1. CQRS is an architectural pattern that separates commands and queries based on the Command Query Responsibility Segregation (CQRS) principle. 2. MediatR is a .NET library that implements the mediator pattern to support request/response and publish/subscribe interactions using CQRS. It allows defining commands, handlers, and processing asynchronously or synchronously in a simple way. 3. Brighter is another .NET library that supports CQRS with additional features like command dispatching, processing pipelines, and quality of service policies for timeout, retry, and circuit breaking patterns. It requires more manual configuration than MediatR.

1. CQRS.frameworks
Max Vasyshchev

2. Agenda
• Standard approach
• CQRS
• Frameworks

3. Application
Standard approach: N-tier architecture
Presentation
Business Logic
Data Access

4. Standard approach: client interaction
Application
Client
Model/Action
Model/View

5. Standard approach:
implementation
ASP.NET application,
SPA + WebAPI
SPA
WebAPI
Services
Controllers
Repositories

6. UI, CRUD-based

7. Task-basedCRUD-based

8. Business logic

9. Problems

10. Standard approach: problems
Shared entities:
• Promote procedural style instead of OO
• Ineffective reads in Data Access layer (ORM loads whole object, even
if you need only some fields)
https://en.wikipedia.org/wiki/Anemic_domain_model
https://martinfowler.com/bliki/AnemicDomainModel.html

11. Too big classes? Split them!
CQRS to the rescque

12. CQRS is not
• New
• Shiny
• Silver bullet

13. CQRS is not
• An architecture
• A guiding principle (CQS is)
• An eventual consistency
• Messaging
• And also does not require message bus
• Having separate models for reading and writing
• Event sourcing
• Linked to DDD

14. CQRS is
• Architectural pattern
• First described by Greg Young
• Based on Command & Query Separation (CQS) principle
• Simple enough to be described in 5 minutes

15. Command & Query Separation principle
• Object’ methods are either Queries or Commands
• Queries return value (data) and do not change system state
• Commands change system state and do not return values

16. CQRS
• Stands for Command Query Responsibility Segregation
• Maintains CQS viewpoint
• Essentially, CQS + splitting object into Command and Query parts

17. CQRS sample

18. MediatR
• Mediator pattern implementation for .NET
• Supports
• Request/response interaction
• Pub/sub interaction
• sync/async processing
• No dependencies
• Easy to setup for: Autofac / LightInject / Castle Windsor / Ninject /
Simple Injector / StructureMap / Unity
• Platforms:
• .NET 4.5 and netstandard 1.1

19. MediatR: request/response in 6 easy steps
1. Define command result
2. Define command parameters

20. MediatR: request/response in 6 easy steps
3. Create command handler
* for synchronous processing here is IRequestHandler interface

21. MediatR: request/response in 6 easy steps
5. Register MediatR in your IoC container
4. Call MediatR instance to invoke handler
6. PROFIT

22. MediatR on practice
Typical
logic

23. Transform
per method:

25. Brighter
• Command Dispatcher, Processor, and Task Queue implementation
• Supports:
• Request/response
• Pub/sub
• Sync/async messaging
• Queues and processing pipelines
• Dependencies:
• Newtonsoft.Json
• Polly
• Platforms:
• .NET 4.5.2 and netstandard 1.6.1

26. Brighter Command

27. Brighter Query

28. Brighter Call

29. Brighter Configuration
• Requires some manual commands and handlers registration
• Also supports quality of service (QoS) policies
• Timeout pattern
• Retry pattern
• Circuit-Breaker pattern
• Is in active development, so there might be a mess in documentation
sometimes, missing/obsolete pieces, etc (issue a pull request )

30. More libs
• Mass Transit
• Rebus
• NServiceBus
All of them are rather messaging frameworks for interprocess
communication

31. Summary
1. CQRS is not for all apps (and not for the whole app)
2. MediatR – concise, simple, easy to start
3. Brighter – seems to be more powerful, sometimes hard to grasp
4. Do manage complexity in your business logic!

32. Links
CQRS:
Greg Young: http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/
https://cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf
Martin Fowler: https://martinfowler.com/bliki/CQRS.html
Udi Dahan: http://udidahan.com/2009/12/09/clarified-cqrs
MS patterns & practices, CQRS Journey book (big): https://msdn.microsoft.com/en-us/library/jj554200.aspx
Libs:
MediatR: https://github.com/jbogard/MediatR/wiki
Brighter: https://brightercommand.github.io/Brighter/index.html
Bonus:
Nice talk by Jimmy Bogard on CQRS and his approach: https://www.youtube.com/watch?v=wTd-VcJCs_M

33. Thanks
See you later