Problem

Mosquitto server start problem:

After a few days of absence, I was unfortunately faced with some problems on my Raspberry smart home server.

Among other things, the Mosquitto MQTT server, which is supposed to receive and process messages from the various iot devices, was not available.

Analysis

After analyzing the process overview with

ps -A | grep mosq

unfortunately no output was shown, i.e. the process had obviously not started.

The manual start with

sudo mosquitto

then led to success. The MQTT server was running and also receiving or processing all messages. Only the service to the Mosquitto caused problems.
After completing the manually started process, another attempt was made with

sudo systemctl start mosquitto.service

then start the service. The subsequent query with

sudo systemctl status mosquitto.service

initially provided a promising answer that the service had started successfully.

Unfortunately not quite successful, there is still no entry for mosquitto in the process overview, and the MQTT message processing did not work either.

Reason

After some troubleshooting, the problem was solved. The cause was an apparently corrupt mosquitto.db that is used by the mosquitto service to cache messages.

Solution

This problem can be easily solved by deleting the corresponding file in the directory / var / lib / mosquitto using:

sudo rm mosquitto.db

If then restarted with

sudo systemctl restart mosquitto.service

 mosquitto works fine. The service is automatically restarted even after a reboot.