AkkaDeveloping SEDA Based

Applications

Me Ben Darfler

@bdarfler http://bdarfler.com Senior Software Engineer at Localytics

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)

Localytics

How to keep up with our growth?

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

AkkaConfigurable● Dispatchers● Mailboxes

Fault Tolerant● Supervisors

Great community● @jboner● @viktorklang

AkkaPerformant

http://blog.jayway.com/2010/08/10/yet-another-akka-benchmark/

SEDAStaged 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."1

1. http://www.eecs.harvard.edu/~mdw/proj/seda/

Backpressure

Whats the big deal?

BackpressureManditory 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)

BackpressureMailboxes

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

BoundedMailbox(false, QUEUE_SIZE, Duration(-1, "millis"))

Backpressure Mailbox

Stages

How do we decompose the problem?

StagesOne actor class per stage

Shared dispatcher

Individually tunable● I/O Bound● CPU Bound

Easier to reason about

Code reuse

DispatchersThreadBasedDispatcher

● Binds one actor to its own thread

ExecutorBasedEventDrivenDispatcher● Must be shared between actors

ExecutorBasedEventDrivenWorkStealingDispatcher● Must be shared between actors of the same type

Queues

SEDA has a queue per stage model

Akka actors have their own mailbox

How do we evenly distribute work?

Work StealingExecutorBasedEventDrivenWorkStealingDispatcher

"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"1

1. http://doc.akka.io/dispatchers-scala

Work StealingReally 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."1

1. http://janvanbesien.blogspot.com/2010/03/load-balancing-actors-with-work.html

Work Stealing

Doesn't that conflict with blocking mailboxes?

Work StealingSending actor will block on the receiving actors mailbox before it can "donate"

Might be fixed in Akka 1.1● I owe @viktorklang a test of his latest changes

Load Balancing

Can we distribute work on the sender side?

Load BalancingRouting.loadBalancerActor()● Creates a new actor that forwards

messages in a load balancing fashion

InfiniteIterator● CyclicIterator● SmallestMailboxFirstIterator

Load Balancing

Doesn't the load balancer need a blocking mailbox?

Load BalancingCan't easily change the load balancer's mailbox

Use SmallestMailboxFirstIterator directly

new SmallestMailboxFirstIterator(List(actor, actor, actor))

Fault ToleranceSupervisors

● Restarts actors● Stops after x times within y milliseconds

Restart Strategies● OneForOne● AllForOne

Fault ToleranceGreat for transient issues

● Network failures

Not great for permanent issues● OutOfMemoryError

Final Product// Actor creationval 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 }}

// Sending a messageval actors = new SmallestMailboxFirstIterator(actorsFor(classOf[MyActor]).toList)def actor = actors.nextactor ! Message()

Thanks

@bdarflerhttp://bdarfler.com