cqrs: an introduction for beginners

Command QueryResponsibility Segregation

Jonathan Oliver

Who Cares?

“This time, it’s going to be different. Trust me.”

“Doctor, it hurts when I do that.”

Why?

UI

Services / Caching

Business Logic

DAL

Database

A Question of Perspective

Why are we doing this?

It’s a “Best Practice”.

The way it’s always been done.

A Few Problems

Bottlenecks Caching

Scalability Translation code Impedance mismatch Race conditions Stale data

UI

Services / Caching

Business Logic

DAL

Database

Stale Data?

Get data Get data

Update data

Stale data

Queries

If it’s stale anyway…

List of Customers…………………….…………………….…………………….…………………….…………………….

(from 10 minutes ago)

A Persistent View Model

Read only. Stored in structure used by view.

Persistent View Model

Query

SELECT * FROM Table WHERE ID = …

That’s Blasphemy!

It’s the simplest thing that could possibly work.

Testability? Maintainability? Security?

Data already shown over the web. It’s already in your cache on the web

tier. Role-based SELECTs.

Commands and User Input

What is the user telling us?

Do we even know?

How?

Task-based UIs vs. CRUD-based UIs.

Capture user intent explicitly.

Validation and Business Rules

Pre-validation from view model.

Validation: Is this command complete? String lengths Required fields Regex, etc.

Business Rules: Can I do what the user wants? Where is the source of truth?

Good Commands

Validated on client.

Revalidated on server.

Reply can be asynchronous: Amazon

CQRS

Image courtesy Udi Dahan

Service Layer

Transaction Script. Active Record / Table Module. Domain Model.

No Getters/Setters

Transactional. Separate database from queries. The “source of truth”.

Questions?