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Fighting Bottlenecks with CQRS/ES ResearchGate Berlin Nov 30, 2013 Mathias Verraes @mathiasverraes verraes.net

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Student of Systems Meddler of Models Labourer of Legacy

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Blog verraes.net DDDBE domaindriven.be Podcast elephantintheroom.io

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Business Problem A Simplified Model Command/Query Responsibility Segregation Event Sourcing Read Model Projections Eventual Consistency

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Business Problem

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Sell a gazillion concert tickets to desperate fans … in a hurry!

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Don’t crash during peaks Don’t annoy our customers Don’t sell more than we have

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A Simplified Model

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Model invariants as a Presale Aggregate?

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#reservedTickets #soldTickets #availableTickets Presale # Tickets

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Model invariants as individual Ticket Aggregates

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reserved sold available Individual Ticket status

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Command/Query Responsibility Segregation

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Client Remote Façade Application Services Domain Model ORM Database

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Highly cohesive Read + Write Model?

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Bottleneck

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Client Database Database Remote Façade Application Services Domain Model ORM Eventually Consistent

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CQRS challenges the assumption that reading and writing are supposed to share the same abstractions.

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CQRS challenges the assumption that reading and writing are supposed to share the same models.

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CQRS challenges the assumption that reading and writing are supposed to share the same databases.

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CQRS challenges the assumption that reading and writing are supposed to share the same applications.

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Segregation of read and write is a radical form of decoupling.

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Client Domain Model Remote Façade Remote Façade Application Svcs Application Svcs Domain Model ORM ORM Database Database

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Radical!

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Client Write Model Read Model

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Client Write Model Read Model DTO Commands Events

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Interact with State Guard Invariants Project State

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Task Based UI Event Store Polyglot Persistence

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No content

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Event Sourcing

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TicketWasReserved [ ticketId, forUser, onDate ]

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Ticket state = Ticket history ! TicketWasSold [ toUser ] TicketWasReserved [ forUser, onDate ] ReservationWasReleased [ ] TicketWasReserved [ forUser, onDate ] TicketWasPrinted [ inPresale ]

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Mature domains already work this way ! AccountWasDebited [ 20 ] AccountWasDebited [ 10 ] AccountWasCredited [ 100 ] AccountWasOpened

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BankAccount { debit(amount) { guardThatAccountIsNotOverdraft(amount) recordThat( new AccountWasDebited(amount) ) } }

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BankAccount { apply(AccountWasDebited event) { amount = amount - event.amount } }

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BankAccount { getRecordedEvents() {} reconstituteFromHistory(eventHistory) {} }

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Read Model Projections

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TicketWasSold TicketWasReserved ReservationWasReleased TicketWasReserved TicketWasPrinted Ticket ! !

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TicketWasSold TicketWasReserved ReservationWasReleased TicketWasReserved TicketWasPrinted AvailableTicket TotalNumberOfAvailableTickets AverageNumberOfTicketsPerBuyer AverageNumberOfReleasesPerTicket

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Eventual Consistency

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Writes are immediately consistent Reads are eventually consistent

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Many issues stemming from eventual consistency can be solved in the user interface

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A Ticket can only be sold once. How do we know it’s available?

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Get a batch of x*2 PossiblyAvailableTickets ! ReserveTicket ack/nack loop while acks < 3 To buy x tickets:

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More? ! Testing Context Mapping Event Storming Process Managers Occasionally Connected Systems Reporting Legacy Integrations …

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Questions? verraes.net @mathiasverraes