pekko/docs/src/main/paradox/testing.md

Testing Classic Actors

@@includeincludes.md { #actor-api } For the new API see @reftesting.

Module info

To use Pekko Testkit, you must add the following dependency in your project:

@@dependency[sbt,Maven,Gradle] { bomGroup=org.apache.pekko bomArtifact=pekko-bom_scala.binary.version bomVersionSymbols=PekkoVersion symbol1=PekkoVersion value1="$pekko.version$" group="org.apache.pekko" artifact="pekko-testkit_$scala.binary.version$" version=PekkoVersion scope="test" }

@@project-info{ projectId="testkit" }

Introduction

As with any piece of software, automated tests are a very important part of the development cycle. The actor model presents a different view on how units of code are delimited and how they interact, which influences how to perform tests.

Pekko comes with a dedicated module pekko-testkit for supporting tests.

Asynchronous Testing: TestKit

Testkit allows you to test your actors in a controlled but realistic environment. The definition of the environment depends very much on the problem at hand and the level at which you intend to test, ranging from simple checks to full system tests.

The minimal setup consists of the test procedure, which provides the desired stimuli, the actor under test, and an actor receiving replies. Bigger systems replace the actor under test with a network of actors, apply stimuli at varying injection points and arrange results to be sent from different emission points, but the basic principle stays the same in that a single procedure drives the test.

The TestKit class contains a collection of tools which makes this common task easy.

Scala

@@snip PlainWordSpec.scala { #plain-spec }

Java

@@snip TestKitSampleTest.java { #fullsample }

The TestKit contains an actor named testActor which is the entry point for messages to be examined with the various expectMsg... assertions detailed below. @scala[When mixing in the trait ImplicitSender this test actor is implicitly used as sender reference when dispatching messages from the test procedure.] @java[The test actor’s reference is obtained using the getRef() method as demonstrated above.] The testActor may also be passed to other actors, as usual, usually subscribing it as notification listener. There is a whole set of examination methods, e.g. receiving all consecutive messages matching certain criteria, receiving a whole sequence of fixed messages or classes, receiving nothing for some time, etc.

The ActorSystem passed to the constructor of TestKit is accessible via the @scala[system member]@java[getSystem() method].

@@@ note

Remember to shut down the actor system after the test is finished (also in case of failure) so that all actors—including the test actor—are stopped.

@@@

Built-In Assertions

The above-mentioned @scala[expectMsg]@java[expectMsgEquals] is not the only method for formulating assertions concerning received messages, the full set is this:

Scala

@@snip TestkitDocSpec.scala { #test-expect }

Java

@@snip TestKitDocTest.java { #test-expect }

In these examples, the maximum durations you will find mentioned below are left out, in which case they use the default value from the configuration item pekko.test.single-expect-default which itself defaults to 3 seconds (or they obey the innermost enclosing Within as detailed @ref:below). The full signatures are:

• @scala[expectMsg[T](d: Duration, msg: T): T]@java[public <T> T expectMsgEquals(Duration max, T msg)] The given message object must be received within the specified time; the object will be returned.
• @scala[expectMsgPF[T](d: Duration)(pf: PartialFunction[Any, T]): T]@java[public <T> T expectMsgPF(Duration max, String hint, Function<Object, T> f)] Within the given time, a message must be received and the given @scala[partial] function must be defined for that message; the result from applying the @scala[partial] function to the received message is returned. @scala[The duration may be left unspecified (empty parentheses are required in this case) to use the deadline from the innermost enclosing @ref:within block instead.]
• @scala[expectMsgClass[T](d: Duration, c: Class[T]): T]@java[public <T> T expectMsgClass(Duration max, Class<T> c)] An object which is an instance of the given Class must be received within the allotted time frame; the object will be returned. Note that this does a conformance check; if you need the class to be equal, @scala[have a look at expectMsgAllClassOf with a single given class argument]@java[you need to verify that afterwards].

@@@ div { .group-scala }

• expectMsgType[T: Manifest](d: Duration) An object which is an instance of the given type (after erasure) must be received within the allotted time frame; the object will be returned. This method is approximately equivalent to expectMsgClass(implicitly[ClassTag[T]].runtimeClass).

