MQTT retain support in Azure Event Grid (preview)

The MQTT Retain feature in Azure Event Grid (Preview) ensures that the last known good value of a topic is stored and readily available for new subscribers. This capability allows new clients to instantly receive the most recent message upon connection, eliminating the need to wait for the next publish. It is beneficial in scenarios such as device state reporting, control signals, or configuration data, where timely access to the latest message is critical.

This article provides an overview of how the MQTT Retain feature works, its billing implications, storage limits, message deletion methods, and retain management considerations.

Billing

Each retained publish counts as two MQTT operations—one for processing the message, and one for storing it.

Storage limits

• Up to 640 MB or 10,000 retain messages per Throughput Unit (TU).
• Maximum size per retain message: 64 KB.

For larger needs, contact Azure Support.

Message deletion

• MQTT 3.1.1: Publish an empty payload to the topic.
• MQTT 5.0: Set expiry or send an empty message to remove it.

Retain management

• Azure portal support for listing retain messages is currently not available.
• Preview doesn't include backfilling retain data for existing namespaces.
• To enable the Retain feature on an existing namespace, do one of the following operations:
  • Perform a control plane operation – Like Throughput Unit (TU) Updates.
  • Add or update the Retain tag on the namespace.