How to clear ALL retained mqtt messages from Mosquitto?

When working with the Mosquitto MQTT broker, there may be occasions when you need to clear all retained messages. The retained message feature allows new subscribers to immediately receive the latest published message, even if it was published before the subscriber subscribed.

To clear all retained messages, you can publish an empty retained message to all relevant topics. Here are the specific steps and an example:

Steps:

1. Determine the topics to clear: Identify the topics for which you want to clear retained messages. If you need to clear all retained messages, you may need to perform the following steps for each known retained message topic.

2. Publish an empty message to the target topic: Use the mosquitto_pub command-line tool or any other MQTT client software to publish an empty retained message to each target topic. This will overwrite the previous retained message, and since the content is empty, no message will be retained for subsequent subscribers.

Example Command:

Suppose you know that the topic topic/status needs to have its retained messages cleared. You can use the following command:

bash
mosquitto_pub -t topic/status -n -r

Here, -t topic/status specifies the MQTT topic, -n indicates that the message is empty, and -r indicates that it is a retained message.

Notes:

• Ensure you have permission to publish to the target topic.
• If you are unsure about all retained topics, you may need to subscribe to a wildcard topic (e.g., #) to observe all messages passing through and identify which ones are retained.
• Clearing retained messages may affect other users or services in the system; evaluate the impact before execution.

By following this method, you can effectively clear all or specified retained MQTT messages in Mosquitto. This is useful for maintaining a clean message system and ensuring only necessary information is transmitted.