Sunday, April 6, 2014

Akka-Persistence and Testing

In my first scala project I had the pleasure not only to learn scala, akka and play but also to work with eventsourced. Eventsourced is an akka-based open-source library for implementing the concepts of event sourcing. The basic idea is that instead of persisting the current state of the application - like you usually do when working with a relational database -, you rather persist the individual messages that cause the state-changes. More details on this concept can be found for example in the documentation for eventsourced, its successor akka-persistence or an article by Martin Fowler. This blog-post focuses on a specific (typical) usage scenario and in particular on testing the various aspects of it that are important for real life projects.

Akka-Persistence and Testing

The example used in this blog-post is supposed to represent a complex Akka-based application (or parts of it) keeping its state in memory and using akka-persistence to persist this state. The tests are integration style tests accessing the application from outside. To keep things simple the application is just modelled as a simple CRUD service for generic items. The service basically delegates all operations to a stateful actor that persists its state through akka-persistence by using it as a write-ahead log (for command-sourcing). So the service stands for the entire interface of the application (respectively the part under test) and the actor for the arbitrarily complex logic behind this interface. In this case the interface is based on regular method calls, but the same ideas apply for a message based interface where actors are addressed directly.

When it comes to testing the integration with akka-persistence, I see 4 important aspects that should be covered:

  1. Are custom serializers used for serializing PersistentMessages into the journal?
  2. Does the application recover its state correctly after a restart when replaying the journal?
  3. Does the application recover its state correctly after a restart when reading state from a snapshot?
  4. Does the application recover its state correctly from an old journal of a previous release?

In this blog-post I just cover the first point and will address the other issues in following posts.

The Demo Application

Let's first have a closer look at the application. As already stated the actor is pretty simplistic:

 1 class ItemActor extends Processor {
 3   import ItemActor._
 5   private var itemById: Map[ItemId, Item] = Map.empty
 6   private var idCounter: Long = 0
 8   def receive = {
 9     case Persistent(CreateItem(template: ItemTemplate), _) =>
10       idCounter += 1
11       sender ! addItem(Item(ItemId(idCounter), template.description))
13     case Persistent(UpdateItem(item: Item), _) =>
14       sender ! (if(itemById.isDefinedAt(
15         addItem(item)
16       else
17         Failure(new NonExistingItemCannotBeUpdatedException(item)))
19     case Persistent(DeleteItem(itemId: ItemId), _) =>
20       val deletedItem = itemById.get(itemId)
21       deletedItem.foreach(itemById -=
22       sender ! deletedItem.fold[Try[Item]](
23         Failure(new NonExistingItemCannotBeDeletedException(itemId)))(
24         Success.apply)
26     case GetItem(itemId: ItemId) =>
27       sender ! itemById.get(itemId)
30     case SaveSnapshot => saveSnapshot(ItemActorSnapshot(itemById, idCounter))
31     case SaveSnapshotSuccess(metadata) =>
32     case SaveSnapshotFailure(metadata, cause) =>
34     case SnapshotOffer(_, ItemActorSnapshot(itemMap, lastId)) =>
35       this.itemById = itemMap
36       this.idCounter = lastId
37     // SNAPSHOT END
38   }
40   private def addItem(item: Item): Try[Item] = {
41     itemById += ( -> item)
42     Success(item)
43   }
44 }

It extends Processor so akka-persistence can take care of persisting messages sent to it. The accepted messages are

  • CreateItem
  • UpdateItem
  • DeleteItem
  • GetItem

(The snapshot related messages can be ignored for the moment.) They return the created, updated, deleted or requested Item if the operation was successful and a failure if it was not. The first three are those that modify the actor's state and thus have to be logged into the journal by akka-persistence. That is why they are wrapped in a Persistent, while the GetItem can be received plainly. The state is basically a Map containing all Items by their ids (itemById in line 5).

The corresponding domain classes Item and ItemTemplate are likewise simplistic:

 1 class ItemId(val id: Long) extends AnyVal with Serializable
 2 object ItemId {
 3   def apply(id: Long): ItemId = new ItemId(id)
 4 }
 5 case class ItemTemplate(description: String) extends CommonItem
 6 case class Item(id: ItemId, description: String) extends CommonItem {
 7   def asTemplate: ItemTemplate = ItemTemplate(description)
 8   def withTemplate(template: ItemTemplate): Item =
 9     copy(description = template.description)
10 }

An Item contains an id and as representative for a more complex structure a single field called description. The ItemTemplate is used for creating new Items when the id is not yet known.

