Loose coupling between MQTT clients and IBM MQ applications

The flow of publications between MQTT clients and IBM MQ applications is loosely coupled. Publications might originate from either an MQTT client or an IBM MQ application, and in no set order. Publishers and subscribers are loosely coupled. They interact with each other indirectly through publications and subscriptions. We can also send messages directly to an MQTT client from an IBM MQ application.

MQTT clients and IBM MQ applications are loosely coupled in two senses:

1. Publishers and subscribers are loosely coupled by the association of a publication and a subscription with a topic. Publishers and subscribers are not normally aware of the address or identity of the other source of a publication or subscription.
2. MQTT clients publish, subscribe, receive publications, and process delivery acknowledgments on separate threads.

An MQTT client application does not wait until a publication has been delivered. The application passes a message to the MQTT client, and then the application continues on its own thread. A delivery-token is used to synchronize the application with the delivery of a publication; see Delivery tokens.

After passing a message to the MQTT client, the application has the choice of waiting on the delivery-token. Rather than waiting, the client can provide a callback method that is called when the publication is delivered to IBM MQ. It can also ignore the delivery-token.

Depending on the quality of service associated with the message, the delivery-token is returned immediately to the callback method, or possibly after some considerable time. The delivery-token might even be returned after the client has disconnected and reconnected. If the quality of service is fire and forget, the delivery-token is returned immediately. In the other two cases, the delivery token is returned only when the client receives acknowledgment that the publication has been sent to subscribers.

Publications sent to an MQTT client as a result of a client subscription, are delivered to the messageArrived callback method. messageArrived runs on a different thread to the main application.

Send messages directly to an MQTT client

We can send a message to a particular MQTT client in one of two ways.

1. An IBM MQ application can send a message directly to an MQTT client without a subscription; see Send a message to a client directly.
2. An alternative approach is to use your ClientIdentifier naming convention. Make all MQTT subscribers create subscriptions using their unique ClientIdentifier as a topic. Publish to ClientIdentifier. The publication is sent to the client that subscribed to the topic ClientIdentifier. Using this technique we can send a publication to a particular MQTT subscriber.

Parent topic: MQ Telemetry overview