Developing MQTT applications

You can find more information about developing MQTT applications in the Mobile Messaging & M2M articles in the IBM® Messaging Community at developerWorks®. Information is also available the following sections of the IBM MQ 7.5 documentation:

• Introduction to IBM MQ Telemetry
• Mobile Messaging & M2M

Consider the following points when you develop MQTT applications:

• The keywords ${UserID}, ${GroupID}, ${ClientID}, and ${CommonName} are used in Eclipse Amlen as substitution variables. Using these keywords in your application can lead to policy validation failures. Therefore, understand how these keywords are used before you use them in your MQTT application. For more information about these substitution variables, see Variable substitution in messaging policies, and Variable substitution in connection policies.
• Each time that you restart an MQTT application that uses memory-based persistence, and uses cleanSession=False, you must clean up old subscription data on Eclipse Amlen for that client application. You can remove old subscription data in one of the following two ways:
  • You can administratively remove any old subscriptions and any previously existing clients that used the same client ID from Eclipse Amlen by completing the following steps:
    1. From the Amlen WebUI, select Monitoring > MQTT Clients. The disconnected MQTT clients are listed in the MQTT client table.
    2. From the MQTT client table, select the client that you want to delete.
    3. Click Actions, and select Delete Client to delete the client, and any subscriptions that are associated with the client.
  • Alternatively, you can create a clean environment by completing the following steps:
    1. Temporarily connect a client with the same client ID and the cleanSession=True flag set.
    2. Disconnect.
    3. Reconnect the client with the cleanSession=False flag set.
• When you develop MQTT QoS 2 applications that interact with JMS applications, you must configure the JMS applications so that QoS 2 is maintained.
  • If your MQTT QoS 2 applications interact with stand-alone JMS applications, complete the following steps to maintain QoS 2 across applications:
    • Ensure that JMS client sessions that receive messages from and send messages to MQTT QoS 2 applications are transacted.
    • Ensure that all messages that are sent from JMS client applications to MQTT QoS 2 applications use the JMSDeliveryMode.PERSISTENT delivery mode.
  • If your MQTT QoS 2 applications interact with Java™ EE applications that use the Eclipse Amlen resource adapter, complete the following steps to maintain QoS 2 across applications:
    • Ensure that your Java EE applications are configured to use global transactions.
    • Ensure that your Java EE applications use the default transaction type of container managed transactions. If you do not want to configure your container managed transaction to use the default transaction attribute of Required, then you must use RequiresNew, Mandatory, or Supports.
    • If your Java EE applications send messages to MQTT QoS 2 applications, ensure that your Java EE applications use the JMSDeliveryMode.PERSISTENT delivery mode.
• When you develop MQTT applications that interact with the Eclipse Amlen resource adapter, consider the following information if you want to maintain message order:
  • For information about configuring the Eclipse Amlen resource adapter to support preservation of message order, see Configuring the Eclipse Amlen resource adapter to preserve message order
  • If you are using MQTT QoS 2, see Message order limitations with the Eclipse Amlen resource adapter for important information about message order limitations. If message order is critical to your application, then you might need to embed sequence numbers in your messages. Sequence numbers can enable your MQTT subscriber applications to manage cases where message order is disrupted.

MQTT has built-in support for high availability (HA). To develop HA applications by using MQTT, you must set up the following configurations:

• Set up a list of server URIs for the Eclipse Amlen servers that make up your HA pair.
• Set up your application to try connecting again after a failure. A call to connect either initially or after a connection lost event is received, automatically works through the list of configured server URIs. This process continues until a successful connection is made or the list is exhausted and a failure is returned. If a failure is returned, try to connect again because during failover you cannot connect to either server. 
• Set cleanSession=0 for HA publisher and subscriber applications. Otherwise, you might lose any messages that were being processed at the time of failover when the new Eclipse Amlen server connects.
• Set a connectTimeout that allows your application to be responsive.
• Set a keepAliveTimeout.

To configure connections in C, you can use the following code snippet:

char* serverURIs[2] = {"tcp://myhaserver1:1883", "tcp://myhaserver2:1883"};
MQTTClient_connectOptions opts = MQTTClient_connectOptions_initializer;
opts.serverURIcount = 2;
opts.serverURIs = serverURIs;
opts.keepAliveInterval = 30;
opts.connectTimeout = 60;
MQTTClient_connect(client, &opts);

To configure connections in Java, you can use the following code snippet:

String[] urls = new String[]{"tcp://myhaserver1:1883", "tcp://myhaserver2:1883"};
MqttConnectOptions options = new MqttConnectOptions();
options.setServerURIs(urls);
options.setConnectionTimeout(value)
options.setKeepAliveInterval(value)
client.connect(options);

To set up this example in JavaScript, you can use the following code snippet:

var defaultServer = "myserver";
var defaultPort = 1883;
var arrHosts = ['myhaserver1', 'myhaserver2'];
var arrPorts = [1883,1883];
		
var client = new Messaging.Client(defaultServer, defaultPort, clientId);
client.connect({
	onSuccess : onConnectSuccess,
	onFailure : onConnectFailure,
	hosts : arrHosts,
	ports : arrPorts
  timeout: <value>
  keepAliveInterval: <value>
});