This document discusses concurrency, scalability, and fault tolerance using actors and software transactional memory (STM) in Akka. It describes how actors provide message-passing concurrency by isolating processes and avoiding shared state. Actors allow easy reasoning about concurrency by preventing issues like race conditions and deadlocks. The document also covers how Akka implements actors, typed actors, remote actors, supervisor hierarchies for fault tolerance, and STM for coordinating concurrent changes to shared state transactionally. Combining actors with STM results in "transactors" that allow building distributed, concurrent, and fault-tolerant applications.

1. Concurrency, Scalability & Fault-tolerance 2.0 with Actors & STMby Mario Fuscomario.fusco@gmail.comtwitter: @mariofusco

2. Which are the biggestchallengesin contemporary software development?

3. Deal with overloadedsystems

4. Do concurrencyeffectively

5. Design, develop and deployapplicationsthat scale well

6. The native Java concurrency modelBased on:ThreadsSemaphoresSynchronizationLocksThey are sometimes plain evil …… and sometimes a necessary pain …… but always the wrong default

7. Actors: a different model of concurrencyImplements Message-Passing Concurrency

8. Share NOTHING

9. Isolated lightweight processes

10. Communicates through messages

11. Asynchronous and non-blocking

12. Each actor has a mailbox (message queue)

13. A single thread can managemany (thousands of) actorsPeople are bad at parallel thinkingWith actors is easier to reason about and easier to avoid: Race conditions, Deadlocks, Starvation and Live locks

14. What is Akka?An actor based concurrency framework

15. Written in Scala, butalsoworking in Java

16. Nowrealeasing 1.0 after 2 years of development (currentlyRC4 available)

17. Better performances and throughput and smallermemoryfootprint (650 bytes) than native scala actors

18. Open source (Apache2)Untyped Actorspublic class MyUntypedActorextends UntypedActor { public void onReceive(Object message) throws Exception { if (message instanceofString) { String msg = (String)message;System.out.println("Received message: " + msg); } }}UntypedActorRef actor = UntypedActor.actorOf(MyUntypedActor.class);actor.start();// --- Do some work here ---actor.stop();

19. Sending message to UntypedActorsThere are 3 different ways to send a message to an actor:Fire-and-forget ( ! In Scala )actor.sendOneWay(msg);Uses a Future under the hood, blocking the sender Actor until eithera reply is received or the Future times out ( !! In Scala )Object res1 = actor.sendRequestReply(msg);Object res2 = actor.sendRequestReply(msg, 1000); // timeoutExplicitly returns a Future ( !!! In Scala )Future future= actor.sendRequestReplyFuture(msg);IMPORTANT: Messages can be any kind of object but have to be immutable. Akkacan’t enforce immutability (yet) so this has to be by convention

20. Typed Actorspublic class CounterImplextends TypedActorimplements Counter { private int counter = 0; public void count() { counter++;System.out.println(counter); } public intgetCountNr() {returncounter; }}Counter counter= (Counter)TypedActor .newInstance(Counter.class, CounterImpl.class, 1000);counter.count(); // Fire-and-forgetintcount = counter.getCountNr(); // uses Future under the hood

21. “Let it crash” fault-tolerance through supervisor hierarchies

22. You can’t avoid failures: shit happens

23. What’swrong in trying to preventerrors?Java and other languages and frameworks (not designed with concurrency in mind) signal faults/errors with exceptionsThrowing an exception in concurrent code will just simply blow up the thread that currently executes the code that threw it:There is no way to find out that things went wrong, apart from inspecting the stack traceThere is nothing you can do to recover from the problem and bring back your system to a normal functioning

24. Fail fast & build self-repairing systems

25. SupervisedactorsActors provide a clean way of getting error notification and do something about it

26. Supervised actors also allow you to create sets (hierarchies) of actors where you can be sure that either all are dead or none are dead

27. That encourages non-defensive programming: don’t try to prevent things from go wrong, because they will. Failures are a natural state in the life-cycle of your app: crash early and let someone else (that sees the whole picture), deal with it

28. A supervisor is responsible for starting, stopping and monitoring its child processes. It can act with 2 fault handling strategies:One-For-Oneor All-For-One

29. Declarative supervised actors configurationSupervisor supervisor = new Supervisor( new SupervisorConfig( new AllForOneStrategy( // or OneForOneStrategy new Class[]{Exception.class}, // Exceptions to handle 3, // maximum number of restart retries 5000), // within time in millis new Supervise[] { new Supervise(UntypedActor.actorOf(MyActor1.class), permanent()), // the actor will always be restarted new Supervise(UntypedActor.actorOf(MyActor2.class), temporary()) // the actor won’t be shut down normally}));// You can also link and unlink actors from a supervisorsupervisor.link(actor1);supervisor.unlink(actor1);

30. Remote actorsAkka provides remote actors in a completely transparent way both in regards to sending messages, error handling and supervision

