akka-kryo-serialization - kryo-based serializers for Scala and Akka

Full test prior to release Latest version

⚠️ We found issues serializing Scala 3 Enums. If you use akka-kryo-serialization with Scala 3 you should upgrade to 2.4.1 or newer asap.

This library provides custom Kryo-based serializers for Scala and Akka. It can be used for more efficient akka actor's remoting.

It can also be used for a general purpose and very efficient Kryo-based serialization of such Scala types like Option, Tuple, Enumeration and most of Scala's collection types.

For upgrading from previous versions see migration-guide. Note that due to the upgrade to Kryo 5 with version 2.0.0, data written with older versions based on Kryo 4 is most likely not readable anymore.


  • It is more efficient than Java serialization - both in size and speed
  • Does not require any additional build steps like compiling proto files, when using protobuf serialization
  • Almost any Scala and Java class can be serialized using it without any additional configuration or code changes
  • Efficient serialization of such Scala types like Option, Tuple, Enumeration, most of Scala's collection types
  • Greatly improves performance of Akka's remoting
  • Supports transparent AES encryption and different modes of compression
  • Apache 2.0 license

Note that this serializer is mainly intended to be used for akka-remoting and not for (long term) persisted data. The underlying kryo serializer does not guarantee compatibility between major versions.

How to use this library in your project

To use this serializer, you need to do two things:

  • Include a dependency on this library into your project: libraryDependencies += "io.altoo" %% "akka-kryo-serialization" % "2.5.0"

  • Register and configure the serializer in your Akka configuration file, e.g. application.conf.

We provide several versions of the library:

Version Akka & Kryo Compatibility Available Scala Versions Tested with
v2.5.x Akka-2.6 and Kryo-5.4 2.12,2.13,3.1 JDK: OpenJdk11,OpenJdk17 Scala: 2.12.17,2.13.10,3.1.3 Akka: 2.6.20
v2.4.x Akka-2.5,2.6 and Kryo-5.3 2.12,2.13,3.0 JDK: OpenJdk11,OpenJdk17 Scala: 2.12.15,2.13.8,3.0.2 Akka: 2.5.32,2.6.18
v2.3.x Akka-2.5,2.6 and Kryo-5.2 2.12,2.13,3.0 JDK: OpenJdk11,OpenJdk15 Scala: 2.12.15,2.13.6,3.0.2 Akka: 2.5.32,2.6.17
v2.2.x Akka-2.5,2.6 and Kryo-5.1 2.12,2.13,3.0 JDK: OpenJdk11,OpenJdk15 Scala: 2.12.14,2.13.6,3.0.0 Akka: 2.5.32,2.6.15
v2.1.x Akka-2.5,2.6 and Kryo-5.0 2.12,2.13 JDK: OpenJdk8,OpenJdk11,OpenJdk15 Scala: 2.12.13,2.13.4 Akka: 2.5.32,2.6.12
v2.0.x Akka-2.5,2.6 and Kryo-5.0 2.12,2.13 JDK: OpenJdk8,OpenJdk11,OpenJdk13 Scala: 2.12.12,2.13.3 Akka: 2.5.32,2.6.10
v1.1.x Akka-2.5,2.6 and Kryo-4.0 2.12,2.13 JDK: OpenJdk8,OpenJdk11,OpenJdk13 Scala: 2.12.11,2.13.2 Akka: 2.5.26,2.6.4
v1.0.x Akka-2.5,2.6 and Kryo-4.0 2.11,2.12,2.13 JDK: OpenJdk8,OpenJdk11 Scala: 2.11.12,2.12.10,2.13.1 Akka: 2.5.25,2.6.0-M7
For past versions see Legacy.md.

From 2.1.0 onward we also provide support for akka-typed. This is done as a separate artifact so that the standard does not pull all the typed akka dependencies.

  • Include: libraryDependencies += "io.altoo" %% "akka-kryo-serialization-typed" % "2.5.0"

Version 2.2.0 requires JDK 11 or higher in favor of optimizations using ByteBuffer.

Note that we use semantic versioning - see semver.org.

