Working with at least once delivery

First, we will use wildcards to subscribe to a topic filter with QoS level 1 and then we will publish one message to a topic name that will match the topic filter with QoS level 1. This way, we will analyze how both publishing and subscription work with QoS level 1.

We will use the mosquitto_sub command-line utility included in Mosquitto to generate a simple MQTT client that subscribes to a topic filter with QoS level 1and prints all the messages it receives. Open a Terminal in macOS or Linux, or a Command Prompt in Windows, go to the directory in which Mosquitto is installed and run the following command:

mosquitto_sub -V mqttv311 -t sensors/+/altitude -q1 -d

The previous command will create an MQTT client that will establish a connection with the local MQTT server and then will make the client subscribe to the topic filter specified after the -t option: sensors/+/altitude. We specify that we want to use QoS level 1 to subscribe to the topic filter with the -q 1 option. We specify the -d option to enable debug messages that will allow us to understand what happens under the hoods and the differences with publishing a message with QoS level 0.

The Terminal or Command Prompt window will display debug messages similar to the following lines. Take into account that the generated ClientId will be different from the one shown after Client mosqsub/1795-Gastons-Ma. Notice that QoS: 1 indicates that the subscription is done with QoS level 1.

Client mosqsub/1795-Gastons-Ma sending CONNECT
Client mosqsub/1795-Gastons-Ma received CONNACK
Client mosqsub/1795-Gastons-Ma sending SUBSCRIBE (Mid: 1, Topic: sensors/+/altitude, QoS: 1)
Client mosqsub/1795-Gastons-Ma received SUBACK
Subscribed (mid: 1): 1

We will use the mosquitto_pub command-line utility included in Mosquitto to generate a simple MQTT client that publishes a message to a topic with QoS level 1 instead of the QoS level 0 that we used when we published messages before. Open a Terminal in macOS or Linux, or a Command Prompt in Windows, go to the directory in which Mosquitto is installed and run the following command:

mosquitto_pub -V mqttv311 -t sensors/drone02/altitude -m  "65 f" -q 1 -d

The previous command will create an MQTT client that will establish a connection with the local MQTT server and then will make the client publish a message to the topic specified after the -t option: sensors/drone02/altitude. We specify the payload for the message after the -m option:"65 f". We specify that we want to use QoS level 1 to publish the message with the -q 1 option. We specify the -d option to enable debug messages that will allow us to understand what happens under the hoods and the differences with publishing a message with QoS level 0.

The Terminal or Command Prompt window will display debug messages similar to the following lines. Take into account that the generated ClientId will be different from the one shown after Client: mosqpub/1516-Gastons-Ma. After publishing the message, the client disconnects.

Client mosqpub/1516-Gastons-Ma sending CONNECT
Client mosqpub/1516-Gastons-Ma received CONNACK
Client mosqpub/1516-Gastons-Ma sending PUBLISH (d0, q1, r0, m1, 'sensors/drone02/altitude', ... (4 bytes))
Client mosqpub/1516-Gastons-Ma received PUBACK (Mid: 1)
Client mosqpub/1516-Gastons-Ma sending DISCONNECT

The previous lines show that the generated MQTT client sends a PUBLISH packet to the MQTT server and then receives a PUBACK package from the server.

Now, go back to the Terminal or Command Prompt window in which you executed the mosquitto_sub command and subscribed to the sensors/+/atitude topic filter. You will see lines similar to the following ones:

Client mosqsub/1795-Gastons-Ma received PUBLISH (d0, q1, r0, m1, 'sensors/drone02/altitude', ... (4 bytes))
Client mosqsub/1795-Gastons-Ma sending PUBACK (Mid: 1)
65 f

The previous lines show that the generated MQTT client, that is, the subscriber, received a PUBLISH packet from the MQTT server and then sent a PUBACK package to the server to acknowledge the message. If we clean up the debug messages that start with the Client prefix, we will see just the last line that shows the payloads for the message that we received as a result of our subscription to the sensors/+/altitude topic filter: 65 f.

The MQTT client that has already established a connection, that is, the publisher, sends a PUBLISH packet to the MQTT server with the header we have already described, QoS set to 1, and including a PacketId numeric value that will be unique for this client. At this time, the publisher will consider the PUBLISH packet identified with the PacketId as an unacknowledged PUBLISH packet.

The MQTT server reads a valid PUBLISH packet and it will respond to the publisher with a PUBACK packet with the same PacketId value that has been used for the PUBLISH packet. Once the publisher receives the PUBACK packet, it discards the message and the MQTT server is responsible of publishing it to the appropriate subscribers.

The following diagram shows the interaction between a publisher and an MQTT server to publish a message with a QoS level of 1.

The MQTT server can start publishing messages to the appropriate subscribers before it sends the PUBACK packet to the publisher. Hence, when the publisher receives the PUBACK packet from the MQTT server, it doesn't mean that all the subscribers have received the message. It is very important to understand the meaning of this PUBACK packet.

For each subscriber to which the message has to be published, the MQTT server will send a PUBLISH packet and the subscriber must acknowledge the reception of the message by sending a PUBACK packet to the MQTT server. The following diagram shows the interaction between an MQTT server and the subscribers when a message is published with a QoS level of 1.

If the application is able to tolerate duplicates and we have to make sure that the messages arrive at least once to the subscribers, QoS level 1 is an excellent choice. In case there is no way to handle duplicates, we have to use QoS level 2.