Akka: Distributed by Design - Björn Antonsson (Typesafe)

Akka - Distributed by DesignBjörn Antonsson

@bantonsson

Wednesday, 30 October 13

@bantonsson

Overview

• Akka• Actors• Distributed by Design• Diving Into The Cluster• What The Future Brings

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• Toolkit and runtime for reactive applications• Write applications that are– Concurrent– Distributed– Fault tolerant– Event-driven

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• Has multiple tools– Actors– Futures– Dataflow– Remoting– Clustering

Actors

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Actors

• Isolated lightweight event-based processes• Share nothing• Communicate through async messages• Each actor has a mailbox (message queue)• Each actor has a parent handling its failures• Location transparent (distributable)

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Actors

• An island of sanity in a sea of concurrency• Everything inside the actor is sequential– Processes one message at a time

• Very lightweight– Create millions– Create short lived

• Inherently concurrent

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Actor code sample

public class Greeting implements Serializable { public final String who; public Greeting(String who) { this.who = who; }}

public class GreetingActor extends UntypedActor { LoggingAdapter log = Logging.getLogger(getContext().system(), this); int counter = 0;

public void onReceive(Object message) { if (message instanceof Greeting) { counter++; log.info("Hello #" + counter + " " + ((Greeting) message).who); } else unhandled(message); }}

Define the message(s) the Actor should be able to respond to

Define the Actor class

Define the Actor’s behavior

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Creating and using Actors

ActorSystem system = ActorSystem.create("MySystem"); ActorRef greeter = system.actorOf( Props.create(GreetingActor.class), "greeter");

greeter.tell(new Greeting("Charlie Parker"), null);

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Actors compared to Objects

• Think of an Actor as an Object• You can't peek inside it• You don't call methods– You send messages (asynchronously)

• You don't get return values– You receive messages (asynchronously)

• The internal state is thread safe

Why should I care?

The world is multicore!

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Amdahl’s Law

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So what's the catch?

• Really no catch• A different programming paradigm• All about tradeoffs– Some things are easier some harder

• Think different

Distributed by Design

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Remote Actors

• Sending messages decouples actors• Local or remote doesn't matter

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NODE 1 NODE 2

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Remote Actors

• Zero code change deployment decision• Add configuration to the Actor Systemakka { actor { provider = akka.remote.RemoteActorRefProvider deployment { /greeter { remote = akka.tcp://MySystem@machine1:2552 } } }}

Configure a Remote Provider

Define Remote Path Protocol Actor System Hostname Port

The "greeter" actor

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Looking up Actors

ActorSelection greeter = system.actorSelection( "akka.tcp://MySystem@machine1:2552/user/greeter");

Can you see the problem?

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Fixed addressesakka { actor { provider = akka.remote.RemoteActorRefProvider deployment { /greeter { remote = akka.tcp://MySystem@machine1:2552 } } }}

ActorSelection greeter = system.actorSelection( "akka.tcp://MySystem@machine1:2552/user/greeter");

Diving Into The Cluster

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Akka Cluster 2.2

• Gossip-Based Cluster Membership• Failure Detector• Cluster DeathWatch• Cluster-Aware Routers

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Cluster Membership

• Node ring à la Riak / Dynamo• Gossip-protocol for state dissemination• Vector Clocks to resolve conflicts• Peer based failure detector

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Node ring with gossiping Members

MemberNode

Gossip

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Gossip Protocol

• Cluster Membership• Leader Determination• Targets Random Node– Partly biased towards nodes with older state

• Push-Pull based– Sender only sends his version number– Receiver asks for newer information

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Vector Clocks

• Partial ordering in a distributed system• Detects causality violations• Used to reconcile and merge cluster state

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Failure Detection

• Uses The Phi Accrual Failure Detector• Peer Based with limited targets– B monitors A– A sends heart beats to B– B samples inter-arrival time to expect next beat– B measures continuum of deadness of A– B marks A as unreachable if A is dead enough

