So, for me I probably started out being biased to Option 1) from a
"simplicity" standpoint, but I'm now leaning towards option 2), though
I'm not quite sure what form the DTW should take, nor how it would be
best presented in the Java API.

One other issue that makes me uncomfortable with Option 1) it is that
by adding .ack() or even (per a previous JIRA) .getSubscription() to
Message we're potentially tying Message directly into Messenger which
is not a dependency I really want to introduce.  If we stick with
Messenger as a value object then we don't have any such coupling
(Messenger depends on Message, but not vice versa).

-- Rob

On 26 October 2012 17:07, Rafael Schloming <> wrote:
> 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

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