MQTT Clients compatibility issue with '.' char and use in Eclipse Tahu MQTT specification

[petedavis]

There is an issue using NATS.io as a MQTT broker for with the Eclipse Tahu specification.

Unfortunately the root namespace used for the topic contains a '.' char. (See page 12 of the specification)

Given that the '/' topic delimiter chars are converted to NATS topic alternative, should the NATS topic delimiter chars be converted or escaped for MQTT clients?

Could there be a known substitution used internally, like converting '.' to '_' and NATS clients would need to know about?

So MQTT client publish to spBv1.0/group/message/node and NATS client subscribe to spBv1_0.group.message.node?

[kozlovic]

As you have probably observed, currently NATS would reject an MQTT topic that contains '.' because it is used as token delimiter in NATS.

If we do any conversion, then it would be ok to document so that NATS users that want to exchange between NATS and MQTT would have to alter their NATS subjects, however, the problem is that internally NATS converts incoming MQTT messages to NATS format, and convert back when sending to MQTT subscribers.

That is, a MQTT publisher that sends on foo.bar/baz would be converted on ingest to foo_bar.baz, matched against any subscription and then sent to MQTT subscribers as foo.bar/baz (reverse conversion).

Seem ok, right? The issue is if the MQTT topic contains the converted character. In the example above, suppose that MQTT publishes on foo.bar/baz/bar_bat, then it would be converted to foo_bar.baz.bar_bat but on the way to the MQTT subscribers it would have to be converted and now would be foo.bar/baz/bar.bat which does not match.

[sandrokeil]

See also #3482