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CQRS & ESWith Akka, Cassandra and

RabbitMQMiel Donkers, codecentric

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Why??

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What is CQRS

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What is CQRSCommand Query Responsibility Segregation

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Traditional Architecture

© Martin Fowler

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CQRS Architecture

© Martin Fowler

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Storing dataSimple approach

Single databaseNormalized tables for write-side dataViews for denormalizing dataKeep queries simple

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CQRS related to DDDDDD - Domain Driven DesignBounded ContextAggregateCommandEvent

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What is ES

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What is ESEvent SourcingEvent describes the state change of aggregateEvent is saved in event storeEvents can be replayed to re-create the current state.

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CQRS & ES Combined

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Architecture with Transactions

© Microsoft - CQRS Journey

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Architecture with Events

© Microsoft - CQRS Journey

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Architecture with Messages

© Microsoft - CQRS Journey

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The hard parts

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ConsistencyKey feature for the Event StoreWithout transactions, much work needed to make asreliableVersioning / timestampsEventual consistency

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Error HandlingDistinguish between business faults and errors.

Errors can be re-tried.Business faults should have pre-determinedreaction.

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Other disadvantagesMany moving parts, makes it hard to debug thesystem as a wholeCQRS not for every bounded context

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Advantages

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AdvantagesTask-based UIRead and write side can be optimized separatelyDebug in local environmentSmall components which are easy to update / fix

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Code

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User AggregateNot Finished �

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User Repositoryprivate var users = Set.empty[User]

override def receiveCommand: Receive = { case GetUsers => sender() ! users case AddUser(name) if users.exists(_.name == name) => sender() ! UserExists(name) case AddUser(user) => persist(user) { persistedUser => receiveRecover(persistedUser) sender() ! UserAdded(persistedUser) } }

override def receiveRecover: Receive = { case user: User => users += user }

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Event Senderprivate var unconfirmed = immutable.SortedMap.empty[Long, ActorPath]

override def receive: Receive = { case Msg(deliveryId, user) => unconfirmed = unconfirmed.updated(deliveryId, sender().path) val headersMap = Map(RabbitMQConstants.MESSAGE_ID -> deliveryId, RabbitMQConstants.CORRELATIONID -> deliveryId) camelSender ! CamelMessage(user.asJson.noSpaces, headersMap)

case CamelMessage(_, headers) => val deliveryId: Long = headers.getOrElse(RabbitMQConstants.MESSAGE_ID, - unconfirmed.get(deliveryId).foreach( senderActor => { unconfirmed -= deliveryId context.actorSelection(senderActor) ! Confirm(deliveryId) })

case Status.Failure(ex) =>

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Event Receiveroverride def receive: Receive = { case msg: CamelMessage => val body: Xor[Error, User] = decode[User](msg.bodyAs[String]) body.fold({ error => sender() ! Failure(error) }, { user => sender() ! Ack }) case _ => log.warning("Unexpected event received") }

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LinksMartin Fowler - CQRSMicrosoft - CQRS JourneyAkkaHeiko Seeberger - Reactive Flows Demo

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Wrapping Up

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Feedback

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The End 

Copyright 2016