As described above the interface to our application is a service that wraps the ItemActor:

 1 class ActorService(itemActor: ActorRef)(implicit timeout: Timeout) extends ItemService {
 2   def find(itemId: ItemId): Future[Option[Item]] =
 3     (itemActor ? GetItem(itemId)).mapTo[Option[Item]]
 5   def create(template: ItemTemplate): Future[Try[Item]] =
 6     (itemActor ? Persistent(CreateItem(template))).mapTo[Try[Item]]
 8   def delete(itemId: ItemId): Future[Try[Item]] =
 9     (itemActor ? Persistent(DeleteItem(itemId))).mapTo[Try[Item]]
11   def update(item: Item): Future[Try[Item]] =
12     (itemActor ? Persistent(UpdateItem(item))).mapTo[Try[Item]]
13 }

For each of the four commands we have seen above it offers corresponding methods. In each case the implementation creates the corresponding command, sends it to the actor (with ask) and returns the result wrapped in a Future to the caller. As we have already seen on the receiving side, those commands that modify the application's state are wrapped in a Persistent.

An ItemApplication rounds off the implementation and takes care of the initialization and dependency injection as well as the proper configuration of the akka-system.

 1 class ItemApplication(overwriteConfig: Config = ConfigFactory.empty()) {
 3   val akkaSystemConfig: Config = overwriteConfig.withFallback(
 4     akkaSerializerConfig[Command, CommandSerializer])
 6   private val config = akkaSystemConfig.withFallback(ConfigFactory.load())
 7   val system = ActorSystem(classOf[ItemApplication].getSimpleName, config)
 8   protected val itemActor =
 9     system.actorOf(ItemActor.props(), classOf[ItemActor].getSimpleName)
10   val itemService = new ActorService(itemActor)(Timeout(5.seconds))
12   private def akkaSerializerConfig[M : ClassTag, S <: Serializer : ClassTag]
13       : Config = {
14     val messageClassName = classTag[M].runtimeClass.getName
15     val serializerClassName = classTag[S].runtimeClass.getName
16     ConfigFactory.parseString(s"""
17         | {
18         |  serializers {
19         |    "$messageClassName" = "$serializerClassName"
20         |  }
21         |  serialization-bindings {
22         |    "$messageClassName" = "$messageClassName"
23         |  }
24         |}
25         |""".stripMargin)
26   }
28   def shutdown(): Unit = {
29     system.shutdown()
30     system.awaitTermination()
31   }
32 }

The interesting part here is the configuration of the serializers for the akka-system. It is created in a hard-coded manner through the method akkaSerializerConfig, but what is it good for?

Customer Serializers

When akka-persistence writes the persistent messages received by a Processor to a journal it makes use of standard akka-serialization. By default this uses java-serialization. That means all messages are first converted to an Array[Byte] through java-serialization and afterwards this array is written to the journal. When the journal is replayed, the data is read from disk as Array[Byte] and this is converted back to messages again through java-(de)serialization. If you just go with the default you can quickly run into compatibility problems when trying to read an existing journal with a new version of the application as plain java-serialization by default does not ensure compatibility of the serialized data to evolved classes. To avoid this kind of problems you can either

  • migrate the journal files to the new format before starting the new application. However when using java-serialization this can be a challenging task by itself, since one application must be able to read an old serialized form and write the new serialized form of instances of the same class (in different versions) or
  • cautiously ensure that your messages stay downwards compatible to their serialized representation (e.g. by declaring serialVersionUID explicitly and things like implementing readObject, writeObject methods) or
  • you can make akka use custom serializers for your messages.

The latter gives you very good control over maintaining compatibility of the serialized representation of your message-instances with the implementation of the corresponding classes. Based on my experience I can recommend using custom serializers. The serializers should of course use a serial representation that can easily be kept downwards compatible. JSON is a valid alternative (even though certainly not one that performs best) as

  • it is very flexible when it comes to migrating structures
  • it might be used already in a program that offers a REST interface where the resources are represented in form of JSON documents

In addition to this it was very convenient to use it for this blog-post as the play-json-lib with macro-inception allows to write JSON (de-)serializers of case-classes in basically one line of code. The corresponding akka-serializer is implemented in the class CommandSerializers for all messages that derive from Command (see line 3-4) which are all messages to be written into the journal in our case. akka allows to configure custom serializers through the configuration object passed to the akka-system. Typically the corresponding configuration is read from a configuration file like application.conf or reference.conf. As the application needs to ensure backwards compatibility of the serialized representation of the message in the journal I do not consider the custom serializers for these classes as part of the configuration that could be modified by an administrator of the application. That is why this configuration is provided in a hard-coded manner in ItemApplication (line 12-26).