Which Maven repository contains this library?

You can find the JARs on Sonatype's Maven repository.

sbt projects

To use the latest stable release of akka-kryo-serialization in sbt projects you just need to add this dependency:

libraryDependencies += "io.altoo" %% "akka-kryo-serialization" % "2.5.0"

To use with Akka 2.5 (or older versions of Akka 2.6) you need to exclude transitive dependencies to prevent unintended inclusion newer versions of Akka:

libraryDependencies += "io.altoo" %% "akka-kryo-serialization" % "2.5.0" excludeAll (ExclusionRule("com.typesafe.akka", "akka-actor_2.13"), ExclusionRule("org.agrona", "agrona"))

maven projects

To use the official release of akka-kryo-serialization in Maven projects, please use the following snippet in your pom.xml

        <name>Maven Central Repository</name>


For snapshots see Snapshots.md

Configuration of akka-kryo-serialization

The following options are available for configuring this serializer:

  • You can add a new akka-kryo-serialization section to the configuration to customize the serializer. Consult the supplied reference.conf for a detailed explanation of all the options available.

  • You should declare in the akka.actor.serializers section a new kind of serializer:

    serializers {
        java = "akka.serialization.JavaSerializer"
        # Define kryo serializer
        kryo = "io.altoo.akka.serialization.kryo.KryoSerializer"
  • As usual, you should declare in the Akka serialization-bindings section which classes should use kryo serialization. One thing to keep in mind is that classes that you register in this section are supposed to be TOP-LEVEL classes that you wish to serialize. I.e. this is a class of object that you send over the wire. It should not be a class that is used internally by a top-level class. The reason for it: Akka sees only an object of a top-level class to be sent. It picks a matching serializer for this top-level class, e.g. a default Java serializer, and then it serializes the whole object graph with this object as a root using this Java serializer.

How do you create mappings or classes sections with proper content?

One of the easiest ways to understand which classes you need to register in those sections is to leave both sections first empty and then set

implicit-registration-logging = true

As a result, you'll eventually see log messages about implicit registration of some classes. By default, they will receive some random default ids. Once you see the names of implicitly registered classes, you can copy them into your mappings or classes sections and assign an id of your choice to each of those classes.

You may need to repeat the process several times until you see no further log messages about implicitly registered classes.

Another useful trick is to provide your own custom initializer for Kryo (see below) and inside it you registerclasses of a few objects that are typically used by your application, for example:


Obviously, you can also explicitly assign IDs to your classes in the initializer, if you wish:

    kryo.register(myObj3.getClass, 123)

If you use this library as an alternative serialization method when sending messages between actors, it is extremely important that the order of class registration and the assigned class IDs are the same for senders and for receivers!

How to customize kryo initialization

To further customize kryo you can extend the io.altoo.akka.serialization.kryo.DefaultKryoInitializer and configure the FQCN under akka-kryo-serialization.kryo-initializer.

Configuring default field serializers

In preInit a different default serializer can be configured as it will be picked up by serailizers added afterwards. By default the com.esotericsoftware.kryo.serializers.FieldSerializer will be used.

The available options are:

  • com.esotericsoftware.kryo.serializers.FieldSerializer
    Serializes objects using direct field assignment. FieldSerializer is generic and can serialize most classes without any configuration. It is efficient and writes only the field data, without any extra information. It does not support adding, removing, or changing the type of fields without invalidating previously serialized bytes. This can be acceptable in many situations, such as when sending data over a network, but may not be a good choice for long term data storage because the Java classes cannot evolve.

  • com.esotericsoftware.kryo.serializers.CompatibleFieldSerializer
    Serializes objects using direct field assignment, providing both forward and backward compatibility. This means fields can be added or removed without invalidating previously serialized bytes. Changing the type of a field is not supported. The forward and backward compatibility comes at a cost: the first time the class is encountered in the serialized bytes, a simple schema is written containing the field name strings.

