pekko/akka-docs/rst/java/serialization.rst

.. _serialization-java:

#####################
 Serialization
#####################

Akka has a built-in Extension for serialization,
and it is both possible to use the built-in serializers and to write your own.

The serialization mechanism is both used by Akka internally to serialize messages,
and available for ad-hoc serialization of whatever you might need it for.

Usage
=====

Configuration
-------------

For Akka to know which ``Serializer`` to use for what, you need edit your :ref:`configuration`,
in the "akka.actor.serializers"-section you bind names to implementations of the ``akka.serialization.Serializer``
you wish to use, like this:

.. includecode:: ../scala/code/docs/serialization/SerializationDocSpec.scala#serialize-serializers-config

After you've bound names to different implementations of ``Serializer`` you need to wire which classes
should be serialized using which ``Serializer``, this is done in the "akka.actor.serialization-bindings"-section:

.. includecode:: ../scala/code/docs/serialization/SerializationDocSpec.scala#serialization-bindings-config

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. ``java.io.Serializable`` and ``MyOwnSerializable`` both apply
and neither is a subtype of the other, a warning will be issued.

.. note::

  If you are using Scala for your message protocol and your messages are contained
  inside of a Scala object, then in order to reference those messages, you will need
  use the fully qualified Java class name. For a message named ``Message`` contained inside
  the Scala object named ``Wrapper`` you would need to reference it as
  ``Wrapper$Message`` instead of ``Wrapper.Message``.

Akka provides serializers for :class:`java.io.Serializable` and `protobuf
<http://code.google.com/p/protobuf/>`_
:class:`com.google.protobuf.GeneratedMessage` by default (the latter only if
depending on the akka-remote module), so normally you don't need to add
configuration for that; since :class:`com.google.protobuf.GeneratedMessage`
implements :class:`java.io.Serializable`, protobuf messages will always be
serialized using the protobuf protocol unless specifically overridden. In order
to disable a default serializer, map its marker type to “none”::

  akka.actor.serialization-bindings {
    "java.io.Serializable" = none
  }

Verification
------------

Normally, messages sent between local actors (i.e. same JVM) do not undergo serialization. For testing, sometimes, it may be desirable to force serialization on all messages (both remote and local). If you want to do this in order to verify that your messages are serializable you can enable the following config option:

.. includecode:: ../scala/code/docs/serialization/SerializationDocSpec.scala#serialize-messages-config

If you want to verify that your ``Props`` are serializable you can enable the following config option:

.. includecode:: ../scala/code/docs/serialization/SerializationDocSpec.scala#serialize-creators-config

.. warning::

   We recommend having these config options turned on **only** when you're running tests. Turning these options on in production is pointless, as it would negatively impact the performance of local message passing without giving any gain.

Programmatic
------------

If you want to programmatically serialize/deserialize using Akka Serialization,
here's some examples:

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: imports

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: programmatic

For more information, have a look at the ``ScalaDoc`` for ``akka.serialization._``


Customization
=============

So, lets say that you want to create your own ``Serializer``,
you saw the ``docs.serialization.MyOwnSerializer`` in the config example above?

Creating new Serializers
------------------------

First you need to create a class definition of your ``Serializer``,
which is done by extending ``akka.serialization.JSerializer``, like this:

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: imports

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: my-own-serializer
   :exclude: ...

The manifest is a type hint so that the same serializer can be used for different
classes. The manifest parameter in ``fromBinaryJava`` is the class of the object that
was serialized. In ``fromBinary`` you can match on the class and deserialize the
bytes to different objects.

Then you only need to fill in the blanks, bind it to a name in your :ref:`configuration` and then
list which classes that should be serialized using it.

.. _string-manifest-serializer-java:

Serializer with String Manifest
-------------------------------

The ``Serializer`` illustrated above supports a class based manifest (type hint).
For serialization of data that need to evolve over time the ``SerializerWithStringManifest``
is recommended instead of ``Serializer`` because the manifest (type hint) is a ``String``
instead of a ``Class``. That means that the class can be moved/removed and the serializer
can still deserialize old data by matching  on the ``String``. This is especially useful
for :ref:`persistence-java`.

The manifest string can also encode a version number that can be used in ``fromBinary`` to
deserialize in different ways to migrate old data to new domain objects.

If the data was originally serialized with ``Serializer`` and in a later version of the
system you change to ``SerializerWithStringManifest`` the manifest string will be the full
class name if you used ``includeManifest=true``, otherwise it will be the empty string.

