One of the [design principles](design principles) of MQTT is that it should be "zero-admin" (as far as possible).

Therefore, topics on a broker are always expected to be "dynamic" and to require no administrative setup.

i.e. a client subscribes on topic x without knowing whether it exists; it doesn't, so broker silently creates that topic; and the client receives publications on that topic, should they be sent (which, of course, in a pub/sub world, they may never be).

Here's a discussion of this subject on the mailing list. A future or clarified version of the specification may make this explicit.

It remains a question of whether this should be a requirement or a recommendation. I think making this a requirement would potentially be restrictive to those adding MQTT support to their existing systems. But dynamic topics are very much in the spirit and ethos of MQTT --- //Nick O'Leary 2012年01月31日 15:25//