orElse

If the primary source completes without emitting any elements, the elements from the secondary source are emitted.

Fan-in operators

Signature

Source.orElseSource.orElse Flow.orElseFlow.orElse

Description

If the primary source completes without emitting any elements, the elements from the secondary source are emitted. If the primary source emits any elements the secondary source is cancelled.

Note that both sources are materialized directly and the secondary source is backpressured until it becomes the source of elements or is cancelled.

Signal errors downstream, regardless which of the two sources emitted the error.

Example

Scala

copysourceval source1 = Source(List("First source"))
val source2 = Source(List("Second source"))
val emptySource = Source.empty[String]

source1.orElse(source2).runWith(Sink.foreach(println))
// this will print "First source"

emptySource.orElse(source2).runWith(Sink.foreach(println))
// this will print "Second source"

Java

copysourceimport akka.stream.javadsl.Keep;
import akka.stream.javadsl.Source;
import akka.stream.javadsl.Sink;

import java.util.*;

Source<String, NotUsed> source1 = Source.from(Arrays.asList("First source"));
Source<String, NotUsed> source2 = Source.from(Arrays.asList("Second source"));
Source<String, NotUsed> emptySource = Source.empty();

source1.orElse(source2).runForeach(System.out::println, system);
// this will print "First source"

emptySource.orElse(source2).runForeach(System.out::println, system);
// this will print "Second source"

Reactive Streams semantics