I'm taking a look at expanding the messenger API to support reliability and so far there seem to be two directions to explore which I'll attempt to describe below:
Option 1) Messenger.ack(Message) or possibly Message.ack() I'll describe this as the simple/expected/conservative option, and those really are its strong points. Some less desirable points are that it takes the Message concept in a bit of a different direction from where it is now. Message is no longer simply a holder of content, but it is also now (at least internally) tracking some kind of delivery state. This is undesirable from a design perspective since you're really merging two separate concepts here, delivery state and message content, e.g. you now end up holding onto the message content to track the delivery state when arguablly the common case is that an app will be done processing the content quickly but may care about the delivery state for longer. Another implication of this merging is that it makes messages harder to reuse, e.g. imagine if you want to receive a message, mutate it a bit, and then resend it or send a number of similar messages. It's also potentially more work from an implementation perspective as the underlying model treats Message as pure content and has delivery factored out as a separate concept, so this would be the start of a bit of an impedence missmatch between layers. It's certainly doable, but might result in more overall code since we'd be expressing similar concepts in two different ways. Option 2) Introduce/surface the notion of a delivery/tracking number through the Messenger API, e.g.: Messenger.put(Message) -> Delivery/Tracking-Number/Whatever Messenger.get(Message) -> Delivery/Tracking-Number/Whatever (I'll abbreviate Delivery/Tracking-Number/Whatever as DTW for now.) There are a couple of choices with this option, DTW could be a value object (i.e. similar to a handle), with actions on the messenger itself, e.g. Messenger.ack(DTW), Messenger.status(DTW). This kind of takes the tracking number analogy and runs with it, you ask your messenger about the status of a given delivery via its tracking number. Alternatively, DTW itself could be a more action oriented interface with its own methods for acking/status/etc. This isn't necessarily an either/or as there are reasons the C API might want to use a handle approach even if we wish to conceptualize/surface DTW as more of an independent thing in the object oriented interfaces. On the negative side this is less traditional/expected relative to Option (1), and it does in total add more surface area to the API. On the positive side however it does provide a lot more capability since the same concept extends quite easily to track the state of outgoing deliveries. From a design perspective this is nicer for a couple of reasons, unlike Option (1) by keeping Message as a pure holder of content you have more flexibility with how you use the API, e.g. you can discard/reuse the Message but still track the status of your deliveries. Also, it seems likely that once the Option 1) path includes the ability to track the status of outgoing deliveries, the total surface area balance might fall more in favor of Option (2). A possible addendum to Option (2) suggested by Rob in order to deal with the surface area concerns is adding some kind of Messenger.ack() that would acknowledge all unacked deliveries pulled from the incoming queue. This would enable you to do basic acking without ever needing to bother with DTWs if you don't care about them. This could in fact be a nice place to start as it doesn't necessarily commit us to either path initially. FWIW, my bias right now is towards exploring Option (2). I think the fact that it is less expected is sufficiently mitigated by the fact that with the addendum there is a very gentle learning curve, and even if you explain all the concepts up front, the whole tracking number analogy makes it fairly intuitive. I can even imagine playing up the difference as a selling point from a technical marketing perspective. --Rafael
