This document discusses Command Query Responsibility Segregation (CQRS). It begins with an agenda and overview of why CQRS is used, including for complex domains, separating read and write boundaries, and enabling scalability. It then explains the typical architecture with one object for data access versus CQRS, which separates reads and writes into different objects or services. The document outlines some of the benefits of CQRS like skill set segregation and enabling divergent models. It also notes that CQRS is not a top-level architecture, but can be applied within bounded contexts, and emphasizes that CQRS is simply separating reads from writes rather than implying specific patterns or technologies.

1. Command Query
Responsibility Segregation
(CQRS)
Derek Comartin
@derek_comartin
CodeOpinion.com

2. Agenda
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Why?
Typical Architecture
What is CQRS
How to apply CQRS
Other benefits
Why Not?
Takeaways

3. Why?
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Complex Domain
Read & Write Boundaries
Divergent Change
Scalability
Skill Set Segregation
Architecture Choices

4. Typical Architecture
Data Storage
Domain
Object
Domain
Object
Application Service
Facade
DTO
DTO
Client

5. Typical Architecture - Client Side
6
Ack/Nak Response
1
5
Request DTO
2
Send DTO to
Server
Display DTO in
View
3
4
Modify/Rebuild
DTO

6. XML of DTO
<BankAccount>
<Id>123</Id>
<CustomerId>456</CustomerId>
<Type>Savings</Type>
<Overdraft>200.00</Overdraft>
<InterestRate>0.0035</InterestRate>
<Active>True</Active>
</BankAccount>

7. Typical Architecture - Analysis
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Simple & Common
Tooling: ActiveRecord, ORM, AutoMapper.
Data Driven
Potentially Anemic Domain Model
Hard to scale data storage

8. Command Query Separation
Every method should either be a command
that performs an action, or a query that
returns data to the caller, but not both.

9. Command Query Responsibility Segregation
”CQRS is simply the creation of two objects
where there was previously only one.”
-Greg Young

10. Typical Service
BankAccountService
{
BankAccount CreateBankAccount(BankAccount);
BankAccount GetBankAccount(AccountId);
decimal UpdateBalance(AccountId, Balance);
void UpdateBankAccount(BankAccount);
decimal GetBankAccountBalance(AccountId);
}

11. CQRS Services
BankAccountReadService
{
BankAccount GetBankAccount(AccountId);
decimal GetBankAccountBalance(AccountId);
}
BankAccountWriteService
{
BankAccount CreateBankAccount(BankAccount);
decimal UpdateBalance(AccountId, Balance);
void UpdateBankAccount(BankAccount);
}

12. "Defining the CQRS pattern is easy. Realizing the
benefits that implementing the CQRS pattern can
offer is not always so straightforward."
– Microsoft Patterns & Practices
"This simple notion leads to some profound
consequences for the design of information
systems."
–Martin Fowler

13. Complex Domain
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(Distributed) Domain Driven Design
Behavior
Intent

14. Typical Architecture
Data Storage
Domain
Object
Domain
Object
Application Service
Facade
DTO
DTO
Client

15. Simple CQRS
Data Storage
Domain
Object
Domain
Object
Application Service
Read Layer
Facade
Facade
Query
DTO
Command
DTO
Client

16. CQRS - Client Side
6
Ack/Nak Response
(optional)
1
5
Request DTO
2
Send Command
Object to Server
Display DTO in
View
3
4
Build Command
Object

17. Command = Intent
DepositCommand
{
Guid AccountId;
decimal DepositAmount;
}

18. CQRS Services
BankAccountReadService
{
BankAccountViewModel GetBankAccount(AccountId);
decimal GetBankAccountBalance(AccountId);
}
BankAccountWriteService
{
void Send(OpenBankAccountCommand);
void Send(DepositCommand);
void Send(WithdrawlCommand);
void Send(DeactivateCommand);
}

19. More Options
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Multiple Models
Domain Model = 3rd Normal Form
Read Model = 1st Normal Form

20. Read & Write Boundaries
Divergent Change
Data Storage
Domain
Object
Domain
Object
Application Service
Read Layer
Facade
Facade
Query
DTO
Command
DTO
Client

21. Skill Set Segregation
Data Storage
Domain
Object
Domain
Object
Application Service
Read Layer
Facade
Facade
Query
DTO
Command
DTO
Client

22. Scalability
Event Storage
Message Queue
Event Handlers
Events
Data Storage
Domain
Object
Data Storage
Domain
Object
Command Handlers
Read Layer
Facade
Message Queue
Command
DTO
Query
DTO
Client

23. Notes
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CRUD (not with DDD)
Not top level architecture
Determine per bounded context

24. Takeaways
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Separate reads and writes between two
objects.
That’s it.
Everything else (DDD, Event Sourcing,
Messaging…) is not CQRS. They just fit
extremely well.

25. Resources
Greg Young
@gregyoung
goodenoughsoftware.net
Udi Dahan
@udidahan
udidahan.com
Microsoft Patterns & Practices
CQRS Journey