cqrs and es with lagom
TRANSCRIPT
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CQRS & EVENTSOURCING
With Lagom
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Miel Donkers@mieldonkers
github.com/mdonkers
@codecentric_nl
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AgendaCQRS conceptsEvent Sourcing conceptsIntroduction Lagom frameworkCQRS & ES as basis for LagomLagom code / demo
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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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DDD example
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Command - Event - State
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Service owns data
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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 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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AdvantagesAuditing and historical tracingRead and write side can be optimized separatelyDebug in local environmentSmall components which are easy to update / fixNo ORM impedance mismatch
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Lagom microservice framework
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Design philosophyDistributedAsynchronousProductivity
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ConceptsMicroservicesPolyglotDesign Driven APIsImmutabilityEvent Sourcing and CQRS
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OpiniatedServiceCall<Source<String, NotUsed>, Source<String, NotUsed>> stream();
@Overridedefault Descriptor descriptor() { return named("stream").withCalls(namedCall("stream", this::stream)) .withAutoAcl(true);}
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Core technologiesAkka
Akka StreamsAkka PersistenceAkka Cluster
PlayConductR
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Lagom persistenceCassandraPersistentEntity
Requires Command, Event and StateSingle instance kept in memory
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Command - Event - State
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Get me all inspections with remarks for a given ship
Query example
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Lagom JDBC supportAvailable in 1.2 releaseSlickStoring and reading entities
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CODE
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Wrapping Up
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Lessons learnedConventions make setup easyFramework on top of many layers
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...[INFO] Service locator is running at http://localhost:8000[INFO] Service gateway is running at http://localhost:9000...[INFO] (Service started, press enter to stop and go back to the console...)[error] c.l.l.i.s.ServiceRegistrationModule$RegisterWithServiceRegistry - Service name=[hello] couldn't register itself to the service locator.com.lightbend.lagom.javadsl.api.transport.TransportException: <head>...[error] a.c.s.PersistentShardCoordinator - Persistence failure when replaying events for persistenceId [/sharding/HelloEntityCoordinator]. Last known sequence number [0]com.lightbend.lagom.javadsl.api.transport.PolicyViolation: <head>...
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LinksMartin Fowler - CQRSMicrosoft - CQRS JourneyLagom FrameworkMiel Donkers - Lagom CQRS Demo
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Thank You!
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