I've been on a couple of larger Grails projects in the past year and half and I'm witnessing a disturbing phenomenon. The allocation of business logic responsibility across the abstractions that Grails provides today is causing considerable pain. Grails provides controllers, services and domain objects where business logic can reside. I'll contend in this blog entry that these abstraction categories work well for small- to medium-sized Grails projects, but thing quickly start to unravel once your application gets to be large.
I see a lot of business logic code in controllers these days. Controllers shouldn't contain any business logic whatsoever. The controller's responsibility is to handle the web request and response. Anything else should be delegated to a collaborator. Don't do it!
The next logical place to put business logic is in the domain class. Allocating responsibility here works to a point, but you will quickly encounter issues when you need business logic that resides in services. I'm not a fan of injecting Grails services into domain classes. This situation quickly spirals out of control and makes unit testing very difficult to perform. For simple per-domain business logic, free free to allocate to the domain class. Anything more, and it belongs in a service (or something else, which we'll discuss in a bit).
So most business logic seems to end up in Grails services these days. That's what the creators of Grails intended. I have no qualms about that. The beef I have with services is that a method is a crappy abstraction for representing business logic in the system. On larger Grails projects, the service approach seems to break down, as services seem to take on more and more responsibility, making them more difficult to test. I'm also witnessing a lot of code duplication; in it's current incarnation, there is no delineation of public API services versus private services which the public API services compose larger sets of business logic with. What we end up with is large, hard-to-test service methods that collaborate with too many dependencies and do too much.
I want an abstraction in Grails that promotes proper factoring of business logic into unit-testable abstractions. These abstractions can be composed into larger abstractions to provide the necessary logic to fulfill the requirements of the system. The chain of responsibility design pattern may offer some value here. Individual commands that have a singular responsibility can be created, unit tested, and finally composed into "chains" of commands that provide the necessary functionality of the system. The command chains can be integration tested to ensure that the individual commands composition provides the functionality required by the customer/business. When new functionality is needed, a new command chain is created, reusing existing commands where appropriate and creating new commands where functionality does not exist. Spring Batch has a similar concept that is core to its design.
I hope to blog a bit more around this in the coming weeks. I really like Grails and would love to see its usage increase in the coming months and years. I think it has some really cool features that allow you to get up and running very quickly. The plugin system alone is a huge advantage to using Grails, because features like a Chain of Responsibility executor can easily be added to the core Grails system.