@@@

• @scala[expectMsgAnyOf[T](d: Duration, obj: T*): T]@java[public Object expectMsgAnyOf(Duration max, Object... msg)] An object must be received within the given time, and it must be equal ( compared with @scala[==]@java[equals()]) to at least one of the passed reference objects; the received object will be returned.
• @scala[expectMsgAnyClassOf[T](d: Duration, obj: Class[_ <: T]*): T]@java[public <T> T expectMsgAnyClassOf(Duration max, Class<? extends T>... c)] An object must be received within the given time, and it must be an instance of at least one of the supplied Class objects; the received object will be returned. Note that this does a conformance check, if you need the class to be equal you need to verify that afterwards.
• @scala[expectMsgAllOf[T](d: Duration, obj: T*): Seq[T]]@java[public List<Object> expectMsgAllOf(Duration max, Object... msg)] Several objects matching the size of the supplied object array must be received within the given time, and for each of the given objects there must exist at least one among the received ones which equals (compared with @scala[==]@java[equals()]) it. The full sequence of received objects is returned in the order received.

@@@ div { .group-scala }

• expectMsgAllClassOf[T](d: Duration, c: Class[_ <: T]*): Seq[T] Several objects matching the size of the supplied Class array must be received within the given time, and for each of the given classes there must exist at least one among the received objects whose class equals (compared with ==) it (this is not a conformance check). The full sequence of received objects is returned.
• expectMsgAllConformingOf[T](d: Duration, c: Class[_ <: T]*): Seq[T] Several objects matching the size of the supplied Class array must be received within the given time, and for each of the given classes there must exist at least one among the received objects which is an instance of this class. The full sequence of received objects is returned.

@@@

• @scala[expectNoMessage(d: Duration)]@java[public void expectNoMessage(Duration max)] No message must be received within the given time. This also fails if a message has been received before calling this method which has not been removed from the queue using one of the other methods.
• @scala[receiveN(n: Int, d: Duration): Seq[AnyRef]]@java[List<Object> receiveN(int n, Duration max)] n messages must be received within the given time; the received messages are returned.

@@@ div { .group-scala }

• fishForMessage(max: Duration, hint: String)(pf: PartialFunction[Any, Boolean]): Any Keep receiving messages as long as the time is not used up and the partial function matches and returns false. Returns the message received for which it returned true or throws an exception, which will include the provided hint for easier debugging.

@@@

In addition to message reception assertions there are also methods which help with message flows:

@@@ div { .group-scala }

• receiveOne(d: Duration): AnyRef Tries to receive one message for at most the given time interval and returns null in case of failure. If the given Duration is zero, the call is non-blocking (polling mode).

@@@

• @scala[receiveWhile[T](max: Duration, idle: Duration, messages: Int)(pf: PartialFunction[Any, T]): Seq[T]]@java[public <T> List<T> receiveWhile(Duration max, Duration idle, Int messages, Function<Object, T> f)] Collect messages as long as

  • they are matching the given partial function
  • the given time interval is not used up
  • the next message is received within the idle timeout
  • the number of messages has not yet reached the maximum All collected messages are returned. @scala[The maximum duration defaults to the time remaining in the innermost enclosing @ref:within block and the idle duration defaults to infinity (thereby disabling the idle-timeout feature). The number of expected messages defaults to Int.MaxValue, which effectively disables this limit.]

• @scala[awaitCond(p: => Boolean, max: Duration, interval: Duration)]@java[public void awaitCond(Duration max, Duration interval, Supplier<Boolean> p)] Poll the given condition every interval until it returns true or the max duration is used up. @scala[The interval defaults to 100 ms and the maximum defaults to the time remaining in the innermost enclosing within block.]

• @scala[awaitAssert(a: => Any, max: Duration, interval: Duration)]@java[public void awaitAssert(Duration max, Duration interval, Supplier<Object> a)] Poll the given assert function every interval until it does not throw an exception or the max duration is used up. If the timeout expires the last exception is thrown. @scala[The interval defaults to 100 ms and the maximum defaults to the time remaining in the innermost enclosing @ref:within block. The interval defaults to 100 ms and the maximum defaults to the time remaining in the innermost enclosing @ref:within block.] Return an arbitrary value that would be returned from awaitAssert if successful, if not interested in such value you can return null.

