Creating an MQTT Trigger

The topic to which the MQTT subscribes does not need to exist prior to creating the trigger. The trigger client creates the topic when it connects to the MQTT server.

MQTT trigger supports the use of the single level wildcard “+” and the multi-level wildcard “#” in topics.

The MQTT connection alias determines whether the subscriptions created by the trigger are persisted across sessions. Make sure to select an MQTT connection alias that supports your requirements for the MQTT trigger.

The QoS that you select for an MQTT trigger applies only to the subscription. That is, the QoS applies to message delivery between the MQTT server and the MQTT trigger. It does not apply to message processing by the trigger or trigger service.

In conjunction with the MQTT connection alias, the QoS for an MQTT trigger determines whether an MQTT trigger can obtain messages published to a topic when the trigger was disconnected. If the MQTT connection alias does not use clean sessions for subscriptions and the QoS for the MQTT trigger is set to At least once (1) or Exactly once (2), the MQTT trigger receives missed messages upon reconnection to the MQTT server.

The QoS for publishing messages to the MQTT server is separate from the QoS for receiving messages. As a result, it is possible for a message to be published to the MQTT server with a higher QoS than which the message is received from the MQTT server. However, it is not possible for a message to be received from the MQTT server at a higher QoS level than which it was published to the MQTT server. When sending the message to the MQTT trigger, the MQTT server uses the lower QoS regardless of whether it was set by the message publisher or by the MQTT trigger.

Make sure to establish package dependencies between the packages that contain the MQTT trigger, the MQTT trigger services, and the package that contains the MQTT connection alias definition.

3. In the Element name field, type a name for the MQTT trigger using any combination of letters, numbers, and/or the underscore character. For information about restricted characters, see About Element Names.

6. In the Select a MQTT connection alias for triggerName dialog box, select the MQTT connection alias that you want this t rigger to use to connect to the MQTT server, establish subscriptions, and receive messages. Click OK.

If a MQTT connection alas as has not yet been configured on Integration Server, Designer displays a message stating that the MQTT subsystem has not been configured. For information about creating an MQTT connection alias, see the webMethods Integration Server Administrator’s Guide.

7. In the Topic field, specify the topic to which the MQTT trigger subscribes. You can specify a single topic string only.

You can subscribe to a topic pattern by including a wildcard in the topic string. MQTT triggers support the single-level wildcard “+” and the multi-level wildcard “#”.

8. In the QoS list, select the quality of service for message delivery between the MQTT server and the MQTT trigger.

QoS Level	Description
At most once (0)	The MQTT server delivers the message to the MQTT trigger once or not at all. This is sometimes referred to as “fire and forget”.
At least once (1)	The MQTT server delivers the message to the MQTT trigger one or more times. This is the default.
Exactly once (2)	The MQTT server delivers the message to the MQTT trigger once and only once.

9. In the Service field, type the fully qualified name of the service to use as the trigger service or click

to navigate to and select the service.

10. In the Decoding Type list, select how Integration Server decodes the payload of the MQTT message when placing it in the input pipeline for the trigger service.

Decoding Type	Description
Payload to byte[]	Integration Server passes the MQTT message payload as a byte[] to the trigger service. This is the default.
Payload to String	Integration Server decodes the MQTT message payload, which is a byte[], into a String before invoking the trigger service.

If the MQTT trigger is saved in an enabled state and the MQTT connection alias is enabled and running, the MQTT trigger starts.

If the MQTT trigger is saved in an enabled state but the MQTT connection alias is disabled or enabled but not running, the MQTT trigger is not connected and does not start.

If the MQTT trigger is missing required information, such as a trigger service or an MQTTT connection alias, Designer prompts you to save the trigger in a disabled state.

The connection client ID of the MQTT trigger is the connection client ID value of the MQTT connection alias plus the trigger name.

When you save an enabled MQTT trigger the MQTT server creates a topic if one does not yet exist. The MQTT server does not create topics if the Topic string uses wildcards.

If the MQTT trigger was not connected to the MQTT server when you saved the trigger, the MQTT server creates the topic the first time the trigger connects.

If the MQTT server is Universal Messaging:

Universal Messaging creates a durable for the MQTT trigger if the trigger has persisted subscriptions. An MQTT trigger has persisted subscriptions if the trigger has a QoS of 1 or 2 and uses an MQTT connection alias that does not use clean sessions (the Use Clean Sessions for Subscriptions check box is not selected).