Clearing MQTT retained messages

Tutorials & Examples
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Clearing retained messages in mosquitto

Purpose

The purpose of this tutorial is to present solutions to clear retained messages in the Eclipse Mosquitto MQTT Broker.

Retained messages can be troublesome and/or unwanted. Many mosquitto tutorials included a retain option in the configuration and setup of mosquitto. This is likely to create problems in the future.

This tutorial is created for Linux users with a package installation on mosquitto.
Other users will have to find the files and paths relevant to their system.

Clearing ALL retained messages

  1. First we have to stop the mosquitto broker.

    sudo systemctl stop mosquitto.service
    or
    sudo service mosquitto stop

  2. Delete the mosquitto.db containing all the stored message data in the persistence. By default, located in /var/lib/mosquitto/mosquitto.db

    sudo rm /var/lib/mosquitto/mosquitto.db

  3. Restart the mosquitto service

    sudo systemctl start mosquitto.service
    or
    sudo service mosquitto start

Clearing SPECIFIC retained messages

In some cases some retained messages can be wanted and we don’t want to clear all the retained messages.

The messages will have to be cleared one by one using the topic

To clear a specific message:

mosquitto_pub -h hostname -t the/topic -n -r -d


If your broker is setup with a username and password then use:

mosquitto_pub -h hostname -t the/topic -u username -P password -n -r -d

  • Replace the hostname with the hostname or IP address of your broker (e.g. localhost)
  • Replace the/topic with the topic that you want the retained messages cleared for
  • Replace the username and password with… Well you know…

Options used in the mosquitto_pub command:

-n = Send a null (zero length) message
-r = Retain the message as a “last known good” value on the broker
-d = Enable debug messages

I hope this is useful.
I have made this thread a wiki so please don’t hesitate to make changes if you see a glaring error or you want to add other operating system instructions.

13 Likes
2

cool !

I used to use another method to clear the retained messages also:

mosquitto_pub -h hostname -t the/topic -m

(-m = message with no content)

I never checked if this worked (since I didn’t set the retain option on the blank message :stuck_out_tongue:)

3

F.Y.I Just sharing: Method I use is with the Windows 10 app “MQTTbox”.

  • inside the APP connect to your MQTT server when connected
  • add publisher with your topic to be cleared, retain=yes and empty payload (payload type “strings…”)
  • publish

MQTTbox also provides a simple method to monitor your MQTT traffic, so far I like MQTTbox over other APPs to do this simple tasks without finding out the correct “mosquitto_pub” string.

4

MQTT Explorer has the ability to recursively delete retained topics. 50 at a time (click)


There’s also MQTT Forget

4 Likes
5

Simple shell script to delete all entries under “homie/examplehomiedev” :

mosquitto_sub -h localhost -W 1 -F '%t' -t '#' | grep '^homie/examplehomiedev' | while read i ; do  mosquitto_pub -h localhost -r -d -t "$i" -n; done

It should be one line in the shell.