public async Task <StreamSubscriptionHandle <T> > SubscribeAsync(
IAsyncObserver <T> observer,
StreamSequenceToken token,
StreamFilterPredicate filterFunc = null,
object filterData = null)
{
if (token != null && !IsRewindable)
{
throw new ArgumentNullException("token", "Passing a non-null token to a non-rewindable IAsyncObservable.");
}
if (logger.IsVerbose)
{
logger.Verbose("Subscribe Observer={0} Token={1}", observer, token);
}
await BindExtensionLazy();
IStreamFilterPredicateWrapper filterWrapper = null;
if (filterFunc != null)
{
filterWrapper = new FilterPredicateWrapperData(filterData, filterFunc);
}
if (logger.IsVerbose)
{
logger.Verbose("Subscribe - Connecting to Rendezvous {0} My GrainRef={1} Token={2}",
pubSub, myGrainReference, token);
}
GuidId subscriptionId = pubSub.CreateSubscriptionId(myGrainReference, stream.StreamId);
await pubSub.RegisterConsumer(subscriptionId, stream.StreamId, streamProviderName, myGrainReference, token, filterWrapper);
return(myExtension.SetObserver(subscriptionId, stream, observer, filterWrapper));
}
private async Task <StreamSubscriptionHandle <T> > SubscribeAsyncImpl(
IAsyncObserver <T> observer,
IAsyncBatchObserver <T> batchObserver,
StreamSequenceToken token,
string filterData = null)
{
if (token != null && !IsRewindable)
{
throw new ArgumentNullException("token", "Passing a non-null token to a non-rewindable IAsyncObservable.");
}
if (observer is GrainReference)
{
throw new ArgumentException("On-behalf subscription via grain references is not supported. Only passing of object references is allowed.", nameof(observer));
}
if (batchObserver is GrainReference)
{
throw new ArgumentException("On-behalf subscription via grain references is not supported. Only passing of object references is allowed.", nameof(batchObserver));
}
if (logger.IsEnabled(LogLevel.Debug))
{
logger.Debug("Subscribe Token={Token}", token);
}
await BindExtensionLazy();
if (logger.IsEnabled(LogLevel.Debug))
{
logger.Debug("Subscribe - Connecting to Rendezvous {0} My GrainRef={1} Token={2}",
pubSub, myGrainReference, token);
}
GuidId subscriptionId = pubSub.CreateSubscriptionId(stream.InternalStreamId, myGrainReference);
// Optimistic Concurrency:
// In general, we should first register the subsription with the pubsub (pubSub.RegisterConsumer)
// and only if it succeeds store it locally (myExtension.SetObserver).
// Basicaly, those 2 operations should be done as one atomic transaction - either both or none and isolated from concurrent reads.
// BUT: there is a distributed race here: the first msg may arrive before the call is awaited
// (since the pubsub notifies the producer that may immideately produce)
// and will thus not find the subriptionHandle in the extension, basically violating "isolation".
// Therefore, we employ Optimistic Concurrency Control here to guarantee isolation:
// we optimisticaly store subscriptionId in the handle first before calling pubSub.RegisterConsumer
// and undo it in the case of failure.
// There is no problem with that we call myExtension.SetObserver too early before the handle is registered in pub sub,
// since this subscriptionId is unique (random Guid) and no one knows it anyway, unless successfully subscribed in the pubsub.
var subriptionHandle = myExtension.SetObserver(subscriptionId, stream, observer, batchObserver, token, filterData);
try
{
await pubSub.RegisterConsumer(subscriptionId, stream.InternalStreamId, myGrainReference, filterData);
return(subriptionHandle);
}
catch (Exception)
{
// Undo the previous call myExtension.SetObserver.
myExtension.RemoveObserver(subscriptionId);
throw;
}
}