pekko/akka-docs/scala/testing.rst

.. _akka-testkit:

##############################
Testing Actor Systems (Scala)
##############################

.. toctree::

   testkit-example

.. sidebar:: Contents

   .. contents:: :local:

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 has an influence on how to
perform tests.

Akka comes with a dedicated module :mod:`akka-testkit` for supporting tests at
different levels, which fall into two clearly distinct categories:

 - Testing isolated pieces of code without involving the actor model, meaning
   without multiple threads; this implies completely deterministic behavior
   concerning the ordering of events and no concurrency concerns and will be
   called **Unit Testing** in the following.
 - Testing (multiple) encapsulated actors including multi-threaded scheduling;
   this implies non-deterministic order of events but shielding from
   concurrency concerns by the actor model and will be called **Integration
   Testing** in the following.

There are of course variations on the granularity of tests in both categories,
where unit testing reaches down to white-box tests and integration testing can
encompass functional tests of complete actor networks. The important
distinction lies in whether concurrency concerns are part of the test or not.
The tools offered are described in detail in the following sections.

.. note::

   Be sure to add the module :mod:`akka-testkit` to your dependencies.

Unit Testing with :class:`TestActorRef`
=======================================

Testing the business logic inside :class:`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 :class:`ActorRef` shields the underlying :class:`Actor` instance
from the outside, the only communications channel is the actor's mailbox. This
restriction is an impediment to unit testing, which led to the inception of the
:class:`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 behaviour (:meth:`receive`). Each one warrants its own
section below.

Obtaining a Reference to an :class:`Actor`
------------------------------------------

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

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#test-actor-ref

Since :class:`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.

Expecting Exceptions
--------------------

Testing that an expected exception is thrown while processing a message sent to
the actor under test can be done by using a :class:`TestActorRef` :meth:`receive` based
invocation:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#test-expecting-exceptions

.. _TestFSMRef:

Testing Finite State Machines
-----------------------------

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

.. includecode:: code/akka/docs/testkit/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 :class:`ActorRef`-inspired ``TestFSMRef[MyFSM]``.
All methods shown above directly access the FSM state without any
synchronization; this is perfectly alright if the
:class:`CallingThreadDispatcher` is used (which is the default for
:class:`TestFSMRef`) and no other threads are involved, but it may lead to
surprises if you were to actually exercise timer events, because those are
executed on the :obj:`Scheduler` thread.

Testing the Actor's Behavior
----------------------------

When the dispatcher invokes the processing behavior of an actor on a message,
it actually calls :meth:`apply` on the current behavior registered for the
actor. This starts out with the return value of the declared :meth:`receive`
method, but it may also be changed using :meth:`become` and :meth:`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 :class:`Actor`
itself. Therefore the :class:`TestActorRef` offers a different mode of
operation to complement the :class:`Actor` testing: it supports all operations
also valid on normal :class:`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 :class:`CallingThreadDispatcher`
described below; this dispatcher is set implicitly for any actor instantiated
into a :class:`TestActorRef`.

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#test-behavior

As the :class:`TestActorRef` is a subclass of :class:`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 :class:`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
:meth:`receiveTimeout`, as including that would entail asynchronous queuing of
:obj:`ReceiveTimeout` messages, violating the synchronous contract.

.. warning::

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

The Way In-Between
------------------

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

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#test-unhandled

The above sample assumes the default behavior for unhandled messages, i.e.
that the actor doesn't swallow all messages and doesn't override :meth:`unhandled`.

Use Cases
---------

You may of course mix and match both modi operandi of :class:`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 Akka forums know about them! Who knows,
common operations might even be worked into nice DSLs.

Integration Testing with :class:`TestKit`
=========================================

When you are reasonably sure that your actor's business logic is correct, the
next step is verifying that it works correctly within its intended environment
(if the individual actors are simple enough, possibly because they use the
:mod:`FSM` module, this might also be the first step). The definition of the
environment depends of course very much on the problem at hand and the level at
which you intend to test, ranging for functional/integration tests 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 :class:`TestKit` class contains a collection of tools which makes this
common task easy.

.. includecode:: code/akka/docs/testkit/PlainWordSpec.scala#plain-spec

The :class:`TestKit` contains an actor named :obj:`testActor` which is the
entry point for messages to be examined with the various ``expectMsg...``
assertions detailed below. When mixing in the trait ``ImplicitSender`` this
test actor is implicitly used as sender reference when dispatching messages
from the test procedure. The :obj:`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.

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 :meth:`expectMsg` is not the only method for formulating
assertions concerning received messages. Here is the full list:

  * :meth:`expectMsg[T](d: Duration, msg: T): T`

    The given message object must be received within the specified time; the
    object will be returned.

