Akka. Developing SEDA Based Applications

Transcription

1 Akka Developing SEDA Based Applications

2 Me Ben Senior Software Engineer at Localytics

3 Localytics Real time mobile analytics platform 40M+ events per day and growing rapidly 3x growth over the past 3 months Heavy users of Scala/Akka/NoSql We are hiring (seriously, come talk to me)

4 Localytics How to keep up with our growth?

5 Actor Model Lock free approach to concurrency No shared state between actors Asynchronous message passing Mailboxes to buffer incoming messages

6 Akka Configurable Dispatchers Mailboxes Fault Tolerant Supervisors

7 Akka Performant

8 SEDA Staged Event Driven Architecture "Decomposes a complex, event-driven application into a set of stages connected by queues." 1 "The most fundamental aspect of the SEDA architecture is the programming model that supports stage-level backpressure and load management."

9 Backpressure Whats the big deal?

10 Backpressure Manditory to prevent OutOfMemoryError Messages backup in memory faster than they can be processed Cassandra was seriously bitten by this Less crappy failure mode when swamped with inserts than "run out of memory and gc-storm to death" (CASSANDRA-401) Add backpressure to StorageProxy (CASSANDRA-685)

11 Backpressure Mailboxes case class UnboundedMailbox(val blocking: Boolean = false) extends MailboxType case class BoundedMailbox( val blocking: Boolean = false, val capacity: Int = { if (Dispatchers.MAILBOX_CAPACITY < 0) Int.MaxValue else Dispatchers.MAILBOX_CAPACITY }, val pushtimeout: Duration = Dispatchers.MAILBOX_PUSH_TIME_OUT ) extends MailboxType Backpressure Mailbox BoundedMailbox(false, QUEUE_SIZE, Duration(-1, "millis"))

12 Stages How do we decompose the problem?

13 Stages One actor class per stage Shared dispatcher Individually tunable I/O Bound CPU Bound Easier to reason about Code reuse

14 Dispatchers ThreadBasedDispatcher Binds one actor to its own thread ExecutorBasedEventDrivenDispatcher Must be shared between actors ExecutorBasedEventDrivenWorkStealingDispatcher Must be shared between actors of the same type

15 Queues SEDA has a queue per stage model Akka actors have their own mailbox How do we evenly distribute work?

16 Work Stealing ExecutorBasedEventDrivenWorkStealingDispatcher "Actors of the same type can be set up to share this dispatcher and during execution time the different actors will steal messages from other actors if they have less messages to process"

17 Work Stealing Really a work "donating" dispatcher "I have implemented a work stealing dispatcher for Akka actors. Although its called "work stealing" the implementation actually behaves more as "work donating" because the victim actor takes the initiative. I.e. it actually donates work to its thief, rather than having the thief steal work from the victim."

18 Work Stealing Doesn't that conflict with blocking mailboxes?

19 Work Stealing Sending actor will block on the receiving actors mailbox before it can "donate" Might be fixed in Akka 1.1 I a test of his latest changes

20 Load Balancing Can we distribute work on the sender side?

21 Load Balancing Routing.loadBalancerActor() Creates a new actor that forwards messages in a load balancing fashion InfiniteIterator CyclicIterator SmallestMailboxFirstIterator

22 Load Balancing Doesn't the load balancer need a blocking mailbox?

23 Load Balancing Can't easily change the load balancer's mailbox Use SmallestMailboxFirstIterator directly new SmallestMailboxFirstIterator(List(actor, actor, actor))

24 Fault Tolerance Supervisors Restarts actors Stops after x times within y milliseconds Restart Strategies OneForOne AllForOne

25 Fault Tolerance Great for transient issues Network failures Not great for permanent issues OutOfMemoryError

26 // Sending a message val actors = new SmallestMailboxFirstIterator(actorsFor(classOf[MyActor]).toList) def actor = actors.next actor! Message() Final Product // Actor creation val supervisor = Supervisor(SupervisorConfig( OneForOneStrategy(List(classOf[Exception]), RETRIES, WITH_IN_TIME), Supervise(myActors)) def myactors : List[Supervise] = { val mailbox = BoundedMailbox(false, QUEUE_SIZE, Duration(-1, "millis")) val dispatcher = Dispatchers.newExecutorBasedEventDrivenDispatcher( "my-dispatcher", 1, mailbox).setcorepoolsize(pool_size).build (1 to POOL_SIZE tolist).foldright(list[supervise]()) { (i, list) => Supervise(actorOf(new MyActor("my-actor-" + i, dispatcher)), Permanent) :: list } }

27