Testing streams
Dependency¶
To use Pekko Stream TestKit, add the module to your project:
val PekkoVersion = "1.1.3"
libraryDependencies += "org.apache.pekko" %% "pekko-stream-testkit" % PekkoVersion % Test
<properties>
<scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.apache.pekko</groupId>
<artifactId>pekko-bom_${scala.binary.version}</artifactId>
<version>1.1.3</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
<dependencies>
<dependency>
<groupId>org.apache.pekko</groupId>
<artifactId>pekko-stream-testkit_${scala.binary.version}</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
def versions = [
ScalaBinary: "2.13"
]
dependencies {
implementation platform("org.apache.pekko:pekko-bom_${versions.ScalaBinary}:1.1.3")
testImplementation "org.apache.pekko:pekko-stream-testkit_${versions.ScalaBinary}"
}
Introduction¶
Verifying behavior of Pekko Stream sources, flows and sinks can be done using various code patterns and libraries. Here we will discuss testing these elements using:
- simple sources, sinks and flows;
- sources and sinks in combination with
TestProbe
from thepekko-testkit
module; - sources and sinks specifically crafted for writing tests from the
pekko-stream-testkit
module.
It is important to keep your data processing pipeline as separate sources, flows and sinks. This makes them testable by wiring them up to other sources or sinks, or some test harnesses that pekko-testkit
or pekko-stream-testkit
provide.
Built-in sources, sinks and operators¶
Testing a custom sink can be as simple as attaching a source that emits elements from a predefined collection, running a constructed test flow and asserting on the results that sink produced. Here is an example of a test for a sink:
sourceval sinkUnderTest =
Flow[Int].map(_ * 2).toMat(Sink.fold(0)(_ + _))(Keep.right)
val future = Source(1 to 4).runWith(sinkUnderTest)
val result = Await.result(future, 3.seconds)
assert(result == 20)
sourcefinal Sink<Integer, CompletionStage<Integer>> sinkUnderTest =
Flow.of(Integer.class)
.map(i -> i * 2)
.toMat(Sink.fold(0, (agg, next) -> agg + next), Keep.right());
final CompletionStage<Integer> future =
Source.from(Arrays.asList(1, 2, 3, 4)).runWith(sinkUnderTest, system);
final Integer result = future.toCompletableFuture().get(3, TimeUnit.SECONDS);
assertEquals(20, result.intValue());
The same strategy can be applied for sources as well. In the next example we have a source that produces an infinite stream of elements. Such source can be tested by asserting that first arbitrary number of elements hold some condition. Here the take
operator and Sink.seq
are very useful.
sourceimport system.dispatcher
import org.apache.pekko.pattern.pipe
val sourceUnderTest = Source.repeat(1).map(_ * 2)
val future = sourceUnderTest.take(10).runWith(Sink.seq)
val result = Await.result(future, 3.seconds)
assert(result == Seq.fill(10)(2))
sourcefinal Source<Integer, NotUsed> sourceUnderTest = Source.repeat(1).map(i -> i * 2);
final CompletionStage<List<Integer>> future =
sourceUnderTest.take(10).runWith(Sink.seq(), system);
final List<Integer> result = future.toCompletableFuture().get(3, TimeUnit.SECONDS);
assertEquals(Collections.nCopies(10, 2), result);
When testing a flow we need to attach a source and a sink. As both stream ends are under our control, we can choose sources that tests various edge cases of the flow and sinks that ease assertions.
sourceval flowUnderTest = Flow[Int].takeWhile(_ < 5)
val future = Source(1 to 10).via(flowUnderTest).runWith(Sink.fold(Seq.empty[Int])(_ :+ _))
val result = Await.result(future, 3.seconds)
assert(result == (1 to 4))
sourcefinal Flow<Integer, Integer, NotUsed> flowUnderTest =
Flow.of(Integer.class).takeWhile(i -> i < 5);
final CompletionStage<Integer> future =
Source.from(Arrays.asList(1, 2, 3, 4, 5, 6))
.via(flowUnderTest)
.runWith(Sink.fold(0, (agg, next) -> agg + next), system);
final Integer result = future.toCompletableFuture().get(3, TimeUnit.SECONDS);
assertEquals(10, result.intValue());
TestKit¶
Pekko Stream offers integration with Actors out of the box. This support can be used for writing stream tests that use familiar TestProbe
from the pekko-testkit
API.
