Confirmed Delivery

This tutorial builds on the basic concepts introduced in Persistence with Queues tutorial and will show you how to properly process publisher acknowledgements. When you receive an acknowledgement for a QoS level 1 message, you have confirmed your message have been properly accepted by the Solace message router and therefore can be guaranteed of no message loss.

Assumptions

This tutorial assumes the following:

• You are familiar with Solace core concepts.

• You have access to Solace messaging with the following configuration:

  • Connectivity information for a Solace message-VPN configured for guaranteed messaging support
  • Enabled client username and password
  • Client-profile enabled with guaranteed messaging permissions.
  • Enabled MQTT service

One simple way to get access to Solace messaging quickly is to create a messaging service in Solace Cloud as outlined here. You can find other ways to get access to Solace messaging below.

Goals

The goal of this tutorial is to understand the following:

1. How to properly handle QoS 1 message acknowledgments on message send

    compile("org.eclipse.paho:org.eclipse.paho.client.mqttv3:1.1.0")

<dependency>
  <groupId>org.eclipse.paho</groupId>
  <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
  <version>1.1.0</version>
</dependency>

Connecting a session to Solace messaging

This tutorial builds on the QoS1Producer introduced in Persistence with MQTT. So connect the MqttClient as outlined in the Persistence with MQTT tutorial.

Tracking the delivery of QoS 1 messages

Similar to the QoS1Producer we will publish a QoS 1 message and then wait for an acknowledgement from the broker confirming the message was received and stored.

In MQTT there are two approaches to track the delivery of messages:

1. Setting an MqttCallback on the client. Once a message has been delivered and stored by Solace messaging, the deliveryComplete(IMqttDeliveryToken) method will be called with delivery token being passed as a parameter.
2. Using an asynchronous MQTT client and the MqttAsyncClient.publish method, which returns a IMqttToken when the publish call returns. The producer can then use the IMqttToken.waitForCompletion method to block until the delivery has been completed and the broker has acknowledge the message.

For the purpose of this tutorial we choose the first approach and set an MqttCallback on the client. The MqttCallback.deliveryComplete method is implemented here to check if the QoS 1 message has been received and stored by Solace messaging.

// Callback - Anonymous inner-class for receiving msg delivery complete token
mqttClient.setCallback(new MqttCallback() {
    public void messageArrived(String topic, MqttMessage message) throws Exception {
    }

    public void connectionLost(Throwable cause) {
        System.out.println("Connection to Solace broker lost!" + cause.getMessage());
        latch.countDown();
    }

    public void deliveryComplete(IMqttDeliveryToken token) {
        System.out.println("\nMessage was successfully delivered to Solace\n");
        latch.countDown(); // unblock main thread
    }
});

The producer code uses a countdown latch to block the main thread until the MqttCallback.deliveryComplete method has been called.

try {
    latch.await(); // block here until message delivery is completed, and latch will flip
} catch (InterruptedException e) {
    System.out.println("I was awoken while waiting");
}

Summarizing

The full source code for this example is available on GitHub. If you combine the example source code shown above results in the following source:

• ConfirmedDeliveryProducer.java

Getting the Source

Clone the GitHub repository containing the Solace samples.

git clone https://github.com/SolaceSamples/solace-samples-mqtt
cd solace-samples-mqtt

Building

The project uses Gradle. To build, execute the following command.

./gradlew build

This builds all of the Java Samples with OS specific launch scripts. The files are staged in the build/staged directory.

Running the Sample

Run the example from the command line as follows.

$ ./build/staged/bin/confirmedDeliveryProducer <host:port> <client-username> <client-password>
ConfirmedDeliveryProducer initializing...
Connecting to Solace messaging at <host:port>
Connected
Publishing message: Hello world from MQTT!

Message was successfully delivered to Solace

Exiting

When the text “Message was successfully delivered to Solace” is printed on screen, you have confirmed that the published QoS 1 message has been delivered successfully to the broker.