Approval testing – value for the money

I am a believer in the value of testing. However not all tests are equal, and actually not all tests provide value at all. Raise your hand if you’ve ever seen (unit) tests that tested every corner case of trivial piece of code that’s used once in a blue moon in an obscure part of the system. Raise your other hand if that test code was not written by human but generated.

As with any type of code, test code is a liability. It takes time to write it, and then it takes even more time to read it and maintain it. Considering time is money, rather then blindly unit testing everything we need to constantly ask ourselves how do we get the best value for the money – what’s the best way to spend time writing code, to write the least amount of it, to best cover the widest range of possible failures in the most maintainable fashion.

Notice we’re optimising quite a few variables here. We don’t want to blindly write plenty of code, we don’t want to write sloppy code, and we want the test code to properly fulfil its role as our safety net, alarming us early when things are about to go belly up.

Testing conventions

What many people seem to find challenging to test is conventions in their code. When all you have is a hammer (unit testing) it’s hard to hit a nail, that not only isn’t really a nail, but isn’t really explicitly there to being with. To make matters worse the compiler is not going to help you really either. How would it know that LoginController not implementing IController is a problem? How would it know that the new dependency you introduced onto the controller is not registered in your IoC container? How would it know that the public method on your NHibernate entity needs to be virtual?

In some cases the tool you’re using will provide some level of validation itself. NHibernate knows the methods ought to be virtual and will give you quite good exception message when you set it up. You can verify that quite easily in a simple test. Not everything is so black and white however. One of diagnostics provided by Castle Windsor is called “Potentially misconfigured components”. Notice the vagueness of the first word. They might be misconfigured, but not necessarily are – it all depends on how you’re using them and the tool itself cannot know that. How do you test that efficiently?

Enter approval testing

One possible solution to that, which we’ve been quite successfully using on my current project is approval testing. The concept is very simple. You write a test that runs producing an output. Then the output is reviewed by someone, and assuming it’s correct, it’s marked as approved and committed to the VCS repository. On subsequent runs the output is generated again, and compared against approved version. If they are different the test fails, at which point someone needs to review the change and either mark the new version as approved (when the change is legitimate) or fix the code, if the change is a bug.

If the explanation above seems dry and abstract let’s go through an example. Windsor 3 introduced way to programmatically access its diagnostics. We can therefore write a test looking through the potentially misconfigured components, so that we get notified if something on the list changes. I’ll be using ApprovalTests library for that.

[Test] 
pub­lic void Approved_potentially_misconfigured_components() 
{ 
    var con­tainer = new Wind­sor­Con­tainer(); 
    container.Install(FromAssembly.Containing<HomeController>());

    var han­dlers = GetPotentiallyMisconfiguredComponents(container); 
    var mes­sage = new String­Builder(); 
    var inspec­tor = new DependencyInspector(message); 
    fore­ach (IEx­poseDe­pen­den­cy­Info han­dler in han­dlers) 
    { 
        handler.ObtainDependencyDetails(inspector); 
    } 
    Approvals.Approve(message.ToString()); 
}

pri­vate sta­tic IHan­dler[] GetPotentiallyMisconfiguredComponents(WindsorContainer con­tainer) 
{ 
    var host = container.Kernel.GetSubSystem(SubSystemConstants.DiagnosticsKey) as IDi­ag­nos­tic­sHost; 
    var diag­nos­tic = host.GetDiagnostic<IPotentiallyMisconfiguredComponentsDiagnostic>(); 
    var han­dlers = diagnostic.Inspect(); 
    return han­dlers; 
}

What’s important here is we’re setting up the container, getting the misconfigured components out of it, produce readable output from the list and passing it down to the approval framework to do the rest of the job.

Now if you’ve set up the framework to pup-up a diff tool when the approval fails you will be greeted with something like this:

You have all the power of your diff tool to inspect the change. In this case we have one new misconfigured component (HomeController) which has a new parameter, appropriately named missingParameter that the container doesn’t know how to provide to it. Now you either slap yourself in the forehead and fix the issue, if that really is an issue, or approve that dependency, by copying the diff chunk from the left pane to the right, approved pane. By doing the latter you’re notifying the testing framework and your teammates that you do know what’s going on and you know it’s not an issue the way things are going to work. Coupled with a sensible commit message explaining why you chose to approve this difference you get a pretty good trail of exception to the rule and reasons behind them.

That’s quite an elegant approach to a quite hard problem. We’re using it for quite a few things, and it’s been giving us really good value for little effort it took to write those tests, and maintain them as we keep developing the app, and the approved files change.

So there you have it, a new, useful tool in your toolbelt.