• @scala[ignoreMsg(pf: PartialFunction[AnyRef, Boolean])]@java[public void ignoreMsg(Function<Object, Boolean> f)] @scala[ignoreMsg]@java[public void ignoreMsg()] @java[There are also cases where not all messages sent to the test kit are actually relevant to the test, but removing them would mean altering the actors under test. For this purpose it is possible to ignore certain messages.] @scala[The internal testActor contains a partial function for ignoring messages: it will only enqueue messages which do not match the function or for which the function returns false. This function can be set and reset using the methods given above; each invocation replaces the previous function, they are not composed. This feature is useful e.g. when testing a logging system, where you want to ignore regular messages and are only interested in your specific ones.]

Expecting Log Messages

Since an integration test does not allow observing the internal processing of the participating actors, verifying expected exceptions cannot be done directly. Instead, use the logging system for this purpose: replacing the normal event handler with the TestEventListener and using an EventFilter allows assertions on log messages, including those which are generated by exceptions:

Scala

@@snip TestkitDocSpec.scala { #event-filter }

Java

@@snip TestKitDocTest.java { #test-event-filter }

If the number of occurrences is specific—as demonstrated above—then intercept will block until that number of matching messages have been received or the timeout configured in pekko.test.filter-leeway is used up (time starts counting after the passed-in block of code returns). In case of a timeout the test fails.

@@@ note

Be sure to exchange the default logger with the TestEventListener in your application.conf to enable this function:

pekko.loggers = [org.apache.pekko.testkit.TestEventListener]

@@@

Overriding behavior

Sometimes, you want to 'hook into' your actor to be able to test some internals. Usually, it is better to test an actors' external interface, but for example if you want to test timing-sensitive behavior this can come in handy. Say for instance you want to test an actor that schedules a task:

Scala

@@snip TestkitDocSpec.scala { #timer }

Java

@@snip TestKitDocTest.java { #timer }

You can override the method that does the scheduling in your test:

Scala

@@snip TestkitDocSpec.scala { #timer-test }

Java

@@snip TestKitDocTest.java { #timer-test }

Timing Assertions

Another important part of functional testing concerns timing: certain events must not happen immediately (like a timer), others need to happen before a deadline. Therefore, all examination methods accept an upper time limit within the positive or negative result must be obtained. Lower time limits need to be checked external to the examination, which is facilitated by a new construct for managing time constraints:

Scala

@@snip TestkitDocSpec.scala { #test-within }

Java

@@snip TestKitDocTest.java { #test-within }

The block @scala[given to]@java[in] within must complete after a duration which is between min and max, where the former defaults to zero. The deadline calculated by adding the max parameter to the block's start time is implicitly available within the block to all examination methods, if you do not specify it, it is inherited from the innermost enclosing within block.

It should be noted that if the last message-receiving assertion of the block is expectNoMessage or receiveWhile, the final check of the within is skipped to avoid false positives due to wake-up latencies. This means that while individual contained assertions still use the maximum time bound, the overall block may take arbitrarily longer in this case.

@@@ note

All times are measured using System.nanoTime, meaning that they describe wall time, not CPU time or system time.

@@@

@@@ div { .group-scala }

Ray Roestenburg has written a great article on using the Akka TestKit (but can also be applied to the Pekko Testkit): https://web.archive.org/web/20180114133958/http://roestenburg.agilesquad.com/2011/02/unit-testing-akka-actors-with-testkit_12.html. His full example is also available @ref:here.

@@@

Accounting for Slow Test Systems

The tight timeouts you use during testing on your lightning-fast notebook will invariably lead to spurious test failures on the heavily loaded Jenkins server (or similar). To account for this situation, all maximum durations are internally scaled by a factor taken from the @ref:Configuration, pekko.test.timefactor, which defaults to 1.

