Manual Publish Acknowledgement Now Available (MQTT5)

[sbSteveK]

We're excited to announce that Manual Publish Acknowledgement is now available in the AWS IoT Device SDK

What is Manual Publish Acknowledgement?

When using MQTT5 with QoS 1 (at-least-once delivery), the broker expects your client to send a PUBACK (publish acknowledgement) packet after receiving each message. By default, the MQTT5 client automatically sends this PUBACK immediately after your message-received callback returns.

Manual Publish Acknowledgement gives you explicit control over when that PUBACK is sent. This means you can defer acknowledgement until your application has fully and durably processed the message. For example, after writing it to a database, forwarding it to a downstream service, or completing any other critical processing step. Only once you're confident the message has been handled should you send the PUBACK, ensuring the broker won't consider the message lost.

Why does this matter?

Without manual acknowledgement, there's a window where your application could receive a message, automatically acknowledge it, and then crash or fail before fully processing it. The broker would then consider the message delivered and would not resend it, potentially causing data loss.

With manual acknowledgement, you stay in control: the broker will continue to redeliver the message until you explicitly acknowledge it, giving you strong at-least-once processing guarantees end-to-end — not just at the network layer.

How to use it

The general pattern of the feature is:

1. Within your publish-received callback, call the acquire method to take manual control of the PUBACK. This prevents the automatic acknowledgement from being sent when the callback returns.
2. Store the returned handle for later use.
3. After your processing is complete, call the invoke method on the client with the stored handle to send the PUBACK.

For detailed instructions and code examples see the Manual Publish Acknowledgement section in the MQTT5 User Guide: MQTT5 User Guide — Manual Publish Acknowledgement