What is retain message in MQTT?
A retained message is a normal MQTT message with the retained flag set to true. Each client that subscribes to a topic pattern that matches the topic of the retained message receives the retained message immediately after they subscribe. The broker stores only one retained message per topic.
How do I publish in MQTT?
An MQTT client can publish messages as soon as it connects to a broker. MQTT utilizes topic-based filtering of the messages on the broker (see part 2 for details). Each message must contain a topic that the broker can use to forward the message to interested clients.
When you publish a message to a topic with the retained flag set what happens to the message?
For a specific topic, the broker will store only one retained message. Hence if another message is published on the same topic with a retain flag as true, then the previous message will be replaced by the new one. This is done so that the client can get the last known value easily.
Which of flag is only used on publish messages?
If the publishing client sends the message again it sets a duplicate (DUP) flag. In QoS 1, this DUP flag is only used for internal purposes and is not processed by broker or client.
When to publish a retained message on MQTT?
You can use my Python script to do this. However this is not recommended on a busy broker. Generally you will publish a message with the retained flag set when the message contains persistent data.
How to create a persistent session in MQTT?
You create an MQTT persistent session by sending a CONNECT message and setting the cleanSession flag to 0. If no session exists for the client sending the CONNECT message, a new persistent session is created. If a session already exists for the client, the client resumes the existing session.
Is the MQTT specification compatible with AWS IoT?
The MQTT specification provides a provision for the publisher to request that the broker retain the last message sent to a topic and send it to all future topic subscribers. AWS IoT does not support retained messages. If a request is made to retain messages, the connection is disconnected.
How to get rid of retained messages in Mosquitto?
A- If you’re not using the persistent database then the easiest way is to stop and restart mosquitto. Otherwise you will need to subscribe to all topics to check and clear the retained messages. You can use my Python script to do this.