  • com.esotericsoftware.kryo.serializers.VersionFieldSerializer
    Serializes objects using direct field assignment, with versioning backward compatibility. Allows fields to have a @Since(int) annotation to indicate the version they were added. For a particular field, the value in @Since should never change once created. This is less flexible than FieldSerializer, which can handle most classes without needing annotations, but it provides backward compatibility. This means that new fields can be added, but removing, renaming or changing the type of any field will invalidate previous serialized bytes. VersionFieldSerializer has very little overhead (a single additional varint) compared to FieldSerializer. Forward compatibility is not supported.

  • com.esotericsoftware.kryo.serializers.TaggedFieldSerializer
    Serializes objects using direct field assignment for fields that have a @Tag(int) annotation. This provides backward compatibility so new fields can be added. TaggedFieldSerializer has two advantages over VersionFieldSerializer:

    1. fields can be renamed
    2. fields marked with the @Deprecated annotation will be ignored when reading old bytes and won't be written to new bytes.

    Deprecation effectively removes the field from serialization, though the field and @Tag annotation must remain in the class. The downside is that it has a small amount of additional overhead compared to VersionFieldSerializer (additional per field variant). Forward compatibility is not supported.

Example for configuring a different field serializer

Create a custom initializer

class XyzKryoInitializer extends DefaultKryoInitializer {
  def preInit(kryo: ScalaKryo): Unit = {

And register the custom initializer in your application.conf by overriding

akka-kryo-serialization.kryo-initializer = "com.example.XyzKryoInitializer"

To configure the field serializer a serializer factory can be used as described here: https://github.com/EsotericSoftware/kryo#serializer-factories

How to configure and customize encryption

Using the DefaultKeyProvider an encryption key can statically be set by defining encryption.aes.password and encryption.aes.salt. Refere to the reference.conf for an example configuration.

Sometimes you need to pass a custom aes key, depending on the context you are in, instead of having a static key. For example, you might have the key in a data store, or provided by some other application. In such instances, you might want to provide the key dynamically to kryo serializer.

You can override the

  encryption.aes.key-provider = "CustomKeyProviderFQCN"

Where CustomKeyProviderFQCN is a fully qualified class name of your custom aes key provider class. The key provider must extend the DefaultKeyProvider and can override the aesKey method.

An example of such a custom aes-key supplier class could be something like this:

class CustomKeyProvider extends DefaultKeyProvider {
  override def aesKey(config: Config): String = "ThisIsASecretKey"

The encryption transformer (selected for aes in post serialization transformations) only supports GCM modes (currently recommended default mode is AES/GCM/NoPadding).

Important: The old encryption transformer only supported CBC modes without manual authentication which is deemed problematic. It is currently available for backwards compatibility by specifying aesLegacy in post serialization transformations instead of aes. Its usage is deprecated and will be removed in future versions.

Resolving Subclasses

If you are using id-strategy="explicit", you may find that some of the standard Scala and Akka types are a bit hard to register properly. This is because these types are exposed in the API as simple traits or abstract classes, but they are actually implemented as many specialized subclasses that are used as necessary. Examples include:

  • scala.collection.immutable.Map
  • scala.collection.immutable.Set
  • akka.actor.ActorRef
  • akka.actor.ActorPath

The problem is that Kryo thinks in terms of the exact class being serialized, but you are rarely working with the actual implementation class -- the application code only cares about the more abstract trait. The implementation class often isn't obvious, and is sometimes private to the library it comes from. This isn't an issue for idstrategies that add registrations when needed, or which use the class name, but in explicit you must register every class to be serialized, and that may turn out to be more than you expect.

For cases like these, you can use the SubclassResolver. This is a variant of the standard Kryo ClassResolver, which is able to deal with subclasses of the registered types. You turn it on by setting

  resolve-subclasses = true

With that turned on, unregistered subclasses of a registered supertype are serialized as that supertype. So for example, if you have registered immutable.Set, and the object being serialized is actually an immutable.Set.Set3 (the subclass used for Sets of 3 elements), it will serialize and deserialize that as an immutable.Set.

If you register immutable.Map, you should use the ScalaImmutableAbstractMapSerializer with it. If you register immutable.Set, you should use the ScalaImmutableAbstractSetSerializer. These serializers are specifically designed to work with those traits.

