What are the pros and cons of automated Unit Tests vs automated Integration tests?
Unit tests localize failures more tightly. Integration-level tests more closely correspond to user requirements and so are better predictor of delivery success. Neither of them is much good unless built and maintained, but both of them are very valuable if properly used.
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The thing with units tests is that no integration level test can exercise all the code as much as a good set of unit tests can. Yes, that can mean that you have to refactor the tests somewhat, but in general your tests shouldn't depend on the internals so much. So, lets say for example that you have a single function to get a power of two. You describe it (as a formal methods guy, I'd claim you specify it)
long pow2(int p); // returns 2^p for 0 <= p <= 30
Your test and your spec look essentially the same (this is sort of pseudo-xUnit for illustration):
assertEqual(1073741824,pow2(30);
assertEqual(1, pow2(0));
assertException(domainError, pow2(-1));
assertException(domainError, pow2(31));
Now your implementation can be a for loop with a multiple, and you can come along later and change that to a shift.
If you change the implementation so that, say, it's returning 16 bits (remember that sizeof(long) is only guaranteed to be no less than sizeof(short)) then this tests will fail quickly. An integration-level test should probably fail, but not certainly, and it's just as likely as not to fail somewhere far downstream of the computation of pow2(28).
The point is that they really test for diferent situations. If you could build sufficiently details and extensive integration tests, you might be able to get the same level of coverage and degree of fine-grained testing, but it's probably hard to do at best, and the exponential state-space explosion will defeat you. By partitioning the state space using unit tests, the number of tests you need grows much less than exponentially.
You are asking pros and cons of two different things (what are the pros and cons of riding a horse vs riding a motorcycle?)
Of course both are "automated tests" (~riding) but that doesn't mean that they are alternative (you don't ride a horse for hundreds of miles, and you don't ride a motorcycle in closed-to-vehicle muddy places)
Unit Tests test the smallest unit of the code, usually a method. Each unit test is closely tied to the method it is testing, and if it's well written it's tied (almost) only with that.
They are great to guide the design of new code and the refactoring of existing code. They are great to spot problems long before the system is ready for integration tests. Note that I wrote guide and all the Test Driven Development is about this word.
It does not make any sense to have manual Unit Tests.
What about refactoring, which seems to be your main concern? If you are refactoring just the implementation (content) of a method, but not its existence or "external behavior", the Unit Test is still valid and incredibly useful (you cannot imagine how much useful until you try).
If you are refactoring more aggressively, changing methods existence or behavior, then yes, you need to write a new Unit Test for each new method, and possibly throw away the old one. But writing the Unit Test, especially if you write it before the code itself, will help to clarify the design (i.e. what the method should do, and what it shouldn't) without being confused by the implementation details (i.e. how the method should do the thing that it needs to do).
Automated Integration Tests test the biggest unit of the code, usually the entire application.
They are great to test use cases which you don't want to test by hand. But you can also have manual Integration Tests, and they are as effective (only less convenient).
Starting a new project today, it does not make any sense not to have Unit Tests, but I'd say that for an existing project like yours it does not make too much sense to write them for everything you already have and it's working.
In your case, I'd rather use a "middle ground" approach writing:
- smaller Integration Tests which only test the sections you are going to refactor. If you are refactoring the whole thing, then you can use your current Integration Tests, but if you are refactoring only -say- the XML generation, it does not make any sense to require the presence of the database, so I'd write a simple and small XML Integration Test.
- a bunch of Unit Tests for the new code you are going to write. As I already wrote above, Unit Tests will be ready as soon as you "mess with anything in between", making sure that your "mess" is going somewhere.
In fact your Integration Test will only make sure that your "mess" is not working (because at the beginning it will not work, right?) but it will not give you any clue on
- why it is not working
- if your debugging of the "mess" is really fixing something
- if your debugging of the "mess" is breaking something else
Integration Tests will only give the confirmation at the end if the whole change was successful (and the answer will be "no" for a long time). The Integration Tests will not give you any help during the refactoring itself, which will make it harder and possibly frustrating. You need Unit Tests for that.
I agree with Charlie about Integration-level tests corresponding more to user actions and the correctness of the system as a whole. I do think there is alot more value to Unit Tests than just localizing failures more tightly though. Unit tests provide two main values over integration tests:
1) Writing unit tests is as much an act of design as testing. If you practice Test Driven Development/Behavior Driven Development the act of writing the unit tests helps you design exactly what you code should do. It helps you write higher quality code (since being loosely coupled helps with testing) and it helps you write just enough code to make your tests pass (since your tests are in effect your specification).
2) The second value of unit tests is that if they are properly written they are very very fast. If I make a change to a class in your project can I run all the corresponding tests to see if I broke anything? How do I know which tests to run? And how long will they take? I can guarantee it will be longer than well written unit tests. You should be able to run all of you unit tests in a couple of minutes at the most.