Sunday, August 4, 2013

When your commands spell CUD

A good while ago, I blogged on commands (and queries). After exploring various flavors, I eventually settled on this one; commands, handlers and an in-memory bus that serves as a command executor.

Commands help you in supporting the ubiquitous language by explicitly capturing user intent at the boundaries of your system - think use cases. You can look at them as messages that are being sent to your domain. In this regard, they also serve as a layer over your domain - decoupling the inside from the outside, allowing you to gradually introduce concepts on the inside, without breaking the outside. The command executor gives you a nice pipeline you can take advantage of to centralize security, performance metrics, logging, session management and so on.

We always need to be critical of abstractions though, and regularly assess their value. A smell that might indicate that commands might not be working for you, or are adding little value, is that the first letters of your commands spell CUD - Create Update Delete.

For example; CreateCarCommand, UpdateCarCommand and DeleteCarCommand.

The language needs attention

Possibly, your team hasn't fully grasped the power of cultivating the ubiquitous language. If you start listening to your domain experts, you might end up with totally different command names; TakeInNewCarCommand, RepaintCarCommand, InstallOptionCommand and RemoveCarFromFleetCommand.

Maybe though, there is no language at all, and you're really just doing CRUD. If the context you are working on is implementing a generic or supporting subdomain this might not be terrible.

If I'm doing CRUD, do I still need commands?

Commands help you decouple the inside from the outside. If there is no domain on the inside though, they can still help you decouple the application layer from other concerns. You might prefer to use another facade to separate concerns though, such as a thin service layer. I don't think the service layer abstraction gives you anything commands don't though. Maybe you don't find any value in separating things at all, and just dump everything in the the application layer.

All of these approaches are valid. You just have to consider the trade-offs. With these last approaches, next to losing decoupling in from out, you also lose that central pipeline that a command executor gives you.

Doesn't my application layer give me this pipeline for free?

It sure can. Looking at modern networking stacks, these all have interception points built in. For example; NancyFx allows you to hook in the request pipeline using before and after hooks; Web API gives your message handlers and action filters; and WCF has a concept of interceptors.

Not all application frameworks do though - think of frameworks targeting desktop software.

The advantage of having your own pipeline, instead of solely having to rely on your application framework's interception points, is that you're boss and don't have to study the ins and outs of each framework, and hope for the framework to have thought of your needs. Also when you need to support multiple application layers, you don't have to implement all features twice.

What about using aspects instead of a pipeline to centralize all these concerns?

You can - instead of a pipeline - use aspects to take care of croscutting concerns such as security, logging, session management.. and have them woven into your executable at compile- or runtime. I think of aspects as if they were macros, which save you on lines of code written, but also often conceal the real problem; missing concepts. While they try to sell you on separation of concerns, notice that you're actually still producing procedural code, but instead of writing it by hand, you're now letting an AOP framework do it for you. Add harder testing, debugging and readability to the bunch, and you can understand why I'm not a fan of AOP. There are scenarios where this all doesn't matter much though, and to strengthen the cliche; granular logging is a good use case.

When your commands spell CUD, it might indicate you could do without them. Do realize what the consequences are of taking them away though;
  • you lose the opportunity to capture user intent at the boundaries of your system, to strengthen the ubiquitous language
  • you may need an alternative facade to decouple in from out
  • you lose that command executor serving as your own pipeline

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