MQTT5 CONNECT and PUBLISH Properties

[pearmaster]

Reason/Context

One of the defining features of MQTTv5 are Properties. (otherwise, you'd probably just use MQTT v3). Properties can be included in almost all MQTT packet types, and can form a significant part of an MQTTv5 API contract.

In issue asyncapi/spec#878 @fmvilas suggested that bindings are the appropriate construct for describing properties.

For more information, this page lists properties by packet type: http://www.steves-internet-guide.com/mqttv5-properties/

Description

I suggest adding additional fields to the MQTT5 server binding for MQTT CONNECT packets:

• Maximum Packet Size
• User Property
• Session Expiry Interval (already included in the MQTT5 0.2.0 binding)

I suggest adding additional fields to the MQTT5 message bindings for MQTT PUBLISH packets:

• Paylod Format Indicator
• Message Expiry Interval
• Response Topic
• Correlation Data
• User Property (These are OK to be message headers per How to describe MQTT v5 user properties in an AsyncAPI document? spec#878 (comment))
• Content Type

I have a pull request that is about ready to go once this issue is approved (according to the contributing guidelines)

Side note: I've used the word "packet" above when the correct MQTT terminology is "message" to disambiguate from AsyncAPI "message" objects.

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[tomkralidis]

Relation to #198

[pearmaster]

@fmvilas While I'm looking at this, I see that mqtt5 doesn't include the bindings that mqtt allows. Do you envision bindings being able to stack like this?:

servers:
  prod:
    bindings:
      mqtt:
        clientId: guest
      mqtt5:
        sessionExpiryInterval: 60

or should the relevant mqtt bindings be copied to the set of mqtt5 bindings?

[fmvilas]

Do you envision bindings being able to stack like this?

I think we somehow defined mqtt for MQTT <5 and mqtt5 for version 5 but, to be honest, I think this is a mistake we did in the past and both bindings should be merged into mqtt. It should be ok to put MQTT 5 stuff into the mqtt binding. We should just make it clear that it's only supported in versions >=5.

[pearmaster]

I'll adapt my MR to merge both binding definitions (plus the new stuff) into mqtt.

[fmvilas]

Awesome, thanks! 🙏

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