This is how a ``SerializerWithStringManifest`` looks like:

.. includecode:: code/jdocs/serialization/SerializationDocTest.java#my-own-serializer2

You must also bind it to a name in your :ref:`configuration` and then list which classes
that should be serialized using it.

It's recommended to throw ``java.io.NotSerializableException`` in ``fromBinary``
if the manifest is unknown. This makes it possible to introduce new message types and
send them to nodes that don't know about them. This is typically needed when performing 
rolling upgrades, i.e. running a cluster with mixed versions for while.
``NotSerializableException`` is treated as a transient problem in the TCP based remoting 
layer. The problem will be logged and message is dropped. Other exceptions will tear down
the TCP connection because it can be an indication of corrupt bytes from the underlying 
transport.


Serializing ActorRefs
---------------------

All ActorRefs are serializable using JavaSerializer, but in case you are writing your
own serializer, you might want to know how to serialize and deserialize them properly.
In the general case, the local address to be used depends on the type of remote
address which shall be the recipient of the serialized information. Use
:meth:`Serialization.serializedActorPath(actorRef)` like this:

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: imports

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: actorref-serializer

This assumes that serialization happens in the context of sending a message
through the remote transport. There are other uses of serialization, though,
e.g. storing actor references outside of an actor application (database, etc.).
In this case, it is important to keep in mind that the
address part of an actor’s path determines how that actor is communicated with.
Storing a local actor path might be the right choice if the retrieval happens
in the same logical context, but it is not enough when deserializing it on a
different network host: for that it would need to include the system’s remote
transport address. An actor system is not limited to having just one remote
transport per se, which makes this question a bit more interesting. To find out
the appropriate address to use when sending to ``remoteAddr`` you can use
:meth:`ActorRefProvider.getExternalAddressFor(remoteAddr)` like this:

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: external-address

.. note::
  
  ``ActorPath.toSerializationFormatWithAddress`` differs from ``toString`` if the
  address does not already have ``host`` and ``port`` components, i.e. it only
  inserts address information for local addresses.
  
  ``toSerializationFormatWithAddress`` also adds the unique id of the actor, which will
  change when the actor is stopped and then created again with the same name.
  Sending messages to a reference pointing the old actor will not be delivered
  to the new actor. If you do not want this behavior, e.g. in case of long term
  storage of the reference, you can use ``toStringWithAddress``, which does not
  include the unique id.


This requires that you know at least which type of address will be supported by
the system which will deserialize the resulting actor reference; if you have no
concrete address handy you can create a dummy one for the right protocol using
``new Address(protocol, "", "", 0)`` (assuming that the actual transport used is as
lenient as Akka’s RemoteActorRefProvider).

There is also a default remote address which is the one used by cluster support
(and typical systems have just this one); you can get it like this:

.. includecode:: code/jdocs/serialization/SerializationDocTest.java
   :include: external-address-default

Deep serialization of Actors
----------------------------

The recommended approach to do deep serialization of internal actor state is to use Akka :ref:`persistence-java`.

A Word About Java Serialization
===============================

When using Java serialization without employing the :class:`JavaSerializer` for
the task, you must make sure to supply a valid :class:`ExtendedActorSystem` in
the dynamic variable ``JavaSerializer.currentSystem``. This is used when
reading in the representation of an :class:`ActorRef` for turning the string
representation into a real reference. :class:`DynamicVariable` is a
thread-local variable, so be sure to have it set while deserializing anything
which might contain actor references.

Serialization compatibility
===========================

It is not safe to mix major Scala versions when using the Java serialization as Scala does not guarantee compatibility
and this could lead to very surprising errors.

If using the Akka Protobuf serializers (implicitly with ``akka.actor.allow-java-serialization = off`` or explicitly with
``enable-additional-serialization-bindings = true``) for the internal Akka messages those will not require the same major
Scala version however you must also ensure the serializers used for your own types does not introduce the same
incompatibility as Java serialization does.

External Akka Serializers
=========================

`Akka-quickser by Roman Levenstein <https://github.com/romix/akka-quickser-serialization>`_


`Akka-kryo by Roman Levenstein <https://github.com/romix/akka-kryo-serialization>`_


`Twitter Chill Scala extensions for Kryo (based on Akka Version 2.3.x but due to backwards compatibility of the Serializer Interface this extension also works with 2.4.x) <https://github.com/twitter/chill>`_