specific MQTT v3 client, to publish a message from one MQTT client to another MQTT client using ClientIdentifier as a topic string.

Example code snippets

The code snippet in Figure 1 shows how an MQTT client written in Java creates a subscription. It also needs a callback method, messageArrived to receive publications for the subscription. The code snippet is extracted from the task, Creating a subscriber for MQ Telemetry Transport using Java.

Figure 1. MQTT v3 client subscriber

String    clientId = String.format("%-23.23s", 
                     System.getProperty("user.name") + "_" + 
                     (UUID.randomUUID().toString())).trim()).replace('-', '_');
MqttClient  client = new MqttClient("localhost", clientId);
String topicString = "MQTT Examples";
int            QoS = 1;
client.subscribe(topicString, QoS);

The code snippet in Figure 2 shows how an WebSphere MQ application written in C sends a publication. The code snippet is extracted from the task, Create a publisher to a variable topic

Figure 2. WebSphere MQ publisher

/* Define and set variables to.defaults */
/* Omitted lines declaring variables    */
char * topicName   = ""
char * topicString = "MQTT Examples"
char * publication = "Hello world!";
do {
  MQCONN(qMgrName, &Hconn, &CompCode, &Reason);
  if (CompCode != MQCC_OK) break;
  td.ObjectType = MQOT_TOPIC;    /* Object is a topic             */
  td.Version = MQOD_VERSION_4;   /* Descriptor needs to be V4     */
  strncpy(td.ObjectName, topicName,  MQ_TOPIC_NAME_LENGTH);
  td.ObjectString.VSPtr = topicString;
  td.ObjectString.VSLength = (MQLONG)strlen(topicString);
  MQOPEN(Hconn, &td, MQOO_OUTPUT | MQOO_FAIL_IF_QUIESCING, &Hobj, &CompCode, &Reason);
  if (CompCode != MQCC_OK) break;
  pmo.Options = MQPMO_FAIL_IF_QUIESCING | MQPMO_RETAIN;
  MQPUT(Hconn, Hobj, &md, &pmo, (MQLONG)strlen(publication)+1, publication, &CompCode, &Reason);
  if (CompCode != MQCC_OK) break;
  MQCLOSE(Hconn, &Hobj, MQCO_NONE, &CompCode, &Reason);
  if (CompCode != MQCC_OK) break;
  MQDISC(&Hconn, &CompCode, &Reason);
} while (0);

When the publication arrives, the MQTT client calls the messageArrived method of the MQTT application client MqttCallback class. The code snippet is extracted from the task, Creating a subscriber for MQ Telemetry Transport using Java.

Figure 3. messageArrived method

public class CallBack implements MqttCallback {
  public void messageArrived(MqttTopic topic, MqttMessage message) {
    try {
      System.out.println("Message arrived: \"" + message.toString()
          + "\" on topic \"" + topic.toString() + "\"");
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
// ... Other callback methods 
}

Figure 4 shows an MQTT v3 publishing a message to the subscription created in Figure 1. The code snippet is extracted from the task, Creating your first MQ Telemetry Transport publisher application using Java.

Figure 4. MQTT v3 client publisher

      String          address = "localhost";
      String         clientId = String.format("%-23.23s", 
                                System.getProperty("user.name") + "_" + 
                                (UUID.randomUUID().toString())).trim()).replace('-', '_');
      MqttClient       client = new MqttClient(address, clientId);
      String      topicString = "MQTT Examples";
      MqttTopic         topic = client.getTopic(Example.topicString);
      String     publication  = "Hello world";
      MqttMessage     message = new MqttMessage(publication.getBytes());
      MqttDeliveryToken token = topic.publish(message);