Retry stage


Some stream use cases may require retrying of requests after a failure response. squbs provides a RetryBidi Akka Streams stage to add a retry capability to streams that need to add retries for any failing requests.


Add the following dependency to your build.sbt or scala build file:

"org.squbs" %% "squbs-ext" % squbsVersion


The retry stage functionality is provided via a BidiFlow that can be connected to flows via the join operator. The retry BidiFlow will perform some specified maximum number of retries of any failures from downstream. A failure is determined by either a passed in failure decider function or a Failure, if a failure decider function is not supplied.

If all the retried attempts fail then the last failure for that request is emitted. The retry stage requires a Context to be carried around for each element. This is needed to uniquely identify each element (see Context to Unique Id Mapping section for more details).

val retryBidi = RetryBidi[String, Long](maxRetries = 10)
val flow = Flow[Try[String], Long].map(s => findAnEnglishWordThatStartWith(s))

Source("a" :: "b" :: "c" :: Nil)
final BidiFlow<Pair[String, Context], Pair[String, Context],
    Pair[Try[String], Context], Pair[Try[String], Context], NotUsed> retryBidi =

final Flow<String, String, NotUsed> flow =
    Flow.<String>create().map(s -> findAnEnglishWordThatStartWith(s));

Source.from(Arrays.asList("a", "b", "c"))
    .runWith(Sink.seq(), mat);

Context to Unique Id Mapping

The Context type itself might be used as a unique id. However, in many scenarios, Context contains more than the unique id itself or the unique id might be retrieved as a mapping from the Context. squbs allows different options to provide a unique id:

  • Context itself is a type that can be used as a unique id, e.g., Int, Long, java.util.UUID
  • Context extends UniqueId.Provider and implements def uniqueId
  • Context is wrapped with UniqueId.Envelope
  • Context is mapped to a unique id by calling a function

With the first three options, a unique id can be retrieved directly through the context.

For the last option, RetryBidi allows a function to be passed in as a parameter.


The following API can be used to pass a uniqueId mapper:

RetryBidi[In, Out, Context](maxRetries: Int, uniqueIdMapper: Context => Option[Any])

This BidiFlow can be joined with any flow that takes in a (In, Context) and outputs a (Out, Context).

case class MyContext(s: String, uuid: UUID)

val retryBidi = RetryBidi[String, String, MyContext](maxRetries = 2, (context: MyContext) => Some(context.uuid))
val flow = Flow[(String, MyContext)].mapAsyncUnordered(10) { elem =>
  (ref ? elem).mapTo[(String, MyContext)]

Source("a" :: "b" :: "c" :: Nil)
  .map( _ -> MyContext("dummy", UUID.randomUUID))

The following API is used to pass a uniqueId mapper:

public class RetryBidi {
    public static <In, Out, Context> BidiFlow<Pair<In, Context>,
                                              Pair<In, Context>,
                                              Pair<Try<Out>, Context>,
                                              Pair<Try<Out>, Context>,
    create(Long maxRetries, Function<Context, Optional<Object>> uniqueIdMapper);

This BidiFlow can be joined with any flow that takes in a akka.japi.Pair<In, Context> and outputs a akka.japi.Pair<Try<Out>, Context>.

class MyContext {
    private String s;
    private UUID uuid;

    public MyContext(String s, UUID uuid) {
        this.s = s;
        this.uuid = uuid;

    public UUID uuid() {
        return uuid;

final BidiFlow<Pair<String, MyContext>, Pair<String, MyContext>,
               Pair<Try<String>, MyContext>, Pair<Try<String>, MyContext>, NotUsed>
                retryBidi = RetryBidi.create(maxRetries, context -> Optional.of(context.uuid));

final Flow<Pair<String, MyContext>, Pair<String, MyContext>, NotUsed> flow =
        Flow.<Pair<String, MyContext>>create()
                .mapAsyncUnordered(10, elem -> ask(ref, elem, 5000))
                .map(elem -> (Pair<String, MyContext>)elem);

Source.from(Arrays.asList("a", "b", "c"))
        .map(s -> new Pair<>(s, new MyContext("dummy", UUID.randomUUID())))
        .runWith(Sink.seq(), mat);

Failure decider

By default, any Failure from the joined Flow is considered a failure for retry purposes. However, the Retry stage also accepts an optional failureDecider parameter to more finely control what elements from the joined Flow should actually be treated as failures that should be retried.


A function of type Try[Out] => Boolean can be provided via the failureDecider parameter. Below is an example where, along with any Failure message, Any response with failing http status code is also considered a failure:

val failureDecider = (tryResponse: Try[HttpResponse]) => tryResponse.isFailure || tryResponse.get().status().isFailure()

val retryBidi = RetryBidi[Request, Response, MyContext](maxRetries = 3, (context: MyContext) => Some(context.uuid), failureDecider = Option(failureDecider), OverflowStrategy.backpressure())


A Function<Try<Out>, Boolean> can be provided via Retry Optional failureDecider parameter to create method. Below is an example where, along with any Failure message, a Success of HttpResponse with status code 400 and above is also considered a failure:

final Function<Try<HttpResponse>, Optional<Object>> failureDecider =
tryResponse -> tryResponse.isFailure() || tryHttpResponse.get().status().isFailure());

final BidiFlow<Pair<HttpResponse, MyContext>, Pair<HttpResponse, MyContext>, Pair<Try<HttpResponse>, MyContext>,
    Pair<Try<HttpResponse>, MyContext>, NotUsed> retryFlow =
    RetryBidi.create(3L, Optional.of(failureDecider), OverflowStrategy.backpressure());