Kevin,
I am very interested in beta testing your client. I have a working MQTT Broker (Mosquito) working with OpenHAB and Smartthings. I have the other code listed above working on Hubitat, but is only one-way.
I have written groovy devices/apps for ST, am familar with HE's custom apps and logs. So I should be able to provide some useful feedback.
Apparently, I am too new to be able to PM. Any other means to contact you?
Contact made.. I’ll send you a link to the download shortly. Unfortunately I have reached the community max number of users in a private topic and @chuck.schwer has not been able to raise it so I can’t include any new people in that thread, but the app will be beta and public fairly soon.
Re OpenHAB - they have found a significant issue with their core MQTT libraries and this is not going to be fixed even in OH2.5M2 so I don’t hold out much hope for my app working well with OH via homie discovery. But HomeAssistant discovery, which OH also supports should work fine.
Can I just link it to a Mosquito MQTT Broker? My link from OH to MQTT has been stable.
Of course..
OH promote their own auto discovery protocol for MQTT - but it's broken in OH 2.4 currently.
I want automagic.
Any word on when this will proceed to public beta? I just set my HA to get my ring devices and drop them out to MQTT. Would like to bring them into HE. Currently using a dozen routines in Alexa to update HE but it is ugly. Thanks!
Blake
Hi Blake, alpha4 will be the last alpha. A few people are pre-testing this currently and then it (alpha4) will be available to all the alpha group which is about 40 strong.
The next release is then intended to be the beta available to all. If you're keen to test then PM me and join the alpha group.
Beta getting closer? 
I guess that's always going to be true 
We're just starting alpha4 which hopefully will be the last one before the public beta. A lot of changes between alpha3c and alpha4 so expecting a few things to hotfix. It's working pretty well for everyone though, based on very few reported bugs, and the number of users.
You're not on my alpha list though ? I guess based on your recent posts you're at least waiting for beta.
I thought the group was full (40 user max on discussion board), so was waiting for the beta.
If not I would poke around on the alpha if available. I have a Node-RED "Hubitat custom node" library in various stages of completion which I may/may not release at some point.
But the more I thought about it, the more I think it is largely unnecessary depending on how well MQTT integration works. So I've been treading water working on that in lieu of waiting for the Hubitat MQTT client.
The Hubitat forum has a private topic limit of 40 - actually it might be 30. In order to jettison some members I created a new topic for alpha4 and it is not yet quite full. But I do have about 75 testers so it will be - that's partly why I want to get to public beta quickly so there is no such limit.
Did you ask me to join you because, if so I apologise, I've missed it. But I have always pointed these enquiries directly to my GitHub for alpha3. I'll check back
Kevin
No, I didn't ask previously - so no apology needed at all.
I'm looking forward to jumping in to alpha 4.
I'm relatively new to MQTT, but in my quest to move all logic out of Hubitat and into Node-RED and Home Assistant, I've been pretty active in it lately. In my typical approach when I get into something new, I've spent a lot of time reading the MQTT release and underlying protocol docs and trying out various brokers. 
We created our own protocol called xAP about 15 years ago to provide agnostic cross device communication using broadcast messages. My large (750 node) multi controller system uses it extensively.
Now I am transitioning all that to MQTT which is similar to xAP but with a data store. Actually I've just bridged xAP to MQTT so that was relatively easy and I am very happy with the results.
It means I can utilise the scheduling / logic engine from any controller on any device and use any device supported by any controller from all the other controllers.
It can be a difficult working out just what did what sometimes as MQTT doesn't provide information on which client initiated a message. Keeps you on your toes but can be a frustration sometimes.
HA has specific support in the MQTT app . HE publishes device information so that HA autodiscovers your HE devices. Conversly HE can listen to the HA statestream messages and create devices within HE.
(same with Athom's Homey and also OpenHAB 'perhaps')
Any way I can get a hold of this and test it out?
clue
Want try this mqtt client? Is it can work directly from HE or do i need make mqtt broker on raspberry pi?
Hehe... You'll have to wait a bit for a PM. Seems like I don't have access to such niceties yet 
That said, I do have a MQTT broker up and running on my raspberry, so no worries there. And the MQTT app seems like the ideal way of integrating my HE with it 
If you’re unable to PM me because of being a new community member let me know here and I’ll get in touch with you...
I would like to test your MQTT client. I already use MQTT between home bridge, a raspberry pi in the closet tied into the original house alarm wiring and home-assistant. Let me know how to get in contact with you.
Download the Hubitat app
So great if could do that.