### Understand the alert

This alert monitors the number of failed v3/v5 PUBLISH operations in the last minute for VerneMQ, an MQTT broker. If you receive this alert, it means that there is an issue with the MQTT message publishing process in your VerneMQ broker.

### What is MQTT?

MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol designed for constrained devices and low-bandwidth, high latency, or unreliable networks. It is based on the publish-subscribe model, where clients (devices or applications) can subscribe and publish messages to topics.

### What is VerneMQ?

VerneMQ is a high-performance, distributed MQTT message broker. It is designed to handle thousands of concurrent clients while providing low latency and high throughput.

### Troubleshoot the alert

1. Check the VerneMQ log files for any error messages or warnings related to the MQTT PUBLISH operation failures. The log files are usually located in the `/var/log/vernemq` directory.

   ```
   sudo tail -f /var/log/vernemq/vernemq.log
   ```

2. Check VerneMQ metrics to identify any bottlenecks in the system's performance. You can do this by using the `vmq-admin` tool, which comes with VerneMQ. Run the following command to get an overview of the broker's performance:

   ```
   sudo vmq-admin metrics show
   ```

   Pay attention to the metrics related to PUBLISH operation failures, such as `mqtt.publish.error_code.*`.

3. Assess the performance of connected clients. Use the `vmq-admin` tool to list client connections along with details like the client's state and the number of published messages:

   ```
   sudo vmq-admin session show --client_id --is_online --is_authenticated --session_publish_errors
   ```

   Investigate the clients with `session_publish_errors` to find out if there's an issue with specific clients.

4. Review your MQTT topic configuration, such as the retained flag, QoS levels, and the permissions for publishing to ensure your setup aligns with the intended behavior.

5. If the issue persists or requires further investigation, consider examining the network conditions, such as latency or connection issues, which might hinder the MQTT PUBLISH operation's efficiency.

### Useful resources

1. [VerneMQ documentation](https://vernemq.com/docs/)
2. [An introduction to MQTT](https://www.hivemq.com/mqtt-essentials/)
