provide a mechanise for one device to *listen* to another devices publishes

[sumnerboy12]

Example - I have a Wemos D1 with the DHT22 shield, publishing temperature and humidity values to separate nodes. I would like to configure another Wemos with the OLED shield to display the values of these sensors. The idea being the sensor lives in some hard-to-reach place, and the display is in my living room for easy viewing.

I can't see a way to do this with Homie currently - unless I am missing something? I can obviously write a custom (non-Homie) sketch which subscribes to those MQTT topics and displays the values, but I thought this would be a good feature to add to homie.

Let me know your thoughts. I am guessing some sort of special subscribe handler which takes the device id, node id and property name, and a handler. Just a wrapper around an MQTT subscribtion which makes it easy to listen to other homie nodes.

[jpmens]

Depending on the number of subscriptions thusly set up, I'd fear a bit of instability on the subscribing device; my experiments with PubSubClient have shown that a few messages in a couple of seconds can cause the device to reset / reboot.

[sumnerboy12]

Understood, but I still think it would be a useful feature - this remote display would be quite useful but currently can't be done within the Homie f/w. If someone subscribes to 45 different feeds, updating every 2s then be it upon their head!

[marvinroger]

This would indeed be useful. However, as @jpmens said, there are issues when subscribing to more than 5 topics (knolleary/pubsubclient#98). This can be fixed by adding a loop() call in between subscribe call. But there is another issue with SSL enabled, we can't subscribe to more than 3 topics (knolleary/pubsubclient#141). Homie already subscribes to node properties, to $ota and to $reset, so I am afraid what you're asking won't be doable.

[jpmens]

I wonder whether the other pubsubclient would mitigate these issues ...

[sumnerboy12]

Looks like I might have to use mqttwarn to pipe the messages from one node to the other. Not very elegant but it should work.

Is there any reason you don't just subscribe to basetopic/devicename/#? This would only require a single subscription leaving room for a couple of user-defined subscribes.

[jpmens]

@sumnerboy12 the publishes from the device would come right back and probably kill it. That, at least is what my tests with PubSubClient (outside of Homie) have shown, but I'm very confident @marvinroger has something more intelligent to say about this. :-)

[sumnerboy12]

good point! will leave this to the experts then.

[marvinroger]

This is is what we did first, but we had to revert this, because as @jpmens said, the device was killing itself.