31. Uses a very efficient and scalable I/O implementation built upon JBossNetty and Google Protocol Buffers

32. Can be secured with a cookie authentication mechanism

33. Can be bothTyped and Untyped

34. Twotypes of remote actors:

35. server-managed: run on the machine thatcreateditwhile client can lookup and obtain a proxy

36. client managed: makethemrunning on a different machine while the localone just hold a proxy to sendthemmessagesServer-managedClient-managedTypedBean actor = TypedActor.newInstance(Bean.class, BeanImpl.class, timeout); Actors.remote().start("localhost", 9999) .registerTypedActor( “my-service", actor );Bean remoteActor = (Bean)TypedActor .newRemoteInstance(Bean.class, BeanImpl.class, timeout, "192.68.23.69", 9999 );Untypedclass MyActor extends UntypedActor {}Actors.remote() .actorOf(MyActor.class, "192.68.23.69", 9999 );class MyActor extends UntypedActor {}Actors.remote() .start("localhost", 9999) .register(“my-service", actorOf(MyActor.class) );

37. Software Transactional MemoryAn STM turns the Java heap into a transactional data set with begin/commit/rollback semantics. Very much like a regular database. It implements the first three letters in ACID; ACI: Atomic: all or none of the changes made during a transaction get applied

38. Consistent: a transaction gets a consistent of reality

39. Isolated: changes made by concurrent execution transactions are not visible to each otherAkka’s STM implements the concept already present in Clojure’s STMThe STM is based on Transactional References (referred to as Refs). Refs: are memory cells, holding an (arbitrary) immutable value

40. implement CAS (Compare-And-Swap) semantics

41. are managed and enforced by the STM for coordinated changes across many Refs

42. are implemented using the excellent Multiverse STM (http://multiverse.codehaus.org)Why we need an STM?public class Container { private int value; private final ReadWriteLock lock = new ReentrantReadWriteLock(); public intgetValue() { lock.readLock().lock(); try { return value; } finally { lock.readLock.unlock(); } } public void setValue(int value) { lock.writeLock().lock(); try { this.value= value; } finally { lock.writeLock.unlock(); } } }What’swrong with it?Ad-hoc and hardlyreusablelogic

43. Unnecessarycomplexity

44. Lockslimitconcurrency and thenthroughputHow does Akka STM work?// Create a Ref with an initial value (also possible without)final Ref<Integer> ref = new Ref<Integer>(0); public int counter() {// A transaction is delimited using an Atomic anonymous inner class return new Atomic<Integer>() { public Integer atomically() { intinc = ref.get() + 1; // Accessing the value of the Refref.set(inc); // Changing the value of the Ref return inc; } }.execute(); }A transaction is automatically retried when it runs into some read or write conflict

45. In this case an (exponential) delay isused to preventfurthercontentions

46. Thereshouldn’t be side-effects inside the transaction to avoid to repeatthemTransactional Datastructuresfinal TransactionalMap<String, User> users = new TransactionalMap<String, User>(); // fill users map (in a transaction)new Atomic() { public Object atomically() { users.put("bill", new User("bill")); users.put("mary", new User("mary")); users.put("john", new User("john")); return null; } }.execute(); // access users map (in a transaction)User user = new Atomic<User>() { public User atomically() { return users.get("bill").get(); } }.execute();

47. Persistent DatastructuresCopyright Rich Hickey 2009

48. Coordinating Actors with STMpublic class Counter extends UntypedActor { private Ref<Integer> count = new Ref(0); private void increment() { count.set(count.get() + 1); } public void onReceive(Object incoming) throws Exception { if (incoming instanceof Coordinated) { Coordinated coordinated = (Coordinated) incoming; Object message = coordinated.getMessage(); if (message instanceof Increment) { Increment increment = (Increment) message; if (increment.hasFriend()) {increment.getFriend() .sendOneWay(coordinated.coordinate(new Increment())); }coordinated.atomic(new Atomically() { public void atomically() { increment(); } });}}}}

49. Actors + STM = Transactorspublic class Counter extends UntypedTransactor { Ref<Integer> count = new Ref<Integer>(0); @Override public Set<SendTo> coordinate(Object message) { if (message instanceof Increment) { Increment increment = (Increment) message; if (increment.hasFriend()) return include(increment.getFriend(), new Increment()); } return nobody(); } public void atomically(Object message) { if (message instanceof Increment) {count.set(count.get() + 1); } }}

50. Other Akka goodiesImplements Advanced Message Queuing Protocol (AMPQ) based on the RabbitMQ

51. Integration with Apache Camel

52. PersistencemodulesupportingmanyNoSQL DB like: Cassandra, MongoDB, Riak, Redis, Terrastore, CouchDB, Voldemort, Hbase

53. Integration betweenAkkaSTM and JTA (Java Transaction API)

54. OSGisupport

55. Integration with Spring allowing to defineTyped Actors as Spring managed bean

56. Integration with Lift Web Framework and Play! FrameworkThanks a lot!Happy Java (or Scala) hingby Mario Fuscomario.fusco@gmail.comtwitter: @mariofusco