Potential issue when mocking in Groovy

The groovy.mock.interceptor.MockFor and grails.test.GrailsMock allow for mock objects in Groovy and Grails, respectively. I've been using both of these classes with good success for a long time. But recently, a small refactoring involving the the removal of a parameter from a method signature has caused me to re-evaluate the mock object usage in a dynamic language like Groovy.

As I said, there was a refactoring done on the public contract of a service, where the method name stayed the same, but a parameter was removed from the method signature. The contract unit tests for this service were changed to drive the refactoring ("test-driving the refactoring"). However, the collaboration unit tests, where this service is now acting a dependency, were not changed and they continued to pass successful. I tried cleaning the old .class files and compiling the Groovy tests, but to no avail, the unit tests which mocked this service continued to pass successfully, even though the method signature no longer existed on the real service implementation. After perusing the javadoc documentation, there does not seem to be any functionality in either of these classes to verify that the type that is being mocked has a method signature that matches the method signature being mocked. Therefore, these classes can mock methods which are non-existent on the real dependency implementations. Fixing the issue involved finding the method signature using a text search.

The whole episode was a bit unsettling; we have a lot of unit tests and we may be testing scenarios which are not representative of the real world. In my case, the real world scenario manifested itself as a runtime exception stating that the method was missing. In the case of Java and Mockito, the method signature change would result in a compilation error where the changed method signature was mocked in unit tests. My takeaway was to be more diligent with my refactoring and really ensure that I have changed all places in the code where a particular method is referenced.

Allocating business logic in Grails

The dilemma

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.

Controllers

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!

Domain objects

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).

Services

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.

The desire

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.

Conclusion

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.