  * :meth:`expectMsgPF[T](d: Duration)(pf: PartialFunction[Any, T]): T`

    Within the given time period, a message must be received and the given
    partial function must be defined for that message; the result from applying
    the partial function to the received message is returned. The duration may
    be left unspecified (empty parentheses are required in this case) to use
    the deadline from the innermost enclosing :ref:`within <TestKit.within>`
    block instead.

  * :meth:`expectMsgClass[T](d: Duration, c: Class[T]): T`

    An object which is an instance of the given :class:`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, have a look at
    :meth:`expectMsgAllClassOf` with a single given class argument.

  * :meth:`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(manifest[T].erasure)``.

  * :meth:`expectMsgAnyOf[T](d: Duration, obj: T*): T`

    An object must be received within the given time, and it must be equal (
    compared with ``==``) to at least one of the passed reference objects; the
    received object will be returned.

  * :meth:`expectMsgAnyClassOf[T](d: Duration, obj: Class[_ <: T]*): T`

    An object must be received within the given time, and it must be an
    instance of at least one of the supplied :class:`Class` objects; the
    received object will be returned.

  * :meth:`expectMsgAllOf[T](d: Duration, obj: T*): Seq[T]`

    A number of 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
    ``==``) it. The full sequence of received objects is returned.

  * :meth:`expectMsgAllClassOf[T](d: Duration, c: Class[_ <: T]*): Seq[T]`

    A number of objects matching the size of the supplied :class:`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.

  * :meth:`expectMsgAllConformingOf[T](d: Duration, c: Class[_ <: T]*): Seq[T]`

    A number of objects matching the size of the supplied :class:`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.

  * :meth:`expectNoMsg(d: Duration)`

    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.

  * :meth:`receiveN(n: Int, d: Duration): Seq[AnyRef]`

    ``n`` messages must be received within the given time; the received
    messages are returned.

  * :meth:`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:

  * :meth:`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).

  * :meth:`receiveWhile[T](max: Duration, idle: Duration, messages: Int)(pf: PartialFunction[Any, T]): Seq[T]`

    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. The maximum duration defaults to the
    time remaining in the innermost enclosing :ref:`within <TestKit.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.

  * :meth:`awaitCond(p: => Boolean, max: Duration, interval: Duration)`

    Poll the given condition every :obj:`interval` until it returns ``true`` or
    the :obj:`max` duration is used up. The interval defaults to 100 ms and the
    maximum defaults to the time remaining in the innermost enclosing
    :ref:`within <TestKit.within>` block.

  * :meth:`ignoreMsg(pf: PartialFunction[AnyRef, Boolean])`

    :meth:`ignoreNoMsg`

    The internal :obj:`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 Exceptions
--------------------

Since an integration test does not allow to 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 :class:`TestEventListener` and using an :class:`EventFilter`
allows assertions on log messages, including those which are generated by
exceptions:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#event-filter

.. _TestKit.within:

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:

.. code-block:: scala

   within([min, ]max) {
     ...
   }

The block given to :meth:`within` must complete after a :ref:`Duration` which
is between :obj:`min` and :obj:`max`, where the former defaults to zero. The
deadline calculated by adding the :obj:`max` parameter to the block's start
time is implicitly available within the block to all examination methods, if
you do not specify it, is is inherited from the innermost enclosing
:meth:`within` block.

It should be noted that if the last message-receiving assertion of the block is
:meth:`expectNoMsg` or :meth:`receiveWhile`, the final check of the
:meth:`within` is skipped in order 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.

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#test-within

.. note::

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

Ray Roestenburg has written a great article on using the TestKit:
`<http://roestenburg.agilesquad.com/2011/02/unit-testing-akka-actors-with-testkit_12.html>`_.
His full example is also available :ref:`here <testkit-example>`.

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`,
``akka.test.timefactor``, which defaults to 1.

You can scale other durations with the same factor by using the implicit conversion
in ``akka.testkit`` package object to add dilated function to :class:`Duration`.

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#duration-dilation

Resolving Conflicts with Implicit ActorRef
------------------------------------------

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

.. includecode:: code/akka/docs/testkit/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 :obj:`testActor` when using the :class:`TestKit` as a mixin. Another
approach is to use it for creation of simple probe actors to be inserted in the
message flows. To make this more powerful and convenient, there is a concrete
implementation called :class:`TestProbe`. The functionality is best explained
using a small example:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala
   :include: imports-test-probe,my-double-echo,test-probe

Here a the system under test is simulated by :class:`MyDoubleEcho`, which is
supposed to mirror its input to two outputs. Attaching two test probes enables
verification of the (simplistic) behavior. Another example would be two actors
A and B which collaborate by A sending messages to B. In order to verify this
message flow, a :class:`TestProbe` could be inserted as target of A, using the
forwarding capabilities or auto-pilot described below to include a real B in
the test setup.

