Build stuff with DDD and CQRS in an onion

F# edition 

Agenda

What are we building?

Layered architecture

Onion 🧅

What if the arrow pointed inwards?

DDD Goal

• Represent domain knowledge in code
• Code is the docs
• Complex business rules are explicit
• Codebase uses words of the domain language

Learn domain before 👩‍💻

• Talk to people (domain experts - product owner etc)!
• Focus on changes to data, not static data
• Questions to answer:
  • What can happen in my domain (events)?
  • How those events are produced (workflow)?
  • How workflows are triggered (commands)?

What's in there?

Use from outside

Code example 💻

Domain

Domain revisit

• All business knowledge is in one place
• Domain experts can understand the code
• Good for complex business rules
• Protect data integrity for writes

CQRS

• Architectural pattern
• Command and queries are different application layers
• Commands change state of the system, effectful
• Queries don't change state, free of side effects

* Command Query Responsibility Segregation

CQRS Ideas

• Different data models for read and write operations

CQRS Why?

• Queries and commands are fundamentally different
  • No restrictions on queries
  • Updates are protected with domain
• Optimisation techniques 
  • Different persistence mechanisms
  • Different types of storage
  • Scale read side

Separate read/write db

Business 🧅

Steps

Code example 💻

Business layer

Business revisit

• Accept commands and triggers workflows
• Glues together persistence and domain
• Defines interface for persistence layer (command side)
• Persistence is implemented somewhere else
• Performs asynchronous communication via events

Outer onion 🧅

Outer layer

Queries

• Drop domain constraints
• Own read models in the presentation
• Potential for optimisation:
  • Drop ORM and use some lightweight SQL
  • Different storage - document db, cache etc
  • Scale by replication

Code example 💻

Presentation layer

Overview

Takeaways

• Domain centric approach
  • domain knowledge in code
  • protect writes
• CQRS
  • Separate writes and reads
  • Simpler code, performance and scalability
• Onion
  • Inner layers
    • don't depend on frameworks
    • define interfaces implemented outside
  • Outer layer
    • depends on frameworks
    • glues stuff together

THANK Y'ALL

https://github.com/MargaretKrutikova/fsharp-ddd-cqrs

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