private IReliableSubscription CreateExternalSubscription()
{
//
// NB: Resolve is synchronous and the environment tries its best to make it non-blocking. In case there's blocking, pause times
// during checkpointing can get unbounded (which we don't want). We should consider going async here, which bubbles up through
// the Start code path (and the subscription visitors), all the way into the recovery and create subscription code paths.
// Because Start may also get called when an inner subscription is created in response to receiving an event, it also spreads
// async to observer methods. With upcoming support for ValueTask in .NET vNext, we could entertain this idea without
// having to worry too much about allocation costs that have prevented us from going down the Task path on the core event
// processing code path.
//
// Going async has other benefits as well, including support for async predicates, selectors, etc. provided we can take care
// of "replay" of such outgoing calls (we need them to be repeatable for recovery purposes, so we either need to persist
// their results or pass in a sequence number so we can ask for a "replay"). In the context of resolution, it can also enable
// powerful dynamic binding, e.g. if an event carries a Uri of another stream to subscribe to:
//
// streamsToSubscribeTo.SelectMany(uri => ctx.GetObservable<T>(uri)).Subscribe(o)
//
// Here, the GetObservable<T>(Uri) call is passed a data-dependent Uri (obtained from an event), so we'd have to dynamically
// bind (and thus possibly externally resolve) the observable, which requires async in an OnNext context.
//
// NB: We've also worked around the synchronous resolver requirement for "anonymous streams" (used for P2P communication between
// engines) by encoding the target QE in the URI of such streams (so the resolver can extract the target QE easily).
//
if (!_serviceResolver.TryResolveReliable(_edge.ExternalUri, out _externalReactiveService))
{
throw new InvalidOperationException(string.Format(CultureInfo.InvariantCulture, "Can't resolve external output '{0}'.", _edge.ExternalUri));
}
var extObserver = _externalReactiveService.GetObserver <T>(_edge.ExternalUri);
var thisObservable = _parentReactiveService.GetObservable <T>(_edge.InternalUri);
var subscription = thisObservable.Subscribe(extObserver, _edge.ExternalSubscriptionUri, state: null);
return(subscription);
}
