Recently, I conducted a test to analyze the bandwidth usage of MQTT, and one feature stood out as particularly impactful: the Topic Alias feature in MQTT 5. This feature can significantly reduce the overhead associated with long topic names, which is especially relevant in UNS (Unified Namespace) implementations using ISA-95-style topics, where topic names tend to be lengthy and sometimes consume more bytes than the payload itself.

💵 It can make an impact if you are being charged based on the amount of data transferred (cloud).

🚨 The Importance of Topic Alias

The Topic Alias feature allows a client to map a topic name to a shorter alias, reducing the amount of data transmitted. This can drastically lower bandwidth usage when transmitting messages with long topic names.

Using Topic Alias during publishing is straightforward but requires the topic-to-alias mapping logic to be implemented in the client program (which is not difficult to do).

On the subscriber side, the implementation should ideally be seamless. In theory, a subscriber needs only to set the maximum allowed alias number during the connection phase. This should make the feature easy to adopt for receiving applications.

👉 During my tests, I discovered something surprising: EMQX was the only broker (of the ones I have tested) to support Topic Alias for subscriptions(!) out of the box. With others I was unable to enable this functionality.

👉 To note: most articles about Topic Alias focus primarily on its use during publishing, not on subscriptions. I was focused on subscriptions.