Support "WAMP AP Retained Events" for pattern-based subscriptions

[jamesremuscat]

(From https://groups.google.com/forum/#!msg/crossbario/xjYjd3oCkQg/bKOl6wTDAgAJ)

I've noticed some interesting edge cases around retained events and pattern-based subscriptions. As an example, take this scenario:

• Node 1 publishes message A to topic myprefix.A with PublishOptions(retain=True)
• Node 1 publishes message B to topic myprefix.B with PublishOptions(retain=True)
• Node 2 subscribes to myprefix with SubscribeOptions(match='prefix', get_retained=True)

My naive expected behaviour was that Node 2 would immediately receive messages A and B. In actual fact, Node 2 receives no messages.

The spec suggests that, at the least, Node 2 should receive message B:

When they opt-in to receiving the Retained Event, the Broker MUST send the Subscriber the most recent Retained Event that they would have received if they were subscribing when it was published.

(emphasis original) - "if they were subscribing when it was published" here includes both messages A and B, the most recent of which is B.

A further observation is that, if now Node 1 publishes message C to topic myprefix.C with PublishOptions(retain=True), then Node 2 receives the message, but the EventDetails object has retained=False, whereas I would have expected it to contain retained=True.

It looks like retained events are only really supported for exact-match subscriptions - https://github.com/crossbario/crossbar/blob/master/crossbar/router/broker.py#L457 doesn't include a match argument in the call to get_observation, so it defaults to exact. Retained events are stored against observations, rather than "bound to the topic it was sent to" (spec).

[jamesremuscat]

Just a note to say that I'm still working around this in user-space, and have needed to on a number of projects over the last 4.5 years; if there was a way of this being resolved in Crossbar I'd be hugely grateful!

[oberstet]

Yeah, there is a whole bunch of such "smaller" issues/gaps/glitches, where only something of limited scope is missing in Crossbar.io:( Lack of resources, curse of OSS, no sponsors, ...