Command and Query
Responsibility Segregation
Cameron Fletcher
Agenda

What is CQRS?

A look at a system that uses CQRS

What have we just seen?

Why would anybody ever do this?
What is CQRS?
Command and Query
Responsibility Segregation
CQRS is simply the creation of two objects where there was
previously only one.
The separation occurs based upon whether the methods are
a command or a query.
Application
Denormalizer
Fully Consistent
Data Store
Eventually Consistent
Data Store
Command Service
Query Service
Client
A Look at a System
that uses CQRS
What Have We Just Seen?
Architecture

Domain

Application

Fully Consistent Write Model

Denormalizer

Eventually Consistent Read Model

Command and Query Services

Task Based User Interface
Putting Things into Context
Why Would I Ever Do This?
Advantages

Performance and Scalability

Separation of Concerns

Reduced Complexity of Domain

Ability to Parallelize Development

Highly Decoupled

Increased Testability

Easier to Adapt to Change
Disadvantages

Increased Complexity of Architecture

Learning Curve for Developers

Fear of Change
When Not To Use CQRS
(In This Way)

Lack of Requirements

No Domain to Work With

If the Benefit isn’t Worth It

When it’s not Useful

On a System as a Whole

Not Just for Scalability

Legacy Systems
Summary
Questions?