One of the more straightforward tests would be to materialize stream to a Future
and then use pipe
pattern to pipe the result of that future to the probe.
sourceimport system.dispatcher
import org.apache.pekko.pattern.pipe
val sourceUnderTest = Source(1 to 4).grouped(2)
val probe = TestProbe()
sourceUnderTest.runWith(Sink.seq).pipeTo(probe.ref)
probe.expectMsg(3.seconds, Seq(Seq(1, 2), Seq(3, 4)))
sourcefinal Source<List<Integer>, NotUsed> sourceUnderTest =
Source.from(Arrays.asList(1, 2, 3, 4)).grouped(2);
final TestKit probe = new TestKit(system);
final CompletionStage<List<List<Integer>>> future =
sourceUnderTest.grouped(2).runWith(Sink.head(), system);
org.apache.pekko.pattern.Patterns.pipe(future, system.dispatcher()).to(probe.getRef());
probe.expectMsg(Duration.ofSeconds(3), Arrays.asList(Arrays.asList(1, 2), Arrays.asList(3, 4)));
Instead of materializing to a future, we can use a Sink.actorRef
that sends all incoming elements to the given ActorRef
. Now we can use assertion methods on TestProbe
and expect elements one by one as they arrive. We can also assert stream completion by expecting for onCompleteMessage
which was given to Sink.actorRef
.
sourcecase object Tick
val sourceUnderTest = Source.tick(0.seconds, 200.millis, Tick)
val probe = TestProbe()
val cancellable = sourceUnderTest
.to(Sink.actorRef(probe.ref, onCompleteMessage = "completed", onFailureMessage = _ => "failed"))
.run()
probe.expectMsg(1.second, Tick)
probe.expectNoMessage(100.millis)
probe.expectMsg(3.seconds, Tick)
cancellable.cancel()
probe.expectMsg(3.seconds, "completed")
sourcefinal Source<Tick, Cancellable> sourceUnderTest =
Source.tick(Duration.ZERO, Duration.ofMillis(200), Tick.TOCK);
final TestKit probe = new TestKit(system);
final Cancellable cancellable =
sourceUnderTest.to(Sink.actorRef(probe.getRef(), Tick.COMPLETED)).run(system);
probe.expectMsg(Duration.ofSeconds(3), Tick.TOCK);
probe.expectNoMessage(Duration.ofMillis(100));
probe.expectMsg(Duration.ofSeconds(3), Tick.TOCK);
cancellable.cancel();
probe.expectMsg(Duration.ofSeconds(3), Tick.COMPLETED);
Similarly to Sink.actorRef
that provides control over received elements, we can use Source.actorRef
and have full control over elements to be sent.
sourceval sinkUnderTest = Flow[Int].map(_.toString).toMat(Sink.fold("")(_ + _))(Keep.right)
val (ref, future) = Source
.actorRef(
completionMatcher = {
case Done =>
CompletionStrategy.draining
},
// Never fail the stream because of a message:
failureMatcher = PartialFunction.empty,
bufferSize = 8,
overflowStrategy = OverflowStrategy.fail)
.toMat(sinkUnderTest)(Keep.both)
.run()
ref ! 1
ref ! 2
ref ! 3
ref ! Done
val result = Await.result(future, 3.seconds)
assert(result == "123")
sourcefinal Sink<Integer, CompletionStage<String>> sinkUnderTest =
Flow.of(Integer.class)
.map(i -> i.toString())
.toMat(Sink.fold("", (agg, next) -> agg + next), Keep.right());
final Pair<ActorRef, CompletionStage<String>> refAndCompletionStage =
Source.<Integer>actorRef(
elem -> {
// complete stream immediately if we send it Done
if (elem == Done.done()) return Optional.of(CompletionStrategy.immediately());
else return Optional.empty();
},
// never fail the stream because of a message
elem -> Optional.empty(),
8,
OverflowStrategy.fail())
.toMat(sinkUnderTest, Keep.both())
.run(system);
final ActorRef ref = refAndCompletionStage.first();
final CompletionStage<String> future = refAndCompletionStage.second();
ref.tell(1, ActorRef.noSender());
ref.tell(2, ActorRef.noSender());
ref.tell(3, ActorRef.noSender());
ref.tell(Done.getInstance(), ActorRef.noSender());
final String result = future.toCompletableFuture().get(1, TimeUnit.SECONDS);
assertEquals("123", result);
Streams TestKit¶
You may have noticed various code patterns that emerge when testing stream pipelines. Pekko Stream has a separate pekko-stream-testkit
module that provides tools specifically for writing stream tests. This module comes with two main components that are TestSource
and TestSink
which provide sources and sinks that materialize to probes that allow fluent API.