You can scale other durations with the same factor by using the @scala[implicit conversion in pekko.testkit package object to add dilated function to Duration]@java[dilated method in TestKit].

Scala

@@snip TestkitDocSpec.scala { #duration-dilation }

Java

@@snip TestKitDocTest.java { #duration-dilation }

@@@ div { .group-scala }

Resolving Conflicts with Implicit ActorRef

If you want the sender of messages inside your TestKit-based tests to be the testActor mix in ImplicitSender into your test.

@@snip PlainWordSpec.scala { #implicit-sender }

@@@

Using Multiple Probe Actors

When the actors under test are supposed to send various messages to different destinations, it may be difficult distinguishing the message streams arriving at the testActor when using the TestKit as @scala[a mixin]@java[shown until now]. Another approach is to use it for the creation of simple probe actors to be inserted in the message flows. @scala[To make this more powerful and convenient, there is a concrete implementation called TestProbe.] The functionality is best explained using a small example:

Scala

@@snip TestkitDocSpec.scala { #imports-test-probe } @@snip TestkitDocSpec.scala { #my-double-echo } @@snip TestkitDocSpec.scala { #test-probe }

Java

@@snip TestKitDocTest.java { #test-probe }

@scala[Here the system under test is simulated by MyDoubleEcho, which is supposed to mirror its input to two outputs. Attaching two test probes enables verification of the (simplistic) behavior]@java[This simple test verifies an equally simple Forwarder actor by injecting a probe as the forwarder’s target]. Another example would be two actors A and B which collaborate by A sending messages to B. To verify this message flow, a TestProbe could be inserted as a target of A, using the forwarding capabilities or auto-pilot described below to include a real B in the test setup.

If you have many test probes, you can name them to get meaningful actor names in test logs and assertions:

Scala

@@snip TestkitDocSpec.scala { #test-probe-with-custom-name }

Java

@@snip TestKitDocTest.java { #test-probe-with-custom-name }

Probes may also be equipped with custom assertions to make your test code even more concise and clear:

Scala

@@snip TestkitDocSpec.scala { #test-special-probe }

Java

@@snip TestKitDocTest.java { #test-special-probe }

You have complete flexibility here in mixing and matching the TestKit facilities with your checks and choosing an intuitive name for it. In real life your code will probably be a bit more complicated than the example given above; just use the power!

@@@ warning

Any message sent from a TestProbe to another actor which runs on the CallingThreadDispatcher runs the risk of dead-lock if that other actor might also send to this probe. The implementation of TestProbe.watch and TestProbe.unwatch will also send a message to the actor being watched, which means that it is dangerous to try watching e.g. TestActorRef from a TestProbe.

@@@

Watching Other Actors from Probes

A @scala[TestProbe]@java[TestKit] can register itself for DeathWatch of any other actor:

Scala

@@snip TestkitDocSpec.scala { #test-probe-watch }

Java

@@snip TestKitDocTest.java { #test-probe-watch }

Replying to Messages Received by Probes

@scala[The probes keep track of the communications channel for replies, if possible, so they can also reply]@java[The probe stores the sender of the last dequeued message (i.e. after its expectMsg* reception), which may be retrieved using the getLastSender() method. This information can also implicitly be used for having the probe reply to the last received message]:

Scala

@@snip TestkitDocSpec.scala { #test-probe-reply }

Java

@@snip TestKitDocTest.java { #test-probe-reply }

Forwarding Messages Received by Probes

@scala[Given a destination actor dest which in the nominal actor network would receive a message from actor source. If you arrange for the message to be sent to a TestProbe probe instead, you can make assertions concerning volume and timing of the message flow while still keeping the network functioning]@java[The probe can also forward a received message (i.e. after its expectMsg* reception), retaining the original sender]:

Scala

@@snip TestkitDocSpec.scala { #test-probe-forward-actors } @@snip TestkitDocSpec.scala { #test-probe-forward }

Java

@@snip TestKitDocTest.java { #test-probe-forward }

@scala[The dest actor will receive the same message invocation as if no test probe had intervened.]

