Reactive Programming in Akka
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The document outlines the Akka toolkit, which is used to build concurrent, distributed, and fault-tolerant applications through an actor model. It details the roles of actors, actor references, message dispatchers, and their lifecycle management, along with fault tolerance strategies and use cases like credit card applications. Additionally, it covers remote actors, routers, and cluster functionalities for scalable systems.
Reactive Programming in Akka
- 1. Presenters: Raja Chattopadhyay and Sovon Nath May 2017
- 3. Akka is a toolkit used to build applications that are: § Concurrent § Distributed § Fault tolerant
- 5. § Concurrent systems are able to process multiple tasks at the same time § Challenge with concurrency is sharing mutable state. § Akka implements concurrency using the Actor model Actor System Actor 1 Actor 2 ActorRef class MyActor extends Actor{ def receive = { case msg => println(s"Message received : $msg") } }
- 6. § Actor Ref: Each actor is represented by an actor ref.You never get access to an Actor instance. An actor reference lets you send messages to the actor § Message Dispatcher: The dispatcher enqueues incoming messages in the mailbox. § Mailbox: It’s the piece which connects sender and receiver. Each actor will have one mail box. External Actor Ref Actor Message Dispatcher Mailbox Send Message Dispatch Runs Invoke
- 7. Supervisor Actor Child Actor 1 Actor Behavior 1 Child Actor 2 Creates / Supervises Sends message Actor Behavior 2Become Actors can perform the following actions: • Send • Create • Become • Supervise A top level actor can be created form the ActorSystem and child actors can be created from another actors Sends message
- 9. § Akka’s philosophy for fault tolerance is “let it crash”. § A supervisor doesn’t try to fix the actor or its state. It tries to recover from the state and deploys a strategy: § Resume § Restart § Stop § Escalate Unlike supervisors, any actor can monitor another actor by calling the watch method on the ActorRef.The only message that they receive is if the actor terminates.
- 10. There can be three events in a lifecycle of an actor: § Start § Restart § Stop Below are the lifecycle hooks: class MyActor extends Actor{ override def preStart(): Unit = { println("Overriding the pre start") super.preStart() } } Start/Stop Restart Start Stop Pre Start Post Stop StartStopRestart Pre StartPost StopPre restart Post restart New Instance
- 11. § Persistent actors have their states persisted. Actors are extended from the PersistentActor § It comes with a few built in persistence plugin: in-memory heap based journal, local file-system based snapshot-store and DB based journal. § Persistent actors have a persistentId, which is used to identify the events of an actor § A persistent actor can act in two ways: either recover from events or process commands class RetrieveBalance extends PersistentActor { override def persistenceId = ”1234” def receiveCommand = … def receiveRecover = … } External Actor Actor Journal Command Save Event Event saved Send Response Generate Event Apply the Event
- 13. § Members apply for a credit card § To approve a credit card, we will look into their bureau details, verification status and then make a decision (ex ‘Approved’ or ‘Declined’) § To gather the information, we will call various actors Decision App Actor Retrieve Bureau Details Actor Retrieve Verification Status Actor Finalize Decision Actor
- 14. Decision Saga Actor RetrieveBureau DetailsActor RetrieveVerifica tionStatusActor FinalizeDecisio nActorCredit card Application Decision Saga Actor RetrieveBureau DetailsActor RetrieveVerifica tionStatusActor FinalizeDecisio nActor 1 2
- 15. Lagom REST: Will be used to create an APIs Akka: Will be used to create & communicate with actors Cassandra: Will be used to store events
- 17. § Remoting is a way to scale out.This is achieved by running actors on various nodes.The nodes interact with each other using akka-remote module. § Remoting provides location transparency, i.e. the caller is not aware of the location of the actor. Remote actors will be called as if, they are local actors. § Remote actors can be referred either by § Locally creating the actor and then deploying it remotely § Look up the actor by its path Sender System (JVM) Netty Serialization Actor Selection Actor Receiving System (JVM) Actor Serialization Netty
- 18. § Routers are needed to route messages to different destination actors.These could be for Performance or Content of the Message. § Akka provides built in router functions.The actor that represents the router, loads the routing function. Built- in routers come in two flavors: § Pool – The router is responsible for creating the routees and removing them from the list when they terminate. § Group – The routees are created by the system.The routers use the actor selection to find the routee. § Some of the inbuilt routing functions available are: § Round Robin § Smallest mail box § Broadcast § Scatter-Gather Router Actor Actor 1 Actor 2 Actor 3
- 19. § Cluster is a collection of nodes § Each node has an actor system.The actor systems have the same name to be a part of the cluster. § A list of member nodes are maintained in the cluster state § The clusters can have two kinds of special nodes called leader node and seed node. § Cluster Sharding – Used to distribute actor instances over a number of nodes and being able to determine where they are, by using their persistence ids. § Cluster Singleton – This allows us to indicate that a single component should only be running on any one node at a given point of time. Joining Down Leaving Exiting Up Removed Unreachable Join (leader action) Leave (leader action) (leader action) (failure detection) (failure detection) (failure detection) (failure detection)
- 20. § Sample Applications § Actors using Scala § https://github.com/richmondtechies/scalaactors § Java Reference Applications with AKKA § https://github.com/richmondtechies/decision-app § https://github.com/richmondtechies/rewards-update-lagom § Reference material: § http://akka.io/docs § Connect with us: § [email protected] § [email protected]