MQTT Node: Subscribing to `+/+/$homie` does not work?

TotallyInformation · 12 May 2020 11:54

Hi, I'm trying to get to grips with Homie better in Node-RED.

The Homie convention says that controllers must subscribe to +/+/$homie in order to auto-discover the homie root topic.

Unfortunately, using the MQTT-in node, it doesn't appear as though this works?

+/<devname>/$homie works as does <something>/+/$homie even <something>/+/+ works, just not +/+/<something>

Is this a bug?

ghayne · 12 May 2020 12:08

Can you have a wildcard at the start of a subscription?

Steve-Mcl · 12 May 2020 12:09

Yeah. Its possible to sub to # at least it is if the broker permits it. Some don't (or it's an option)

TotallyInformation · 12 May 2020 12:10

Yes, that is fine. And +/something/somethingelse works.

Steve-Mcl · 12 May 2020 12:10

Have you tried another client?

TotallyInformation · 12 May 2020 12:12

Yes, works fine with MQTT-Spy.

In fact +/+/+ also works in MQTT-Spy

Trying_to_learn · 12 May 2020 12:18

Well, I shall step up and say I am seeing similar things with what I am doing.

I won't say it is exactly the same, but I am seeing problems with retained messages not being seen on a machine when it boots.
ie: I boot a machine and it doesn't seem to get all/any of the retained MQTT messages that are on the BROKER (main MQTT computer).

But I'll shut up now as I don't want to confuse the problem.

TotallyInformation · 12 May 2020 12:24

Oh! +/+/+ actually works in Node-RED. Even stranger then.

Urm, this is bonkers. Other things like +/+/$state work, just not +/+/$homie

knolleary · 12 May 2020 12:30

Hi Julian,

I can't reproduce this.

Here I have a node subscribed to +/+/$homie and a node subscribed to +/b/$homie
When I publish to a/b/$homie I see both nodes receive the message.

[{"id":"fe3d0902.971a58","type":"inject","z":"683f0662.e6d798","name":"","repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":360,"y":320,"wires":[["ef4eae89.c5447"]]},{"id":"a1656b27.2292e8","type":"mqtt in","z":"683f0662.e6d798","name":"","topic":"+/+/$homie","qos":"2","datatype":"auto","broker":"8db984ef.0e77e8","x":350,"y":100,"wires":[["3d3ace8e.2ade72"]]},{"id":"3d3ace8e.2ade72","type":"debug","z":"683f0662.e6d798","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":560,"y":100,"wires":[]},{"id":"539a1f25.cede7","type":"mqtt in","z":"683f0662.e6d798","name":"","topic":"+/b/$homie","qos":"2","datatype":"auto","broker":"8db984ef.0e77e8","x":350,"y":180,"wires":[["5d7eed7.e838814"]]},{"id":"5d7eed7.e838814","type":"debug","z":"683f0662.e6d798","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":550,"y":180,"wires":[]},{"id":"ef4eae89.c5447","type":"mqtt out","z":"683f0662.e6d798","name":"","topic":"a/b/$homie","qos":"","retain":"","broker":"8db984ef.0e77e8","x":550,"y":320,"wires":[]},{"id":"8db984ef.0e77e8","type":"mqtt-broker","z":"","name":"","broker":"localhost","port":"1883","clientid":"","usetls":false,"compatmode":false,"keepalive":"60","cleansession":true,"birthTopic":"","birthQos":"0","birthPayload":"","closeTopic":"","closeQos":"0","closePayload":"","willTopic":"","willQos":"0","willPayload":""}]

I've tried various combinations of topic strings - some using $, some not (just incase it was confusing the regex the nodes use). All work as expected.

TotallyInformation · 12 May 2020 12:31

Hi Nick, yes, I've just discovered that everything apart from +/+/$homie works!

It isn't to do with retention as other retained topics work OK.

knolleary · 12 May 2020 12:31

But +/+/$homie works for me in the flow I just shared.