Auto-Pilot

Receiving messages in a queue for later inspection is nice, but to keep a test running and verify traces later you can also install an AutoPilot in the participating test probes (actually in any TestKit) which is invoked before enqueueing to the inspection queue. This code can be used to forward messages, e.g. in a chain A --> Probe --> B, as long as a certain protocol is obeyed.

Scala

@@snip TestProbeSpec.scala { #autopilot }

Java

@@snip TestKitDocTest.java { #test-auto-pilot }

The run method must return the auto-pilot for the next message, @scala[which can be KeepRunning to retain the current one or NoAutoPilot to switch it off]@java[wrapped in an Option; setting it to None terminates the auto-pilot].

Caution about Timing Assertions

The behavior of within blocks when using test probes might be perceived as counter-intuitive: you need to remember that the nicely scoped deadline as described @ref:above is local to each probe. Hence, probes do not react to each other's deadlines or the deadline set in an enclosing TestKit instance:

Scala

@@snip TestkitDocSpec.scala { #test-within-probe }

Java

@@snip TestKitDocTest.java { #test-within-probe }

Here, the @scala[expectMsg]@java[expectMsgEquals] call will use the default timeout.

Testing parent-child relationships

The parent of an actor is always the actor that created it. At times this leads to a coupling between the two that may not be straightforward to test. There are several approaches to improve the testability of a child actor that needs to refer to its parent:

1. when creating a child, pass an explicit reference to its parent
2. create the child with a TestProbe as parent
3. create a fabricated parent when testing

Conversely, a parent's binding to its child can be lessened as follows:

4. when creating a parent, tell the parent how to create its child

For example, the structure of the code you want to test may follow this pattern:

Scala

@@snip ParentChildSpec.scala { #test-example }

Java

@@snip ParentChildTest.java { #test-example }

Introduce a child to its parent

The first option is to avoid the use of the context.parent function and create a child with a custom parent by passing an explicit reference to its parent instead.

Scala

@@snip ParentChildSpec.scala { #test-dependentchild }

Java

@@snip ParentChildTest.java { #test-dependentchild }

Create the child using @scala[TestProbe]@java[TestKit]

The @scala[TestProbe]@java[TestKit] class can create actors that will run with the test probe as a parent. This will cause any messages the child actor sends to @scala[context.parent]@java[getContext().getParent()] to end up in the test probe.

Scala

@@snip ParentChildSpec.scala { #test-TestProbe-parent }

Java

@@snip ParentChildTest.java { #test-TestProbe-parent }

Using a fabricated parent

If you prefer to avoid modifying the parent or child constructor you can create a fabricated parent in your test. This, however, does not enable you to test the parent actor in isolation.

Scala

@@snip ParentChildSpec.scala { #test-fabricated-parent }

Java

@@snip ParentChildTest.java { #test-fabricated-parent-creator } @@snip ParentChildTest.java { #test-fabricated-parent }

Externalize child making from the parent

Alternatively, you can tell the parent how to create its child. There are two ways to do this: by giving it a Props object or by giving it a function which takes care of creating the child actor:

Scala

@@snip ParentChildSpec.scala { #test-dependentparent }

Java

@@snip ParentChildTest.java { #test-dependentparent } @@snip ParentChildTest.java { #test-dependentparent-generic }

Creating the @scala[Props]@java[Actor] is straightforward and the function may look like this in your test code:

Scala

@@snip ParentChildSpec.scala { #child-maker-test }

Java

@@snip ParentChildTest.java { #child-maker-test }

And like this in your application code:

Scala

@@snip ParentChildSpec.scala { #child-maker-prod }

Java

@@snip ParentChildTest.java { #child-maker-prod }

Which of these methods is the best depends on what is most important to test. The most generic option is to create the parent actor by passing it a function that is responsible for the Actor creation, but @scala[the]@java[using TestProbe or having a] fabricated parent is often sufficient.

CallingThreadDispatcher

The CallingThreadDispatcher runs invocations on the current thread only. This dispatcher does not create any new threads.

It is possible to use the CallingThreadDispatcher in unit testing, as described above, but originally it was conceived to allow uninterrupted stack traces to be generated in case of an error. As this special dispatcher runs everything which would normally be queued directly on the current thread, the full history of a message's processing chain is recorded on the call stack, so long as all intervening actors run on this dispatcher.

