The promise approach is more elegant and efficient. Historically it has
sometimes led to complications, though. Particularly tricky is
bidirectional streaming.
subscribe(downstream :Downstream) -> (upstream :Upstream)
The problem here is that all messages sent to `upstream` will normally be
held until `subscribe()` itself returns a capability which they can be sent
to. So if `subscribe()` doesn't return until the downstream direction is
done (or canceled), then the upstream doesn't work.
HOWEVER, if you're using the C++ implementation, a solution to this was
introduced two or so years ago. You can use `context.setPipeline()` to
connect the pipelined capabilities without actually completing the RPC.
So, assuming you can rely on `setPipeline()` being available, then I would
lean towards the promise approach rather than the handle approach.
-Kenton
On Wed, Aug 24, 2022 at 11:00 AM Vaci <v...@vaci.org> wrote:
> If a capability accepts a callback as a method param, such as for
> subscriptions, then it seems there are (at least) two possible ways to
> handle cancellation:
>
> a) return a handle that can be dropped to cancel the subscription
>
> interface Foo {
> subscribe (callback :Callback) -> (handle :Capability);
> }
>
> or b)
>
> fulfil the promise returned by the method call at EOF, and drop the
> promise to cancel the subscription.
>
> interface Foo {
> subscribe (callback :Callback);
> }
>
> I'm curious to understand fully the pros and cons of each approach,
> especially when the callback is a ByteStream, where it seems natural to
> return the pumpTo promise as the result of the subscribe call's
> implementation?
>
> Cheers,
> Vaci
>
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