Command-Query-Responsibility-Segregation (CQRS) Makes Domain Models Practical
As I've been learning more about software application architecture and trying to get my feet wet with real Object Oriented Programming (OOP), one of the biggest stumbling blocks has been trying to reconcile the constrained nature of a domain model with the rather numerous and diverse set of user interfaces (UI) that leverage said domain model. In my experimentation, as each UI was added to the application, it seemed that the domain model would have to grow in order to deliver the new data requirements. This eventually created a large and sloppy domain model that loaded far more information than could ever be needed by any single request. The situation seemed hopeless. But then, I started reading about Command-Query-Responsibility-Segregation (CQRS) and I had an enormous "Ah-ha!" moment of clarity.
According to people like Martin Fowler and Udi Dahan, Command-Query-Responsibility-Segregation (CQRS) is a rather involved concept that makes use of things like application events and different, "eventually consistent," database stores; but for me, I like to dumb it down quite considerably such that my unfrozen caveman brain can comprehend it. When I think about CQRS, I sum it up as follows:
Commands (ie. requests to change the state of an application) are handled differently than requests to query the state of an application.
This is huge! This separation of responsibilities suddenly makes the domain model practical. Domain models are awesome for making sure that the application stays in a valid state. Databases, on the other hand, are awesome for gathering relational data. CQRS speaks to both of these strengths. If you want to change the state of the application, go through the domain model; if you want to query the state of the application, go directly to the most optimized source of data!
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When I first started learning about Object Oriented Programming (OOP), people told me to stop thinking about the database. And, I tried - I really tried. The problem with this, however, is that you also stop thinking about SQL. And, subsequently, you forget that SQL is really awesome at doing a lot of what you need to do in your application - namely, present data. CQRS gives you a way to keep a concentrated domain model while allowing you to grow your "query layer" as the needs of your user interface evolve.
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