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MemberNode

Heartbeat

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MemberNode

Heartbeat

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Member States

• Joining• Up• Leaving• Exiting• Down• Removed• Unreachable*

unreachable*

joining

leaving

exitingdown

(leader action)

removed

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Leader

• No SPOF• Can be any node• No handover involved• Deterministically recognized by all nodes – always the first member in the sorted

membership ring

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Leader Duties

• Shift members from– Joining to Up– Exiting to Removed– Up to Down (auto-downing) to Removed

• Can only be performed on convergence

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Cluster Convergence

• A Node sees that all other Nodes have seen this version of the gossip

• Is always local to that Node• Unreachable Nodes blocks this• Mark Unreachable Nodes as Down to proceed– Manual Ops intervention– Automatic action

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Cluster Metrics

• Gossip based• Metrics about the Nodes in the Cluster– Load– CPU Usage– Processors– Heap Memory• Used, Committed, Max

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Cluster Roles

• Assign roles to Nodes (named tags)• Nodes can have multiple roles• Restrict work to certain roles• Deterministically recognized role leader

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Cluster Events

• Subscribe to be notified• Membership Changes– Up, Exited, Removed

• Leader Changed• Metrics Changed• Role Leader Changed• Member Unreachable

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Cluster DeathWatch

• Triggered by marking node «A» Down– Tell parents of their lost children on «A»– Kill all children of actors on «A»– Send Terminated for actors on «A»

Building on The Cluster

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Load Balancing

• Cluster aware routers– Round Robin Router– Consistent Hashing Router– Adaptive Load Balancing Router• Use Cluster Metrics to select target• CPU/Memory/Load

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Cluster Contributions/Patterns

• Distributed Pub/Sub Mediator– Publish and Subscribe to message flows

• Cluster Singleton– HA singleton actor instance within the cluster

• Cluster Client– Let other systems connect to the cluster

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DistributedPubSubMediator

Frontend Master

Mediator Mediator

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Frontend Master

Mediator Mediator

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Frontend Master

Mediator Mediator

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Frontend Master

Mediator Mediator

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ClusterSingleton

ClusterSingletonManager

MasterMaster(Standby)

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ClusterSingleton

MasterMaster(Standby)

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ClusterSingleton

Master Master

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ClusterClient & ClusterSingleton

Master

Mediator

Receptionist

Master(Standby)

Receptionist

ClusterClient

Worker

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Typesafe ActivatorDistributed Workers Cluster Template

• http://typesafe.com/platform/getstarted

What The Future Brings

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Gossip Optimizations

• Several times faster Vector Clock comparison• Fewer Vector Clock comparisons• Gossip message size cut in half• Gossip message scrubbing• Lazy deserialization of Gossip messages

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Return from Unreachable

• Unreachableto Reachable

• Cluster ismore resilientto fluctuations

unreachable*

joining

leaving

exitingdown

(leader action)

removed

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Rebuilt Routers

• Rebuilt from the ground• Routing logic usable in Actors• Actor Selection as routees• Improved Cluster behavior

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Persistence

• New module akka-persistence• Command sourcing & event sourcing• Based on the proven Eventsourced library• Migrate actors by persisting their state

Resources

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Survey and Resources

• Help Akka get better. Fill out the survey!– http://tinyurl.com/akka-survey

• Akka Cluster Documentation– http://tinyurl.com/akka-cluster

• Akka Cluster in Production Blog Post – Ryan Tanner• http://tinyurl.com/akka-at-conspire

@bantonsson

Coursera Course

• Principles of Reactive Programming byMartin Odersky, Erik Meijer and Roland Kuhn– Starts 4th of November 2013– 7 weeks– Workload: 5-7 hours a week– Free as in free beer

• https://www.coursera.org/course/reactive

Björn Antonsson@bantonsson @akkateam

bjorn.antonsson@typesafe.com

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