The SubclassResolver approach should only be used in cases where the implementation types are completely opaque, chosen by the implementation library, and not used explicitly in application code. If you have subclasses that have their own distinct semantics, such as immutable.ListMap, you should register those separately. You can register both a higher-level class like immutable.Map and a subclass like immutable.ListMap -- the resolver will choose the more-specific one when appropriate.

SubclassResolver should be used with care -- even when it is turned on, you should define and register most of your classes explicitly, as usual. But it is a helpful way to tame the complexity of some class hierarchies, when that complexity can be treated as an implementation detail and all of the subclasses can be serialized and deserialized identically.

Using serializers with different configurations

There may be the need to use different configurations for different use cases. To support this the KryoSerializer can be extended to use a different configuration path.

Define a custom configuration:

akka-kryo-serialization-xyz = ${akka-kryo-serialization} {
  # configuration overrides like...
  # id-strategy = "explicit"

Create new serializer subclass overriding the config key to the matching config section.

package xyz

class XyzKryoSerializer(system: ExtendedActorSystem) extends KryoSerializer(system) {
  override def configKey: String = "akka-kryo-serialization-xyz"

And finally declare the custom serializer in the akka.actor.serializers section:

    serializers {
        kryo = "io.altoo.akka.serialization.kryo.KryoSerializer"
        # define additional kryo serializer
        kryo-xyz = "xyz.XyzKryoSerializer"

Enum Serialization

Serialization of Java and Scala 3 enums is done by name (and not by index) to avoid having reordering of enum values breaking serialization. As of 2.5.0, the default serializer for scala.Enumeration has been switched to a name base serializer for consistency.

To change current default back to old index based (order of definition) serialization, a custom Kryo initializer must be defined and the following override has to be done:

protected override def defaultEnumerationSerializer: Class[_ <: Serializer[Enumeration#Value]] = classOf[EnumerationSerializer]

Note: this serializer will be removed in the future. If needed, you can reimplement it in your own project.

Using Kryo on JDK 17

Kryo needs modules to be opened for reflection when serializing basic JDK classes. Those options have to be passed to the JVM, for example in sbt:

javaOptions ++= Seq("--add-opens", "java.base/java.util=ALL-UNNAMED", "--add-opens", "java.base/java.util.concurrent=ALL-UNNAMED", "--add-opens", "java.base/java.lang=ALL-UNNAMED", "--add-opens", "java.base/java.lang.invoke=ALL-UNNAMED", "--add-opens", "java.base/java.math=ALL-UNNAMED"),

To use unsafe transformations, the following access must be granted:

javaOptions ++= Seq("--add-opens", "java.base/java.nio=ALL-UNNAMED", "--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED"),

Kryo shaded and ASM (pre 2.x)

Kryo depends on ASM, which is used by many different projects in different versions. This can lead to unintended version conflicts. To avoid this, Kryo provides a shaded version to work around this issue. For easier usage we depend on the shaded Kryo. Note that only the ASM dependency is shaded and not kryo itself.

If you prefer to re-use Kryo you can override the dependency (but be sure to pick compatible versions):

lazy val kryoSerializationDeps = Seq(
  "io.altoo" %% "akka-kryo-serialization" % "1.0.0" excludeAll ExclusionRule("com.esotericsoftware", "kryo-shaded"),
  "com.esotericsoftware" % "kryo" % "5.4.0"

If this would be a common use case we could provide different artifacts with both dependencies.

How do I build this library on my own?

If you wish to build the library on your own, you need to check out the project from Github and do

sbt compile publish-local

If you wish to use it within an OSGi environment, you can add OSGi headers to the build by executing:

sbt osgi-bundle publish-local

Note that the OSGi build uses the sbt-osgi plugin, which may not be available from Maven Central or the Typesafe repo, so it may require a local build as well. sbt-osgi can be found at sbt/sbt-osgi.

What about Akka BSL?

We will keep depending on the last Open Source Akka release 2.6.20 and intend to provide support for its successor Pekko in the future.