Akka.Net Streams - Source stops pulling elements when buffer throws error

Question

I've been playing a little with the streams extension package for Akka.Net and noticed this error at attempting to combine buffer and throttle methods:

using (var system = ActorSystem.Create("test-system"))
using (var materializer = system.Materializer(GetSettings(system)))
{
            int index = 0;
            var sink = Sink.ActorRefWithAck<KeyValue>(
                system.ActorOf<Writer>(), 
                new OnInitMessage(), 
                new OnAcknowledgeMessage(), 
                OnComplete.Instance, 
                exception => new OnError(exception));

            ServiceBusSource
                .Create(client, message =>
                {
                    var json = new StreamReader(message.GetBody<Stream>(), Encoding.UTF8).ReadToEnd();
                    var result = JsonConvert.DeserializeObject<KeyValue>(json);

                    message.Complete();

                    return result;
                })
                .WithLogger(system, entity => $"{entity.Key} => {entity.Value}")
                .Buffer(1, OverflowStrategy.Fail)
                .Throttle(1, TimeSpan.FromSeconds(5), 3, ThrottleMode.Shaping)
                .ToMaterialized(sink, Keep.Right)
                .Run(materializer);

            Console.ReadLine();
}

I'm using ServiceBusSource from Alpakka These are the packages I'm referencing:

• Akka.Streams: 1.3.1
• Akka.Streams.Azure.ServiceBus: 0.1.0
• WindowsAzure.ServiceBus: 4.1.3

I'm intentionally making it fail in order to see how behaves BUT, after failing from buffer's strategy, the stream completes and no more elements are being pulled.

KeyValue.cs

public class KeyValue
{
    public int Id { get; set; }

    public string Key { get; set; }

    public string Value { get; set; }

    public DateTime Produced { get; set; }

    public DateTime Emitted { get; set; }

    public override string ToString()
    {
        return $"[{Produced}] - [{Emitted}] => {Id} {Key}:{Value}";
    }
}

GetSettings Method:

ActorMaterializerSettings GetSettings(ActorSystem system)
        {
            return ActorMaterializerSettings.Create(system)
                .WithSupervisionStrategy(cause =>
                {
                    system.Log.Error(cause, "Failed");
                    return Directive.Resume;
                });
        }

Answer 1

There are several ways of handling errors inside of a stream - most of them described in docs:

1. Use Recover to make a fallback event from error.
2. Use RecoverWithRetries to allow to redirect to a different stream upon error.
3. Use Restart.WithBackoff to rebuild a retry stream after exponential backoff delay.
4. Use WithSupervisionStrategy - which is a very limited option, as it works only on stages that refer to it explicitly (as explained in docs).

Your case is by design - when you use OverflowStrategy.Fail it means, that once overflow is reached, an error will be produced. Reaction of most of the akka stages is to close stream immediately upon failure.