Error Handling in Streams

When a stage in a stream fails this will normally lead to the entire stream being torn down. Each of the stages downstream gets informed about the failure and each upstream stage sees a cancellation.

In many cases you may want to avoid complete stream failure, this can be done in a few different ways:

• Recover to emit a final element then complete the stream normally on upstream failure
• RecoverWithRetries to create a new upstream and start consuming from that on failure
• Restarting sections of the stream after a backoff
• Using a supervision strategy for stages that support it

In addition to these built in tools for error handling, a common pattern is to wrap the stream inside an actor, and have the actor restart the entire stream on failure.

Recover

Recover allows you to emit a final element and then complete the stream on an upstream failure. Deciding which exceptions should be recovered is done through a delegate. If an exception does not have a matching case the stream is failed.

Recovering can be useful if you want to gracefully complete a stream on failure while letting downstream know that there was a failure.

Source.From(Enumerable.Range(0, 6)).Select(n =>
    {
        if (n < 5)
            return n.ToString();

        throw new ArithmeticException("Boom!");
    })
    .Recover(exception =>
    {
        if (exception is ArithmeticException)
            return new Option<string>("stream truncated");
        return Option<string>.None;
    })
    .RunForeach(Console.WriteLine, materializer);

This will output:

0
1
2
3
4
stream truncated

Recover with Retries

RecoverWithRetries allows you to put a new upstream in place of the failed one, recovering stream failures up to a specified maximum number of times.

Deciding which exceptions should be recovered is done through a delegate. If an exception does not have a matching case the stream is failed.

var planB = Source.From(new List<string> {"five", "six", "seven", "eight"});

Source.From(Enumerable.Range(0, 10)).Select(n =>
    {
        if (n < 5)
            return n.ToString();

        throw new ArithmeticException("Boom!");
    })
    .RecoverWithRetries(attempts: 1, partialFunc: exception =>
    {
        if (exception is ArithmeticException)
            return planB;
        return null;
    })
    .RunForeach(Console.WriteLine, materializer);

This will output:

0
1
2
3
4
five
six
seven
eight

Delayed Restarts with a Backoff Stage

Just as Akka provides the backoff supervision pattern for actors, Akka streams also provides a RestartSource, RestartSink and RestartFlow for implementing the so-called exponential backoff supervision strategy, starting a stage again when it fails or completes, each time with a growing time delay between restarts.

This pattern is useful when the stage fails or completes because some external resource is not available and we need to give it some time to start-up again. One of the prime examples when this is useful is when a WebSocket connection fails due to the HTTP server it's running on going down, perhaps because it is overloaded. By using an exponential backoff, we avoid going into a tight reconnect look, which both gives the HTTP server some time to recover, and it avoids using needless resources on the client side.

The various restart shapes mentioned all expect an Akka.Stream.RestartSettings which configures the restart behavior.

Configurable parameters are:

• minBackoff is the initial duration until the underlying stream is restarted
• maxBackoff caps the exponential backoff
• randomFactor allows addition of a random delay following backoff calculation
• maxRestarts caps the total number of restarts
• maxRestartsWithin sets a time-frame during which restarts are counted towards the same total for maxRestarts

The following snippet shows how to create a backoff supervisor using Akka.Streams.Dsl.RestartSource which will supervise the given Source. The Source in this case is a HttpResponseMessage, produced by HttpCLient. If the stream fails or completes at any point, the request will be made again, in increasing intervals of 3, 6, 12, 24 and finally 30 seconds (at which point it will remain capped due to the maxBackoff parameter):

var httpClient = new HttpClient();

var settings = RestartSettings.Create(
    minBackoff: TimeSpan.FromSeconds(3), 
    maxBackoff: TimeSpan.FromSeconds(30),
    randomFactor: 0.2 // adds 20% "noise" to vary the intervals slightly
).WithMaxRestarts(20, TimeSpan.FromMinutes(5)); // limits the amount of restarts to 20 within 5 minutes

var restartSource = RestartSource.WithBackoff(() =>
{
    // Create a source from a task
    return Source.FromTask(
        httpClient.GetAsync("http://example.com/eventstream") // Make a single request
    )
    .Select(c => c.Content.ReadAsStringAsync())
    .Select(c => c.Result);
}, settings);

Using a randomFactor to add a little bit of additional variance to the backoff intervals is highly recommended, in order to avoid multiple streams re-start at the exact same point in time, for example because they were stopped due to a shared resource such as the same server going down and re-starting after the same configured interval. By adding additional randomness to the re-start intervals the streams will start in slightly different points in time, thus avoiding large spikes of traffic hitting the recovering server or other resource that they all need to contact.

The above RestartSource will never terminate unless the Sink it's fed into cancels. It will often be handy to use it in combination with a KillSwitch, so that you can terminate it when needed:

var killSwitch = restartSource
    .ViaMaterialized(KillSwitches.Single<string>(), Keep.Right)
    .ToMaterialized(Sink.ForEach<string>(evt => Console.WriteLine($"Got event: {evt}")), Keep.Left)
    .Run(Materializer);

DoSomethingElse();

killSwitch.Shutdown();

Sinks and flows can also be supervised, using Akka.Streams.Dsl.RestartSink and Akka.Streams.Dsl.RestartFlow. The RestartSink is restarted when it cancels, while the RestartFlow is restarted when either the in port cancels, the out port completes, or the out port sends an error.

