When Microservices met Event Sourcing

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When Microservices met Event Sourcing Vinicius Gomes

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Vinicius Gomes ! Software developer at ThoughtWorks ! vvgomes.com/blog ! twitter.com/vvgomes

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Agenda ! The Traditional Approach ! Introduction to Event Sourcing ! Command Query Responsibility Segregation ! Implementing the "Event Sourced Microservices" Architecture

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The Traditional Approach

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Example: Online Restaurant

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Online Restaurant

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Online Restaurant ! Open an order

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Online Restaurant ! Add Menu Item

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Benefits ! Simple ! HTTP ! Technology agnostic

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Challenges ! Coupling

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Challenges ! Coupling ! Resilience

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Challenges ! Coupling ! Resilience ! Response time

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Challenges ! Coupling ! Resilience ! Response time ! User intent POST /orders/8659a0d6/items/ “Add item to order”

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Challenges ! Coupling ! Resilience ! Response time ! User intent ! Current / mutable state

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Event Sourcing

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Domain Events ! Model the past ! Immutable ! Immortal

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Event Store ! Append only ! Sequential ! History

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Aggregates ! Applying events

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Aggregates ! Applying events

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Aggregates ! Applying events

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Aggregates ! Applying events

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Aggregates ! Applying events

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Aggregates ! Applying events

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Aggregates ! Applying events

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Commands ! Represent user intent ! Cause new events

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Commands ! Commands are more meaningful than HTTP methods POST /orders/8659a0d6/items/ AddItemToOrderCommand as opposed to

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Commands How do clients send commands?

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Commands ! A collection resource that accepts multiple commands POST /orders/8659a0d6/commands/

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Commands ! Command URI POST /orders/8659a0d6/orderItems/commands/add

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Commands

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Commands

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Commands Queries?

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Queries ! Event Store is not good for queries ! Most applications ! A few writes ! A lot of reads

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Command Query Responsibility Segregation

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CQRS

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CQRS

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CQRS ! Command Model ! Commands definitions ! Events definitions ! The aggregate ! Aggregate repository ! Write only API ! Query Model ! Event listeners ! Query entities ! Repositories ! Read only API

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Sample Implementation

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Example: Online Restaurant

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Example: Online Restaurant

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Example: Online Restaurant

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Example: Online Restaurant

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Technologies ! Java 8 ! Spring Boot ! Axon Framework ! Spring Data REST ! RabbitMQ

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Conclusions

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Microservices + Event Sourcing ! Benefits

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Microservices + Event Sourcing ! Benefits ! History based queries

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent ! Decoupling

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent ! Decoupling ! Resilience

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent ! Decoupling ! Resilience ! Challenges

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent ! Decoupling ! Resilience ! Challenges ! Complexity ! Snapshots

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent ! Decoupling ! Resilience ! Challenges ! Complexity ! Snapshots ! Upcasting

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Microservices + Event Sourcing ! Benefits ! History based queries ! Audit log by design ! Immutability ! User intent ! Decoupling ! Resilience ! Challenges ! Complexity ! Snapshots ! Upcasting ! Race conditions