MQTT.js and shared subscriptions

[jkinghed]

Several MQTT brokers are now supporting shared subscriptions (HiveMQ, VerneMQ, EMQ) from the v5 MQTT standard.

The syntax of subscribing to a shared topic seem currently differs based on broker:

HiveMQ
$share:sharename:topic/subtopic/etc (details)

VerneMQ
$share/sharename/topic/subtopic/etc (details)

EMQ
$queue/topic/subtopic/etc
or
$share/sharename/topic/subtopic/etc
(details)

The MQTT v5 standard specifies
$share/sharename/topic/subtopic/etc (details)

So, at least VerneMQ and EMQ both works with the standard.

Any messages published to topic topic/subtopic/etc should then be distributed by the broker to the different subscribers using the $share/sharename/topic/subtopic/etc subscription, but only 1 of the subscribers will get the message. (Which subscriber depends on broker and settings.)

Using this feature makes load balancing very easy.

However, it seems MQTT.js does not yet support this as messages published to a topic with shared subscriptions from MQTT.js are ignored.

Are there any plans to implement this?

[mcollina]

Are there any plans to implement this?

Would you like to contribute? See #651 .

[jkinghed]

Not sure I have the required skills, but I'll try to find time to look into it. Thanks.

[markusapfler]

Is there any progress on the implementation of this feature?

[jkinghed]

Is there any progress on the implementation of this feature?

It has been implemented in Node-Red - source - which uses MQTT.js.

The issue is that MQTT.js default client treats $share/sharename/topic/subtopic/etc as a literal topic, so when something is published on topic/subtopic/etc it is ignored, as the subscription is for $share/sharename/topic/subtopic/etc

However, in Node-Red - and I'm not sure I'm correct on the sequence of things here - it seems that the handler for the subscription overrides the default and validates each message against the actual topic without $share/sharename/ prefix. code

So if you were to do something similar in your client, you might get it to work...

[Paic]

Hey 👋
We had this problem today (can't remember the version, I'll update my post later) and I just tested right now with package version 3.0.0 and it looks like everything is fine (using VerneMQ, even though it doesn't really matter).

Can someone confirm this is fixed ?

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[SaumilShah-7]

Unable to use shared subscription in the latest version of MQTT.js. Client keeps subscribing in loop and doesn't consume any message. Will this be supported or is there a work-around ?

[fp-caregility]

Unable to use shared subscription in the latest version of MQTT.js. Client keeps subscribing in loop and doesn't consume any message. Will this be supported or is there a work-around ?

I am also seeing this behavior. Did you find a solution?

[robertsLando]

Could someone share a minumum reproduction script?