[hono-dev] Hono-Kura integration

["De Alti, Cristiano" <Cristiano.DeAlti@xxxxxxxxxxxx>]

Hi,
  I wonder if there is still room for an improvement in the MQTT topic format for telemetry and event.

Kura would always authenticate to Hono, and does not use the Hono “gateway” concept.
Having to use the “extended” topic form is unfortunate as it wastes two extra bytes (“t/”) compared to the Kura/Kapua “data messages”.

Shrinking the topic to the minimum will benefit people connecting over a cellular link on a cheap (and limited) data plan, and being able to use “t/${topic-suffix}” would represent a significant cost saving.

As this “compact” form is ambiguous and cannot be used, I wonder If one of the following alternatives (in order of preference) could be added to the current topic rules

1. t//$topic-suffix}. Two adjacent topic separators are explicitly allowed in section 4.7.1.1, Topic level separator, of the 3.1.1 spec, delimiting a zero-length topic level.
It would indicate “this tenant-id and device-id”
2. t///$topic-suffix}. As above but with two zero-length topic levels indicating “this tenant-id” and “this device-id” respectively
3. t/_/$topic-suffix}. The “_” placeholder is used to indicate “this tenant-id and device-id”
4. t/_/_/$topic-suffix}. Same as above but using two levels to indicate “this tenant-id” and “this device-id” respectively

As said above, 1 and 2 would be preferred but I’ve never tried to publish such topics, so I don’t know yet if they could be problematic for some client implementations.
The last two forms, 3 and 4, are definitely less problematic.
Maybe all four should be supported.

Regards,
  Cristiano