These actions are responsible for fetching the data, but not changing it. The Get methods have no side effects (they do not mutate the state). We are certain that this function will have side effects as it will mutate data and does not have a return value. The Edit function does not return a value, its only job is to edit a book by name. So, what does that really mean? In its most basic form, you separate queries from commands by function: Simply put: a query should never mutate state, while a command can mutate state but should never have a return value. Methods should return a value only if they create no side effects. It all comes down to two principles.Įvery method should either be a command that performs an action, or a query that returns data to the caller, but never both. In case it's the first time you have ever heard about this term, I encourage you to read the formal definition of CQS on Martin Fowler’s blog. Command Query Separation (CQS)īertrand Meyer devised CQS, or Command Query Separation, as part of his work on the Eiffel programming language.
Subscribe to this magazine for FREE and receive all previous, current and upcoming editions, right in your Inbox. NET Core, MVC, Azure, DevOps, ALM, TypeScript, Angular, React, and more. This magazine is aimed at Developers, Architects and Technical Managers and covers C#, Patterns. This tutorial is from the DotNetCurry(DNC) Magazine with in-depth tutorials and best practices in. This will allow us to tackle a lot of these goals and to keep code readable, structured, reusable and less error prone. In this tutorial, I'll focus on how to design software using the Command Query Separation (CQS) pattern. These factors, let’s call them architectural goals, include (but are not limited to) : When you choose an architecture, a lot of factors come into play.