Command Query Responsibility Segregation (CQRS)

CQRS is architectural pattern that separate responsibility of handling Command(write operation) from Query (read operation). Note that, CQRS ≠ DDD : While CQRS can be used with DDD, but they are completely different approach, DDD is not about applying CQRS, focus on modeling teh BS domain, CQRS focuses on seperating operations. It just depending on us to use or not. Main principles:

• ~ Speration: Command represent an action to system, changes system state (create/update/delete), may have side effects. Query represent a request for retrieves data, no side effects, read-only.
• ~ Commands: Usually contain necessary data to perform an action. Does not return data, exception confirmation or success status.
• ~ Command Handler: This class contain BS logic for handling Command, each Command has one handler and this handler interact with repositories (or UOF).
• ~ Query: Are used to fetch data from system and It has own model separated from Commands. Unlike Commands it does not change state, only read-only operations included here.
• ~ Query Handler: This class contain BS logic for fetching data from source, return it in a suitable format.
• ~ Models: In traditional architecture , the same model is used for both, reading and writing operation. In CQRS these responsibilities are separated, allowing better performance. The Models for Command and Query are different, not reflect with each other's structure. Model for Commands focus on modifying state, Model for Query focus on data retrieval.

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MediatR

The problem with CQRS is while it sepates operation for reading and writing, which increase the complexity of architecture. Managing two seperate pipelines, makes system harder to understand and especially for smaller project, and MediatR helps to solve some of issues comes with CQRS. It is popular .NET library used to implement mediator pattern, which decouple the sender (eg: controller) from receiver (eg: command/query handler). Act as a mediator between request and handler, instead of directly invoke handler from Controller, we send request to MediatR and It will route it to coreccet handler. This help to keep clean seperation of concern, and make code easier to test.

All diagrams, Folder structure, Code example in this blog were created by me using Lucid , Canva and IDE If you'd like to contribute, request more diagrams or text, or suggest improvements, feel free to contact me

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