Do you want to control devices in a secondary location? MQTT is perfect for this. But it took me a while to figure out exactly how to set this up, so here’s an example configuration.
This example assumes the following:
- an MQTT broker configured with the name mosquitto.
- a working openHAB system at each location.
- let’s call our two locations Home and Vacation, to keep things simple.
Concepts
The hardest concept for me to grasp here, was that each device actually has two separate MQTT topics–the state, and the command. If you try and use a single topic, you’ll end up with infinite loop issues.
Say you have a light switch in your vacation property. You need to define that light switch as an item on both openHAB systems.
- On the home system, the item acts as a proxy.
- On the vacation system, the item represents the actual switch.
But each item communicates differently via MQTT:
| Inbound Topic | Inbound Trigger | Outbound Topic | Outbound Trigger | |
|---|---|---|---|---|
| Home | state | state | command | command |
| Vacation | command | command | state | state |
Because the home item is a proxy, it doesn’t receive commands. Instead, we configure it to listen for state updates from the physical switch, and update its state accordingly. Meanwhile, the vacation item only listens for commands, since it doesn’t care about any state other than that of the physical switch it represents. Likewise, any command received by the vacation item is sent out via the state topic, since that’s what the home item is listening for.
I know this is confusing, but just remember this: the physical item sends status and receives commands; the proxy item sends commands and receives status.
As you go through and configure all your proxy items, this concept will start to click and you won’t have to think that hard about it.
Code Example
NOTE: This section has been edited from my original post to provide a simpler, more concise syntax relying on the defaults. If you prefer the original, more verbose syntax, see the optional section below.
First, assuming you’ve already defined your items on the Vacation system, let’s add our MQTT messaging configuration. In our example we’ll extend an existing Z-Wave switch.
On your Vacation system, edit the file items/default.items:
Switch FrontLights_Switch "Front Lights" { channel="zwave:device:abcdef00:node1:switch_binary", mqtt=">[mosquitto:Vaca/FrontLights/state:state:*:default], <[mosquitto:Vaca/FrontLights/command:command:default]" }
Next, on your Home system, define a “proxy” item.
On your Home system, create the file items/vacation.items:
Switch Vaca_FrontLights_Switch "Front Lights" { mqtt=">[mosquitto:Vaca/FrontLights/command:command:*:default], <[mosquitto:Vaca/FrontLights/state:state:default]" }
Optional: Verbose, Command-Specific Syntax
The above example is as simple as it gets. If you want to specify different commands and/or mappings, you can use these examples instead.
On the Vacation system, edit the file items/default.items:
Switch FrontLights_Switch "Front Lights" { channel="zwave:device:abcdef00:node1:switch_binary", mqtt=">[mosquitto:Vaca/FrontLights/state:state:OFF:0], >[mosquitto:Vaca/FrontLights/state:state:ON:1], <[mosquitto:Vaca/FrontLights/command:command:MAP(binary.map)]" }
On the Home system, edit or create items/vacation.items:
Switch Vaca_FrontLights_Switch "Front Lights" { mqtt=">[mosquitto:Vaca/FrontLights/command:command:OFF:0], >[mosquitto:Vaca/FrontLights/command:command:ON:1], <[mosquitto:Vaca/FrontLights/state:state:MAP(binary.map)]" }
If you’re going to follow my example, here’s the contents of transform/binary.map:
key=value
1=ON
0=OFF
ON=1
OFF=0
Some Helpful Links
Here are some links that helped me figure all of this out.
- openHAB Docs - MQTT Binding
- openHAB Community - OpenHAB MQTT Inbound and Outbound - A helpful forum post, but it misses the need for separate topics for command & state. Also, ignore the bit about autoupdate=“false” unless you know you need it.
- openHAB Community - State / Command in MQTT Messages - This forum post really helped me understand the concept of separate message topics for command & state, which is the key to this whole thing.