What’s happening with RSMB?

Some people will be aware that I wrote a small MQTT broker in about 2008, made available on IBM’s alphaWorks website. It was called RSMB (Really Small Message Broker) when released, because the name we used first for it, Nanobroker, was already taken. Somewhat amusingly, an IBM website for RSMB still exists.

A year or two later, Roger Light asked Andy Stanford-Clark why RSMB wasn’t open source (not my decision), so Andy suggested Roger write his own. And that’s how Mosquitto started, as a drop in replacement for RSMB. When MQTT software was being contributed to the Eclipse foundation, IBM contributed a Java client and my C client to the Paho project, and Mosquitto was contributed as the broker. IBM did contribute the RSMB source to the Mosquitto project on my encouragement, to serve as a repository of potentially useful code, and because it had support for MQTT-SN.

So in my mind, Mosquitto became the official replacement for RSMB, and I expected RSMB to outlive its usefulness pretty quickly. As it happens, MQTT-SN support in Mosquitto has been on the back burner ever since, because Roger wanted to rebase the internals of Mosquitto on an event library before tackling it. Unfortunately, this ran into a number of issues, social and technical. I’m still hoping that it will happen.

But one of the alternative approaches to MQTT-SN support is now available in Paho (written by Tomoaki Yamaguchi) – a transparent gateway which converts MQTT-SN into MQTT. Transparent because it creates a new MQTT connection for each MQTT-SN client, so that the MQTT broker has visibility of those clients. RSMB acts as an aggregating gateway, where one MQTT bridge connection carries the traffic for all MQTT-SN clients, and the MQTT broker sees only one.

I do wonder if an MQTT-SN to MQTT gateway will in fact be a better solution because it may allow easier support of additional underlying transports. The gateway has UDP and XBee right now, others such as BLE and even serial could be useful.

Mosquitto in the meantime has added further capabilities such as TLS and WebSocket support, and many more. If there were a niche that Mosquitto has left open then I would be happy to support RSMB in that, but I don’t think there is. The combination of Mosquitto and the Paho transparent gateway will do a better job all round.