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

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala
   :include: test-special-probe

You have complete flexibility here in mixing and matching the :class:`TestKit`
facilities with your own 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!

Replying to Messages Received by Probes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The probes keep track of the communications channel for replies, if possible,
so they can also reply:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#test-probe-reply

Forwarding Messages Received by Probes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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 :class:`TestProbe` ``probe`` instead, you can make assertions
concerning volume and timing of the message flow while still keeping the
network functioning:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala
   :include: test-probe-forward-actors,test-probe-forward

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 in order to
keep a test running and verify traces later you can also install an
:class:`AutoPilot` in the participating test probes (actually in any
:class:`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.

.. includecode:: ../../akka-testkit/src/test/scala/akka/testkit/TestProbeSpec.scala#autopilot

The :meth:`run` method must return the auto-pilot for the next message, wrapped
in an :class:`Option`; setting it to :obj:`None` terminates the auto-pilot.

Caution about Timing Assertions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The behavior of :meth:`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 <TestKit.within>` is local to each probe. Hence, probes
do not react to each other's deadlines or to the deadline set in an enclosing
:class:`TestKit` instance::

  class SomeTest extends TestKit(_system: ActorSystem) with ImplicitSender {

    val probe = TestProbe()

    within(100 millis) {
      probe.expectMsg("hallo")  // Will hang forever!
    }
  }

This test will hang indefinitely, because the :meth:`expectMsg` call does not
see any deadline. Currently, the only option is to use ``probe.within`` in the
above code to make it work; later versions may include lexically scoped
deadlines using implicit arguments.

CallingThreadDispatcher
=======================

The :class:`CallingThreadDispatcher` serves good purposes in unit testing, as
described above, but originally it was conceived in order to allow contiguous
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:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#calling-thread-dispatcher

How it works
------------

When receiving an invocation, the :class:`CallingThreadDispatcher` checks
whether the receiving actor is already active on the current thread. The
simplest example for 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 simply after the actor finishes its
previous work. In the other case, the invocation is simply processed
immediately on the current thread. Futures scheduled via this dispatcher are
also executed immediately.

This scheme makes the :class:`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 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
:meth:`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 :meth:`resume` will collect all currently
queued invocations from all threads into its own queue and process them.

Limitations
-----------

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

.. code-block:: scala

   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 :class:`CallingThreadDispatcher` is not a
general-purpose replacement for the normal dispatchers. On the other hand it
may be quite useful to run your actor network on it for testing, because if it
runs without dead-locking chances are very high that it will not dead-lock in
production.

.. warning::

   The above sentence is unfortunately not a strong guarantee, because your
   code might directly or indirectly change its behavior when running on a
   different dispatcher. If you are looking for a tool to help you debug
   dead-locks, the :class:`CallingThreadDispatcher` may help with certain error
   scenarios, but keep in mind that it has may give false negatives as well as
   false positives.

Benefits
--------

To summarize, these are the features with the :class:`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

.. _actor.logging:

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 Akka toolkit offers the following
options:

* *Logging of exceptions thrown within Actor instances*

  This is always on; in contrast to the other logging mechanisms, this logs at
  ``ERROR`` level.

* *Logging of message invocations on certain actors*

  This is enabled by a setting in the :ref:`configuration` — namely
  ``akka.actor.debug.receive`` — which enables the :meth:`loggable`
  statement to be applied to an actor’s :meth:`receive` function:

.. includecode:: code/akka/docs/testkit/TestkitDocSpec.scala#logging-receive

.
  If the abovementioned setting is not given in the :ref:`configuration`, this method will
  pass through the given :class:`Receive` function unmodified, meaning that
  there is no runtime cost unless actually 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 :class:`EventHandler`
  listeners.

* *Logging of special messages*

  Actors handle certain special messages automatically, e.g. :obj:`Kill`,
  :obj:`PoisonPill`, etc. Tracing of these message invocations is enabled by
  the setting ``akka.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 ``akka.actor.debug.lifecycle``; this, too, is enabled
  uniformly on all actors.

All these messages are logged at ``DEBUG`` level. To summarize, you can enable
full logging of actor activities using this configuration fragment::

  akka {
    loglevel = DEBUG
    actor {
      debug {
        receive = on
        autoreceive = on
        lifecycle = on
      }
    }
  }