Publish on topics with leading $dollar sign?

[Sebastian Raff]

Hi!

I realized that (at least with mosquitto) I can publish on $test/foo/bar and subscribers to # don't receive these messages, they have to explicitly subscribe to e.g. $test/#. This is really useful for me, with one of my applications (mqttDB) I want to publish a bunch of relative big messages with payload sizes up to a megabyte, if some other application subscribes to # this could lead to problems and the $-topics seem to be a neat solution for my use case.

Now to my question: I'm aware of the special topics on $SYS/# and afaik this is not covered by the current standard but quite common on most brokers. I heard that some brokers don't allow clients to publish on $-topics, is that true? Is there a special reason to forbid this? Would you advise me not to use $-topics, although mosquitto seems to have no problem with that? Or do you think it's "ok-ish" to publish on $...? Will the upcoming MQTT 4 standard specify $-topics?

Regards,

Sebastian