Problem: processing subscriptions with producers one doesn't control

[yrashk]

Currently, the server will not send the topic name over the TCP connection. That is done in order to avoid imposing rigidity and overhead, so that clients can elect how they compose their own messages to recognize their routing properties once received.

This concept doesn't work well with messages sent by producers not controlled by the same entity. It will be nearly impossible to detect what those messages mean once they are received.

Proposed solution: implement something like SUBSCRIBE/FILTER (or SUBSCRIBE/TRANSFORM) instruction, which, in addition to topic name, will take a close that takes topic name and the message itself and produces either a message or nothing. This way one can subscribe to any topic and process messages in any way before they are passed over to the server. Even more so, if this instruction does allow to drop the message altogether, it can be served as a true filter that will prevent some messages from ever being processed. Also, additionally, this can be even used to route messages to any other topics (if one will use PUBLISH in that closure).