How to use it

Just set the dispatcher as you normally would:

Scala

@@snip TestkitDocSpec.scala { #calling-thread-dispatcher }

Java

@@snip TestKitDocTest.java { #calling-thread-dispatcher }

How it works

When receiving an invocation, the CallingThreadDispatcher checks whether the receiving actor is already active on the current thread. The simplest example of this situation is an actor which sends a message to itself. In this case, processing cannot continue immediately as that would violate the actor model, so the invocation is queued and will be processed when the active invocation on that actor finishes its processing; thus, it will be processed on the calling thread, but after the actor finishes its previous work. In the other case, the invocation is processed immediately on the current thread. Futures scheduled via this dispatcher are also executed immediately.

This scheme makes the CallingThreadDispatcher work like a general purpose dispatcher for any actors which never block on external events.

In the presence of multiple threads, it may happen that two invocations of an actor running on this dispatcher happen on two different threads at the same time. In this case, both will be processed directly on their respective threads, where both compete for the actor's lock and the loser has to wait. Thus, the actor model is left intact, but the price is the loss of concurrency due to limited scheduling. In a sense, this is equivalent to traditional mutex style concurrency.

The other remaining difficulty is correct handling of suspend and resume: when an actor is suspended, subsequent invocations will be queued in thread-local queues (the same ones used for queuing in the normal case). The call to resume, however, is done by one specific thread, and all other threads in the system will probably not be executing this specific actor, which leads to the problem that the thread-local queues cannot be emptied by their native threads. Hence, the thread calling resume will collect all currently queued invocations from all threads into its queue and process them.

Limitations

@@@ warning

In case the CallingThreadDispatcher is used for top-level actors, without going through TestActorRef, then there is a time window during which the actor is awaiting construction by the user guardian actor. Sending messages to the actor during this time will result in them being enqueued and then executed on the guardian’s thread instead of the caller’s thread. To avoid this, use TestActorRef.

@@@

If an actor's behavior blocks on something which would normally be affected by the calling actor after having sent the message, this will dead-lock when using this dispatcher. This is a common scenario in actor tests based on CountDownLatch for synchronization:

val latch = new CountDownLatch(1)
actor ! startWorkAfter(latch)   // actor will call latch.await() before proceeding
doSomeSetupStuff()
latch.countDown()

The example would hang indefinitely within the message processing initiated on the second line and never reach the fourth line, which would unblock it on a normal dispatcher.

Thus, keep in mind that the CallingThreadDispatcher is not a general-purpose replacement for the normal dispatchers. If you are looking for a tool to help you debug dead-locks, the CallingThreadDispatcher may help with some error scenarios, but keep in mind that it may give false negatives as well as false positives.

Thread Interruptions

If the CallingThreadDispatcher sees that the current thread has its isInterrupted() flag set when message processing returns, it will throw an InterruptedException after finishing all its processing (i.e. all messages which need processing as described above are processed before this happens). As tell cannot throw exceptions due to its contract, this exception will then be caught and logged, and the thread’s interrupted status will be set again.

If during message processing an InterruptedException is thrown then it will be caught inside the CallingThreadDispatcher’s message handling loop, the thread’s interrupted flag will be set and processing continues normally.

@@@ note

In summary, if the current thread is interrupted while doing work under the CallingThreadDispatcher, then that will result in the isInterrupted flag to be true when the message send returns and no InterruptedException will be thrown.

@@@

Benefits

To summarize, these are the features that CallingThreadDispatcher has to offer:

• Deterministic execution of single-threaded tests while retaining nearly full actor semantics
• Full message processing history leading up to the point of failure in exception stack traces
• Exclusion of certain classes of dead-lock scenarios

Tracing Actor Invocations

The testing facilities described up to this point were aiming at formulating assertions about a system’s behavior. If a test fails, it is usually your job to find the cause, fix it and verify the test again. This process is supported by debuggers as well as logging, where the Pekko toolkit offers the following options:

• Logging of exceptions thrown within Actor instances It is always on; in contrast to the other logging mechanisms, this logs at ERROR level.

