MQTT Messages are not being delivered and are expiring.

What is MQTT Messages are not being delivered and are expiring.

Understanding MQTT and Its Purpose

MQTT, which stands for Message Queuing Telemetry Transport, is a lightweight messaging protocol designed for constrained devices and low-bandwidth, high-latency, or unreliable networks. It is widely used in IoT (Internet of Things) applications to facilitate communication between devices and servers. The protocol is designed to be simple and efficient, making it ideal for scenarios where bandwidth and power consumption are critical considerations.

Identifying the Symptom: Message Expiry

One common issue encountered when using MQTT is message expiry. This occurs when messages are not delivered to the intended recipient before a specified time limit, resulting in the messages being discarded. This can lead to data loss and communication breakdowns in IoT systems.

What You Might Observe

When message expiry occurs, you may notice that certain messages are missing or not reaching their destination. This can be particularly problematic in systems where timely data delivery is crucial, such as in monitoring or alerting applications.

Exploring the Root Cause: Broker Settings

The primary cause of message expiry in MQTT is often related to the broker's settings. MQTT brokers have a feature called message expiry interval, which defines how long a message should be retained before it is considered expired. If this interval is set too short, messages may expire before they can be delivered, especially in networks with high latency or when the recipient is temporarily unavailable.

Understanding Message Expiry Interval

The message expiry interval is a setting that can be configured on the broker. It determines the maximum time a message should be retained in the broker's queue before being discarded. If the interval is too short, messages may not have enough time to be delivered, leading to expiry.

Steps to Fix the Issue: Adjusting Message Expiry Settings

To resolve the issue of message expiry, you need to adjust the message expiry settings on your MQTT broker. Here are the steps to do so:

Step 1: Access Broker Configuration

First, access the configuration settings of your MQTT broker. This can typically be done through a configuration file or a management interface provided by the broker. For example, if you are using the Mosquitto MQTT broker, you can edit the mosquitto.conf file.

Step 2: Modify the Message Expiry Interval

Locate the setting for the message expiry interval. This may be labeled as message_expiry_interval or something similar, depending on your broker. Increase the value to allow more time for message delivery. For instance, you might set it to 60 seconds or more, depending on your network conditions and application requirements.

# Example for Mosquittomessage_expiry_interval 60

Step 3: Restart the Broker

After making changes to the configuration, restart your MQTT broker to apply the new settings. This can usually be done with a command like:

sudo systemctl restart mosquitto

Additional Resources

For more information on configuring MQTT brokers and handling message expiry, consider visiting the following resources:

MQTT Essentials: Publish/Subscribe Eclipse Paho Project

By following these steps and adjusting the message expiry settings, you can ensure that your MQTT messages are delivered reliably and reduce the risk of data loss due to message expiry.