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[mosquitto-dev] v5 API message modification

With excitement I've been watching the newly improved vr5 api for Mosquitto. Kudos to Roger for such a great work.

There is one feature I loved: the ability to build plugins that can modify messages, altering payloads and topics. I've been doing many experiments with it, and I see a lot of potential. But I couldn't help noticing that message modification happens only when messages come in (and not when they go out).

Potential uses of modifying outgoing messages:
  • Selective modification of properties based on destination.
  • Topic remapping.
  • "Redacting" portions of the payload.
  • Decompression of retained messages (compressed when they come in) to reduce memory footprint.
I found a way to circumvent the lack of modification on outgoing messages. I intercept them on the read ACL callback and re-publish them to the specific client ID. This works fine as long as the message is not stale (retained). In this case the retain bit is always forced to 0, and this breaks the rule that "stale" messages must be marked with a 1.

I can imagine at least 3 reasons why modification on output is not implemented:
  • Is not implemented "yet"
  • There are reasons to consider is a bad idea
  • There are ways to achieve the same without it
If anyone knows of a way to achieve this without further modifications to mosquitto, in a way that is compatible with retained/stale, could you please share your ideas.

If the whole concept is a bad idea, could someone mention why?

And if it's just a missing feature, I'm thinking of prototyping an implementation, but looking for good ideas first.


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