The document discusses Command Query Responsibility Segregation (CQRS) and event-driven architecture. It defines CQRS as separating commands and queries, outlines its benefits like scalability and flexibility, and notes potential downsides like complexity. The document recommends applying CQRS for collaborative domains with parallel operations, and avoiding it for simple CRUD apps. It also discusses how event-driven architecture uses immutable states and loosely coupled applications, and how it pairs well with CQRS.

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CQRS and Event Driven
Architecture
Andre Eberhardt

2. Globalcode – Open4education
whoami
/in/andreeberhardt
@aeberha
/eberhara
Software Specialist @ ADP

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motivation

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agenda
cqrs
what’s it?
why does it matter?
when should (or not) it be applied?
event driven architecture
intro
usage with cqrs
sample app...

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what’s cqrs?
Command
Query
Responsibility
Segregation

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an architectural model?
NO

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a pattern?
YES!

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it’s not this...

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it could be this...

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... or even this!

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so, why does it matter?
Scalability
Reduce complexity (domain level)
Flexibility
Focus on domain logic / proper technology
Task-based operations / Stale data

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but... and the downsides?
Learning curve / fear of change
Complexity
Eventual consistency
"Most people using CQRS (and event sourcing too) shouldn't have done so." —Udi Dahan

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so when should I apply it?
Decision per Bounded Context
Collaborative / complex domains
Multiple operations in parallel (stale data)
Distributed / specialized teams
Command vs Read performance

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and when avoid it?
Simple / static CRUD
Sequential operations
Non-collaborative bounded contexts
Eventual consistency not acceptable

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event driven architecture

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what’s that now?
State changes
Event consumers vs producers
Fire-and-forget
Happens in the past
“User created”, “User deleted”, ...

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and why is that good?
Immutable states
Loosely coupled apps
Fits well with some rock stars (CQRS and DDD)
Async process
State reproducibility

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request-driven soa

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event driven architecture

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cqrs without events...

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cqrs and events?!

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lessons learned
Start small and extensible
Scale when you need to scale
Avoid huge / complex architectures for small problems
Avoid scaling when you don’t need to

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gurus
Greg Young
Martin Fowler
Udi Dahan

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sample app...
https://github.com/eberhara/sample-cqrs

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thank you :)
/in/andreeberhardt
@aeberha
/eberhara