The test

The tricky part about this configuration is, that akka will ignore it silently if it contains errors. Thus we we will not notice any problems during runtime, if the configuration is wrong. In that case akka will silently use the default java-serialization, a fact that might remain unnoticed until the first migration problem appears. Even if the code-based configuration ensures that we cannot introduce typos in class-names there are still other things that might go wrong as we will see soon. That is why it is important to test that the configured serializers are actually used by akka.

For this we have the following test:

 1           in { application =>
 2         val serializer = SerializationExtension(application.system)
 4         Seq(CreateItem(newItemTemplate()),
 5             UpdateItem(newItem()),
 6             DeleteItem(newId)).foreach { expected =>
 8           val bytes = successOf(serializer.serialize(expected))
 9           val actual = new CommandSerializer()
10               .fromBinary(bytes, Some(expected.getClass))
12           actual should be (expected)
13         }
14       }

At first this test retrieves the SerializationExtension from the akka-system of the application. Then it uses it to serialize each of the possible command-objects and afterwards de-serializes them with the CommandSerializer that is also expected to be used by the SerializationExtension. If the de-serialized command equals the original one we can be sure that the serialization config was correct.

The first test-run discloses that there is indeed an error in the configuration:

[WARN] [...] [...] [akka.serialization.Serialization (akka://ItemApplication)]
 Multiple serializers found for class
 choosing first: Vector(
  (interface de...ItemActor$Command,de...CommandSerializer@635eed0))

Unexpected character ('�' (code 65533 / 0xfffd)): expected a valid value
 (number, String, array, object, 'true', 'false' or 'null')

As you can see bytes does not contain valid JSON and the parser complains about an unexpected character. However the more interesting output is the line above that. akka warns about the fact that there are multiple serializers for the command-object and it chooses an arbitrary one which happens to be the java-serializer in this case. If there are multiple alternative serializers for a class the akka documentation states:

You only need to specify the name of an interface or abstract base class of the messages. In case of ambiguity, i.e. the message implements several of the configured classes, the most specific configured class will be used, i.e. the one of which all other candidates are superclasses. If this condition cannot be met, because e.g. and MyOwnSerializable both apply and neither is a subtype of the other, a warning will be issued.

That means our interface (Command) must be more specific than Serializable. We can easily achieve this if we make Command extend Serializable. As soon as we have this the test runs through just fine.

Note that for this test to be relevant, it is important that it uses the same configuration as is used in production and not a configuration that is specifically made up for the test. In our case this is ensured by the ItemApplicationFixture that prepares and cleans the ItemApplication up for each test. Let's have a look at the central method:

 1   def withApplication[A](persistDir: File)(block: TestApplication => A) = {
 2     val tmpDirPersistenceConfig = ConfigFactory.parseMap(
 3       Map(JournalDirConfig -> new File(persistDir, "journal").getPath,
 4         NativeLevelDbConfig -> false.toString,
 5         SnapshotDirConfig -> new File(persistDir, "snapshots").getPath)
 6         .asJava)
 7     val application = newItemApplication(tmpDirPersistenceConfig)
 8     ultimately(application.shutdown())(block(application))
 9   }
11   def newItemApplication(config: Config) =
12     new ItemApplication(config) with ItemApplicationTestExtensions

As you can see it uses the real ItemApplication (line 12) that is also used for production. It does provide test-specific extensions (ItemApplicationTestExtensions) and config (line 2-6) to ItemApplication but just to ease writing the tests and to ensure that the journals and snapshots for the tests are written to a temporary folder that is cleaned up after the test and that the java-leveldb implementation is used instead of the native one which is perfectly fine for testing and typically comes with less problems when running the tests in different environments.

This concludes the first blog-post about akka-persistence and testing. I will cover the other important topics about recovery, snapshots and migration in following posts so stay tuned.

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