Reverse tunnel SSH was used for remote management. Once we replaced it with MQTT, we realized we could use it for IPC, too. I was not too clear about this in the post above.

Before MQTT we used whatever we had at hand; the touchscreen frontend (that ran Qt, not an embedded browser) talked to the local backend with HTTP. SMS messages were first sent to another local HTTP gateway and then via UNIX shm to a master process that templated the message to the smstools3 spool directory. A customer-facing remote management GUI ran on the device's built-in HTTP server but I no longer remember how it interfaced with the rest of the system. The touchscreen GUI might actually have had its own HTTP server for that, because it also handled Modbus communication to external I/O.

This was back in the day when everyone was building their own IoT platforms and obviously we were required to dogfood our own product, trying to make it do things it was never meant for. The gateway was originally designed to be an easy solution for AV integrators to remotely manage whole campuses from a single GUI. I'm pretty sure no customer ever wrote a line of code on the platform.

Ah okay, that makes more sense. We used wireguard for remote management and we never got into MQTT (I wish we had).