Supervision Strategies

Note

The stages that support supervision strategies are explicitly documented to do so, if there is nothing in the documentation of a stage saying that it adheres to the supervision strategy it means it fails rather than applies supervision..

The error handling strategies are inspired by actor supervision strategies, but the semantics have been adapted to the domain of stream processing. The most important difference is that supervision is not automatically applied to stream stages but instead something that each stage has to implement explicitly.

For many stages it may not even make sense to implement support for supervision strategies, this is especially true for stages connecting to external technologies where for example a failed connection will likely still fail if a new connection is tried immediately.

For stages that do implement supervision, the strategies for how to handle exceptions from processing stream elements can be selected when materializing the stream through use of an attribute.

There are three ways to handle exceptions from application code:

• Stop - The stream is completed with failure.
• Resume - The element is dropped and the stream continues.
• Restart - The element is dropped and the stream continues after restarting the stage. Restarting a stage means that any accumulated state is cleared. This is typically performed by creating a new instance of the stage.

By default the stopping strategy is used for all exceptions, i.e. the stream will be completed with failure when an exception is thrown.

var source = Source.From(Enumerable.Range(0, 6)).Select(x => 100/x);
var result = source.RunWith(Sink.Aggregate<int, int>(0, (sum, i) => sum + i), materializer);
// division by zero will fail the stream and the
// result here will be a Task completed with Failure(DivideByZeroException)

The default supervision strategy for a stream can be defined on the settings of the materializer.

Decider decider = cause => cause is DivideByZeroException
    ? Directive.Resume
    : Directive.Stop;
var settings = ActorMaterializerSettings.Create(system).WithSupervisionStrategy(decider);
var materializer = system.Materializer(settings);

var source = Source.From(Enumerable.Range(0, 6)).Select(x => 100/x);
var result = source.RunWith(Sink.Aggregate<int, int>(0, (sum, i) => sum + i), materializer);
// the element causing division by zero will be dropped
// result here will be a Task completed with Success(228)

Here you can see that all DivideByZeroException will resume the processing, i.e. the elements that cause the division by zero are effectively dropped.

Note

Be aware that dropping elements may result in deadlocks in graphs with cycles, as explained in Graph cycles, liveness and deadlocks.

The supervision strategy can also be defined for all operators of a flow.

Decider decider = cause => cause is DivideByZeroException
    ? Directive.Resume
    : Directive.Stop;

var flow = Flow.Create<int>()
    .Where(x => 100 / x < 50)
    .Select(x => 100 / (5 - x))
    .WithAttributes(ActorAttributes.CreateSupervisionStrategy(decider));
var source = Source.From(Enumerable.Range(0, 6)).Via(flow);
var result = source.RunWith(Sink.Aggregate<int, int>(0, (sum, i) => sum + i), materializer);
// the elements causing division by zero will be dropped
// result here will be a Future completed with Success(150)

Restart works in a similar way as Resume with the addition that accumulated state, if any, of the failing processing stage will be reset.

Decider decider = cause => cause is ArgumentException
    ? Directive.Restart
    : Directive.Stop;

var flow = Flow.Create<int>()
    .Scan(0, (acc, x) =>
    {
      if(x < 0)
            throw new ArgumentException("negative not allowed");
        return acc + x;
    })
    .WithAttributes(ActorAttributes.CreateSupervisionStrategy(decider));
var source = Source.From(new [] {1,3,-1,5,7}).Via(flow);
var result = source.Limit(1000).RunWith(Sink.Seq<int>(), materializer);
// the negative element cause the scan stage to be restarted,
// i.e. start from 0 again
// result here will be a Task completed with Success(List(0, 1, 4, 0, 5, 12))

Errors From SelectAsync

Stream supervision can also be applied to the tasks of SelectAsync and SelectAsyncUnordered even if such failures happen in the task rather than inside the stage itself. .

Let's say that we use an external service to lookup email addresses and we would like to discard those that cannot be found.

We start with the tweet stream of authors:

var authors = tweets
    .Where(t => t.HashTags.Contains("Akka.Net"))
    .Select(t => t.Author);

Assume that we can lookup their email address using:

Task<string> LookupEmail(string handle)

The Task is completed with Failure if the email is not found.

Transforming the stream of authors to a stream of email addresses by using the LookupEmail service can be done with SelectAsync and we use Deciders.ResumingDecider to drop unknown email addresses:

var emailAddresses = authors.Via(
    Flow.Create<Author>()
        .SelectAsync(4, author => AddressSystem.LookupEmail(author.Handle))
        .WithAttributes(ActorAttributes.CreateSupervisionStrategy(Deciders.ResumingDecider)));

If we would not use Resume the default stopping strategy would complete the stream with failure on the first Task that was completed with Failure.