@@@ div { .group-scala }

• Logging of message invocations on certain actors This is enabled by a setting in the @ref:Configuration — namely pekko.actor.debug.receive — which enables the loggable statement to be applied to an actor’s receive function:

@@snip TestkitDocSpec.scala { #logging-receive }

If the aforementioned setting is not given in the @ref:Configuration, this method will pass through the given Receive function unmodified, meaning that there is no runtime cost unless enabled.

The logging feature is coupled to this specific local mark-up because enabling it uniformly on all actors is not usually what you need, and it would lead to endless loops if it were applied to event bus logger listeners.

@@@

• Logging of special messages Actors handle certain special messages automatically, e.g. Kill, PoisonPill, etc. Tracing of these message invocations is enabled by the setting pekko.actor.debug.autoreceive, which enables this on all actors.
• Logging of the actor lifecycle Actor creation, start, restart, monitor start, monitor stop and stop may be traced by enabling the setting pekko.actor.debug.lifecycle; this, too, is enabled uniformly on all actors.

Logging of these messages is at DEBUG level. To summarize, you can enable full logging of actor activities using this configuration fragment:

pekko {
  loglevel = "DEBUG"
  actor {
    debug {
      receive = on
      autoreceive = on
      lifecycle = on
    }
  }
}

@@@ div { .group-scala }

Different Testing Frameworks

Pekko’s test suite is written using ScalaTest, which also shines through in documentation examples. However, the TestKit and its facilities do not depend on that framework, so you can essentially use whichever suits your development style best.

This section contains a collection of known gotchas with some other frameworks, which is by no means exhaustive and does not imply an endorsement or special support.

When you need it to be a trait

If for some reason it is a problem to inherit from TestKit due to it being a concrete class instead of a trait, there’s TestKitBase:

@@snip TestkitDocSpec.scala { #test-kit-base }

The implicit lazy val system must be declared exactly like that (you can of course pass arguments to the actor system factory as needed) because trait TestKitBase needs the system during its construction.

Warning: use of the trait is discouraged because of potential issues with binary backwards compatibility in the future, use at own risk.

Specs2

Some Specs2 users have contributed examples of how to work around some clashes which may arise:

• Mixing TestKit into org.specs2.mutable.Specification results in a name clash involving the end method (which is a private variable in TestKit and an abstract method in Specification); if mixing in TestKit first, the code may compile but might then fail at runtime. The workaround—which is beneficial also for the third point—is to apply the TestKit together with org.specs2.specification.Scope.
• The Specification traits provide a Duration DSL which uses partly the same method names as scala.concurrent.duration.Duration, resulting in ambiguous implicits if scala.concurrent.duration._ is imported. There are two workarounds:
  • either use the Specification variant of Duration and supply an implicit conversion to the Pekko Duration. This conversion is not supplied with the Pekko distribution because that would mean that our JAR files would depend on Specs2, which is not justified by this little feature.
  • or mix org.specs2.time.NoTimeConversions into the Specification.
• Specifications are by default executed concurrently, which requires some care when writing the tests or the sequential keyword.

@@@

Configuration

There are several configuration properties for the TestKit module, please refer to the @ref:reference configuration.

@@@ div { .group-scala }

Example

Ray Roestenburg's example code from his blog, which unfortunately is only available on web archive, adapted to work with Akka 2.x.

@@snip TestKitUsageSpec.scala { #testkit-usage }

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Synchronous Testing: TestActorRef

Testing the business logic inside Actor classes can be divided into two parts: first, each atomic operation must work in isolation, then sequences of incoming events must be processed correctly, even in the presence of some possible variability in the ordering of events. The former is the primary use case for single-threaded unit testing, while the latter can only be verified in integration tests.

Normally, the ActorRef shields the underlying Actor instance from the outside, the only communications channel is the actor's mailbox. This restriction impedes unit testing, which led to the inception of the TestActorRef. This special type of reference is designed specifically for test purposes and allows access to the actor in two ways: either by obtaining a reference to the underlying actor instance or by invoking or querying the actor's behavior (receive). Each one warrants its section below.

