By   October 13, 2015
0

MQTT Config

So as I work to integrate MQTT into my automation environment, I find myself wishing for some sort of device discovery mechanism. Possibly also a device configuration mechanism.

Here’s the problem as I see it.

If I know the topic of a device, I can subscribe to it and I can even get its last data. I can also subscribe to all topics or a subset of topics. But then I have to wait for a device to publish something in order to discover it.

Unfortunately, I can’t ask Mosquitto what devices it knows about.

There are many use-cases where this just isn’t sufficient:

  • A device that isn’t publishing when you reboot.
  • A device that is turned off and only publishes when it gets turned on (like a device that uses a tilt sensor to power itself on).
  • A device that doesn’t publish at all (a subscribe only device like a Relay).

It would also be nice to be able to re-configure some attributes of a device though I can see how this could be abused in an InternetOfThings scenario.

So I believe I’m going to define a ‘discovery’ record and experiment with that. I’m thinking something like this:

/Device_ID/config/[Name]/[Attribute]/[Value]

Examples:

/Kitchen/config/Temperature/type/int
/Kitchen/config/Temperature/direction/input
/Kitchen/config/Temperature/unit/celsius
/Kitchen/config/Humidity/type/float
/Kitchen/config/Humidity/direction/input
/Kitchen/config/Humidity/unit/percent
/Garage/config/Relay/type/bool
/Garage/config/Relay/direction/output
/Garage/config/RelayToggle/type/int
/Garage/config/RelayToggle/direction/output
/Garage/config/RelayToggle/unit/seconds
/Garage/config/Relay/type/bool
/Garage/config/Relay/direction/input

This tells me that is a sensor named “Kitchen” that publishes a temperature as an int in degrees celsius… There’s also a control named “Garage” that publishes a “Relay” state but it also has an output that it subscribes to called “Relay”… They both provide/take a boolean. There’s also a control named “Garage” that subscribes to “RelayToggle” that will conceivably cycle a relay on/off or off/on for some number of seconds.

I’ll experiment with that and see if I can make it useful… It will allow me to maintain an inventory of devices that are available… I will have to come up with some sort of expiry mechanism of course and if an attribute changes, I’ll need to override the old config with the new one presumably.