Using the TestKit¶
A sink returned by TestSink.probe
allows manual control over demand and assertions over elements coming downstream.
sourceval sourceUnderTest = Source(1 to 4).filter(_ % 2 == 0).map(_ * 2)
sourceUnderTest.runWith(TestSink[Int]()).request(2).expectNext(4, 8).expectComplete()
sourcefinal Source<Integer, NotUsed> sourceUnderTest =
Source.from(Arrays.asList(1, 2, 3, 4)).filter(elem -> elem % 2 == 0).map(elem -> elem * 2);
sourceUnderTest
.runWith(TestSink.probe(system), system)
.request(2)
.expectNext(4, 8)
.expectComplete();
A source returned by TestSource.probe
can be used for asserting demand or controlling when stream is completed or ended with an error.
sourceval sinkUnderTest = Sink.cancelled
TestSource.probe[Int].toMat(sinkUnderTest)(Keep.left).run().expectCancellation()
sourcefinal Sink<Integer, NotUsed> sinkUnderTest = Sink.cancelled();
TestSource.<Integer>probe(system)
.toMat(sinkUnderTest, Keep.left())
.run(system)
.expectCancellation();
You can also inject exceptions and test sink behavior on error conditions.
sourceval sinkUnderTest = Sink.head[Int]
val (probe, future) = TestSource.probe[Int].toMat(sinkUnderTest)(Keep.both).run()
probe.sendError(new Exception("boom"))
assert(future.failed.futureValue.getMessage == "boom")
sourcefinal Sink<Integer, CompletionStage<Integer>> sinkUnderTest = Sink.head();
final Pair<TestPublisher.Probe<Integer>, CompletionStage<Integer>> probeAndCompletionStage =
TestSource.<Integer>probe(system).toMat(sinkUnderTest, Keep.both()).run(system);
final TestPublisher.Probe<Integer> probe = probeAndCompletionStage.first();
final CompletionStage<Integer> future = probeAndCompletionStage.second();
probe.sendError(new Exception("boom"));
ExecutionException exception =
Assert.assertThrows(
ExecutionException.class, () -> future.toCompletableFuture().get(3, TimeUnit.SECONDS));
assertEquals("boom", exception.getCause().getMessage());
Test source and sink can be used together in combination when testing flows.
sourceval flowUnderTest = Flow[Int].mapAsyncUnordered(2) { sleep =>
pattern.after(10.millis * sleep, using = system.scheduler)(Future.successful(sleep))
}
val (pub, sub) = TestSource.probe[Int].via(flowUnderTest).toMat(TestSink[Int]())(Keep.both).run()
sub.request(n = 3)
pub.sendNext(3)
pub.sendNext(2)
pub.sendNext(1)
sub.expectNextUnordered(1, 2, 3)
pub.sendError(new Exception("Power surge in the linear subroutine C-47!"))
val ex = sub.expectError()
assert(ex.getMessage.contains("C-47"))
sourcefinal Flow<Integer, Integer, NotUsed> flowUnderTest =
Flow.of(Integer.class)
.mapAsyncUnordered(
2,
sleep ->
org.apache.pekko.pattern.Patterns.after(
Duration.ofMillis(10),
system.scheduler(),
system.dispatcher(),
() -> CompletableFuture.completedFuture(sleep)));
final Pair<TestPublisher.Probe<Integer>, TestSubscriber.Probe<Integer>> pubAndSub =
TestSource.<Integer>probe(system)
.via(flowUnderTest)
.toMat(TestSink.<Integer>probe(system), Keep.both())
.run(system);
final TestPublisher.Probe<Integer> pub = pubAndSub.first();
final TestSubscriber.Probe<Integer> sub = pubAndSub.second();
sub.request(3);
pub.sendNext(3);
pub.sendNext(2);
pub.sendNext(1);
sub.expectNextUnordered(1, 2, 3);
pub.sendError(new Exception("Power surge in the linear subroutine C-47!"));
final Throwable ex = sub.expectError();
assertTrue(ex.getMessage().contains("C-47"));
Fuzzing Mode¶
For testing, it is possible to enable a special stream execution mode that exercises concurrent execution paths more aggressively (at the cost of reduced performance) and therefore helps exposing race conditions in tests. To enable this setting add the following line to your configuration:
pekko.stream.materializer.debug.fuzzing-mode = on
Never use this setting in production or benchmarks. This is a testing tool to provide more coverage of your code during tests, but it reduces the throughput of streams. A warning message will be logged if you have this setting enabled.