@@@ note

It is highly recommended to stick to traditional behavioral testing (using messaging to ask the Actor to reply with the state you want to run assertions against), instead of using TestActorRef whenever possible.

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@@@ warning

Due to the synchronous nature of TestActorRef, it will not work with some support traits that Pekko provides as they require asynchronous behaviors to function properly. Examples of traits that do not mix well with test actor refs are @ref:PersistentActor and @ref:AtLeastOnceDelivery provided by @ref:Pekko Persistence.

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Obtaining a Reference to an Actor

Having access to the actual Actor object allows the application of all traditional unit testing techniques on the contained methods. Obtaining a reference is done like this:

Scala

@@snip TestkitDocSpec.scala { #test-actor-ref }

Java

@@snip TestKitDocTest.java { #test-actor-ref }

Since TestActorRef is generic in the actor type it returns the underlying actor with its proper static type. From this point on you may bring any unit testing tool to bear on your actor as usual.

@@@ div { .group-scala }

Testing Finite State Machines

If your actor under test is an FSM, you may use the special TestFSMRef which offers all features of a normal TestActorRef and besides allows access to the internal state:

@@snip TestkitDocSpec.scala { #test-fsm-ref }

Due to a limitation in Scala’s type inference, there is only the factory method shown above, so you will probably write code like TestFSMRef(new MyFSM) instead of the hypothetical ActorRef-inspired TestFSMRef[MyFSM]. All methods shown above directly access the FSM state without any synchronization; this is perfectly alright if the CallingThreadDispatcher is used and no other threads are involved, but it may lead to surprises if you were to exercise timer events, because those are executed on the Scheduler thread.

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Testing the Actor's Behavior

When the dispatcher invokes the processing behavior of an actor on a message, it calls apply on the current behavior registered for the actor. This starts with the return value of the declared receive method, but it may also be changed using become and unbecome in response to external messages. All of this contributes to the overall actor behavior and it does not lend itself to easy testing on the Actor itself. Therefore the TestActorRef offers a different mode of operation to complement the Actor testing: it supports all operations also valid on normal ActorRef. Messages sent to the actor are processed synchronously on the current thread and answers may be sent back as usual. This trick is made possible by the CallingThreadDispatcher described below (see @ref:CallingThreadDispatcher); this dispatcher is set implicitly for any actor instantiated into a TestActorRef.

Scala

@@snip TestkitDocSpec.scala { #test-behavior }

Java

@@snip TestKitDocTest.java { #test-behavior }

As the TestActorRef is a subclass of LocalActorRef with a few special extras, also aspects like supervision and restarting work properly, but beware that execution is only strictly synchronous as long as all actors involved use the CallingThreadDispatcher. As soon as you add elements which include more sophisticated scheduling you leave the realm of unit testing as you then need to think about asynchronicity again (in most cases the problem will be to wait until the desired effect had a chance to happen).

One more special aspect which is overridden for single-threaded tests is the receiveTimeout, as including that would entail asynchronous queuing of ReceiveTimeout messages, violating the synchronous contract.

@@@ note

To summarize: TestActorRef overwrites two fields: it sets the dispatcher to CallingThreadDispatcher.global and it sets the receiveTimeout to None.

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The Way In-Between: Expecting Exceptions

If you want to test the actor behavior, including hot swapping, but without involving a dispatcher and without having the TestActorRef swallow any thrown exceptions, then there is another mode available for you: use the receive method on TestActorRef, which will be forwarded to the underlying actor:

Scala

@@snip TestkitDocSpec.scala { #test-expecting-exceptions }

Java

@@snip TestKitDocTest.java { #test-expecting-exceptions }

Use Cases

You may mix and match both modi operandi of TestActorRef as suits your test needs:

• one common use case is setting up the actor into a specific internal state before sending the test message
• another is to verify correct internal state transitions after having sent the test message

Feel free to experiment with the possibilities, and if you find useful patterns, don't hesitate to let the Pekko forums know about them! Who knows, common operations might even be worked into nice DSLs.