TotallyInformation · 12 May 2020 12:34

Yes, so maybe it is something to do with the data or some hidden problem with the topic? I've copied the topic from another MQTT viewer and it still doesn't work properly.

Colin · 12 May 2020 12:52

I don't know what it might be but have you restarted/rebooted everything?
Also have a look in the node red and mosquitto logs (if it is mosquitto), though I have no idea what you might find there.

zenofmud · 12 May 2020 12:53

Jullian, do you have any homie nodes installed?
what are the QOS and retained settings/

Christian-Me · 12 May 2020 15:11

Hi,

I use homie now for a long time (currently 1331 retained messages). I can confirm +/+/$homie works as expected with the mqtt in node & mosquitto.

Important is if your messages are realy "retained". First I would take a look in the broker $sys topics

and at the messages itself:

There is a link between QoS and the retained flag in the mqtt spec especially when using QoS 0. More infos how you to configure mosquitto to work perfectly with homie and QoS=0 devices you can find here:

mosquitto config tips here:

github.com

Christian-Me/node-red-contrib-home/blob/master/Mosquitto/readme.md

# mosquitto MQTT broker

## introduction

The homie convention auto discover feature is based on retained messages. The mqtt broker is therefor used as a persistance database to collect all device information and make them available when needed. It is **absolutely necessary** to make sure that the messages are flagged as `retained`. The mqtt specification makes a link between QoS (Quality of Service) and the retained flag. Basicity it is up to the broker what to do with messages with the retained flag set but send wis QoS 0. Some Brokers ignore the flag others like mosquitto stores the in memory but don`t write them to disk. 

## enable **retained** messages when send with QoS=0

Small microcontrollers like the esp8266 using the common *pubsub* client library is unable to send proper QoS>0 messages due to memory and performance limitations. Below you find the necessary settings in the mosquitto.conf file to enable all features you need to store retained messages with QoS=0.

Adjust max_queued_messages as you need, use mqtt-Explorer to determine how many messages needed (some devices can easily need 50 or more messages). Give some "headroom" for future devices. There should not be not to much demand on the database if every device behaves according to the convention. Raspberry Pi users be aware to perhaps increase the autosave interval to avoid excessive IO on your SD card.


```
max_queued_messages 1000
queue_qos0_messages true
autosave_interval 1800
persistence true
persistence_file mosquitto.db
```

This file has been truncated. show original

If you like to dig deeper:

sobscribing to +/+/$homie gives you the info that there is a homie device present. To read all the data you have to subscribe to homie/#

TotallyInformation · 12 May 2020 15:43

No, the only Homie output I have at the moment is a test ESP8266 device running my own code and the homie esp8266 library. That particular attribute is automatically created by the library.

TotallyInformation · 12 May 2020 15:47

How are you generating the device data?

Yes, this is really retained but I don't generate this attribute, it is generated by the library. I'm not creating the device from Node-RED. The problem with the Node-RED nodes is that they require a fixed device name which isn't very helpful when you have a variable number of devices.

The point is that other tools work with +/+/$homie just fine, it is only Node-RED that isn't working.

Christian-Me · 12 May 2020 15:47

$state should be updated reguarly. $homie not! So I suspect your problem
it is related to the "problem" that your messages are not stored as retained in your broker (perhaps because they are sent with QoS=0 or the number of retained messages are limited.

TotallyInformation · 12 May 2020 15:49

The data appears to be correct in both MQTT Explorer and MQTT-Spy:

Christian-Me · 12 May 2020 15:57

the broker is the "center" of my system for commands and states. Even Node-RED is a homie device nor (using my homie-node node) All the rest are ESP8266 devices running ESPEasy and homebrew using the ESP-homie library. If you are using ESP8266 you probably have the QoS = 0 problem! because it uses the pubsub library

The "tools" record the messages / topics as they arrive (including retained flag). What happens when you restart MQTT Explorer? Are all messages pop up instantainosly?

The mqtt in node should emit all your retained messages after every deploy. In my case 1331 messages!

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MQTT Node: Subscribing to `+/+/$homie` does not work?

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