Reviewed and improved transactors doc
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Patrik Nordwall
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**<span style="font-size: 150%;">Transactors (Java)</span>**
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============================================================
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Transactors (Java)
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==================
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.. sidebar:: Contents
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.. contents:: :local:
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Module stability: **SOLID**
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Why Transactors?
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================
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----------------
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Actors are excellent for solving problems where you have many independent processes that can work in isolation and only interact with other Actors through message passing. This model fits many problems. But the actor model is unfortunately a terrible model for implementing truly shared state. E.g. when you need to have consensus and a stable view of state across many components. The classic example is the bank account where clients can deposit and withdraw, in which each operation needs to be atomic. For detailed discussion on the topic see `this JavaOne presentation <http://www.slideshare.net/jboner/state-youre-doing-it-wrong-javaone-2009>`_.
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@ -15,21 +19,21 @@ Akka's Transactors combine Actors and STM to provide the best of the Actor model
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If you need Durability then you should not use one of the in-memory data structures but one of the persistent ones.
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Generally, the STM is not needed very often when working with Akka. Some use-cases (that we can think of) are:
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# When you really need composable message flows across many actors updating their **internal local** state but need them to do that atomically in one big transaction. Might not often, but when you do need this then you are screwed without it.
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# When you want to share a datastructure across actors.
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# When you need to use the persistence modules.
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- When you really need composable message flows across many actors updating their **internal local** state but need them to do that atomically in one big transaction. Might not often, but when you do need this then you are screwed without it.
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- When you want to share a datastructure across actors.
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- When you need to use the persistence modules.
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Actors and STM
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--------------
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^^^^^^^^^^^^^^
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You can combine Actors and STM in several ways. An Actor may use STM internally so that particular changes are guaranteed to be atomic. Actors may also share transactional datastructures as the STM provides safe shared state across threads.
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It's also possible to coordinate transactions across Actors or threads so that either the transactions in a set all commit successfully or they all fail. This is the focus of Transactors and the explicit support for coordinated transactions in this section.
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----
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Coordinated transactions
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========================
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------------------------
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Akka provides an explicit mechanism for coordinating transactions across actors. Under the hood it uses a ``CountDownCommitBarrier``, similar to a CountDownLatch.
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@ -40,9 +44,11 @@ Here is an example of coordinating two simple counter UntypedActors so that they
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import akka.actor.ActorRef;
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public class Increment {
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private ActorRef friend = null;
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private final ActorRef friend;
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public Increment() {}
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public Increment() {
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this.friend = null;
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}
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public Increment(ActorRef friend) {
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this.friend = friend;
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@ -59,9 +65,7 @@ Here is an example of coordinating two simple counter UntypedActors so that they
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.. code-block:: java
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import akka.actor.ActorRef;
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import akka.actor.UntypedActor;
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import static akka.actor.Actors.*;
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import akka.stm.Ref;
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import akka.transactor.Atomically;
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import akka.transactor.Coordinated;
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@ -88,11 +92,8 @@ Here is an example of coordinating two simple counter UntypedActors so that they
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}
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});
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}
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} else if (incoming instanceof String) {
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String message = (String) incoming;
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if (message.equals("GetCount")) {
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getContext().replyUnsafe(count.get());
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}
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} else if (incoming.equals("GetCount")) {
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getContext().replyUnsafe(count.get());
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}
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}
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}
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@ -104,7 +105,7 @@ Here is an example of coordinating two simple counter UntypedActors so that they
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counter1.sendOneWay(new Coordinated(new Increment(counter2)));
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To start a new coordinated transaction set that you will also participate in, just create a ``Coordinated`` object:
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To start a new coordinated transaction that you will also participate in, just create a ``Coordinated`` object:
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.. code-block:: java
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@ -116,7 +117,7 @@ To start a coordinated transaction that you won't participate in yourself you ca
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actor.sendOneWay(new Coordinated(new Message()));
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To include another actor in the same coordinated transaction set that you've created or received, use the ``coordinate`` method on that object. This will increment the number of parties involved by one and create a new ``Coordinated`` object to be sent.
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To include another actor in the same coordinated transaction that you've created or received, use the ``coordinate`` method on that object. This will increment the number of parties involved by one and create a new ``Coordinated`` object to be sent.
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.. code-block:: java
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@ -134,10 +135,9 @@ To enter the coordinated transaction use the atomic method of the coordinated ob
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The coordinated transaction will wait for the other transactions before committing. If any of the coordinated transactions fail then they all fail.
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----
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UntypedTransactor
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=================
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-----------------
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UntypedTransactors are untyped actors that provide a general pattern for coordinating transactions, using the explicit coordination described above.
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@ -146,10 +146,12 @@ Here's an example of a simple untyped transactor that will join a coordinated tr
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.. code-block:: java
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import akka.transactor.UntypedTransactor;
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import akka.stm.Ref;
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public class Counter extends UntypedTransactor {
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Ref<Integer> count = new Ref<Integer>(0);
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@Override
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public void atomically(Object message) {
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if (message instanceof Increment) {
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count.set(count.get() + 1);
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@ -174,7 +176,8 @@ Example of coordinating an increment, similar to the explicitly coordinated exam
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public class Counter extends UntypedTransactor {
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Ref<Integer> count = new Ref<Integer>(0);
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@Override public Set<SendTo> coordinate(Object message) {
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@Override
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public Set<SendTo> coordinate(Object message) {
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if (message instanceof Increment) {
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Increment increment = (Increment) message;
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if (increment.hasFriend())
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@ -183,6 +186,7 @@ Example of coordinating an increment, similar to the explicitly coordinated exam
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return nobody();
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}
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@Override
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public void atomically(Object message) {
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if (message instanceof Increment) {
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count.set(count.get() + 1);
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@ -190,14 +194,13 @@ Example of coordinating an increment, similar to the explicitly coordinated exam
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}
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}
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To execute directly before or after the coordinated transaction, override the ``before`` and ``after`` methods. These methods also expect partial functions like the receive method. They do not execute within the transaction.
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To execute directly before or after the coordinated transaction, override the ``before`` and ``after`` methods. They do not execute within the transaction.
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To completely bypass coordinated transactions override the ``normally`` method. Any message matched by ``normally`` will not be matched by the other methods, and will not be involved in coordinated transactions. In this method you can implement normal actor behavior, or use the normal STM atomic for local transactions.
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----
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Coordinating Typed Actors
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=========================
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-------------------------
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It's also possible to use coordinated transactions with typed actors. You can explicitly pass around ``Coordinated`` objects, or use built-in support with the ``@Coordinated`` annotation and the ``Coordination.coordinate`` method.
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}
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}
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`<code format="java">`_
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Counter counter1 = (Counter) TypedActor.newInstance(Counter.class, CounterImpl.class);
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Counter counter2 = (Counter) TypedActor.newInstance(Counter.class, CounterImpl.class);
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.. code-block:: java
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Coordination.coordinate(true, new Atomically() {
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Counter counter1 = (Counter) TypedActor.newInstance(Counter.class, CounterImpl.class);
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Counter counter2 = (Counter) TypedActor.newInstance(Counter.class, CounterImpl.class);
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Coordination.coordinate(true, new Atomically() {
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public void atomically() {
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counter1.increment();
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counter2.increment();
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counter1.increment();
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counter2.increment();
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}
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});
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});
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TypedActor.stop(counter1);
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TypedActor.stop(counter2);
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TypedActor.stop(counter1);
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TypedActor.stop(counter2);
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`<code>`_
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