Unit testing handling of degraded network stack, file corruption, and other imperfections

Question

I\'m primarily a C++ coder, and thus far, have managed without really writing tests for all of my code. I\'ve decided this is a Bad Idea(tm), after adding new features that subt

Answer 1

You start by talking about unit tests, then talk about entire applications; it seems you are a little confused about what unit testing is. Unit testing by definition is about testing at the most fine grained level, when each "unit" of the software is being tested. In common use, a "unit" is an individual function, not an entire application. Contemporary programming style has short functions, each of which does one well defined thing, which is therefore easy to unit test.

Answer 2

It sounds as if you answered your own question.

Mocks/stubs are the key to testing difficult to test areas. For all of your examples, the manual approach of say creating a website with dodgy data, or causing network failure could be done manually. However it would be very difficult and tedious to do so, not something anyone would recommend. In fact, doing some would mean you are not actually unit testing.

Instead you'd use mock/stubs to pretend such scenarios have happened allowing you to test them. The benefit of using mocks is that unlike the manual approach you can guarantee that each time you run your tests the same procedure will be carried out. The tests in turn will be much faster and stable because of this.

Edit - With regards the updated question.

Just as a disclaimer my networking experience is very limited, therefore I can't comment on the technical side of your issues. However, I can comment on the fact you sound as if you are testing too much. In other words, your tests cover too much of a wide scope. I don't know what your code base is like but given functions/objects within that, you should still be able to provide fake input that will allow you to test that your objects/functions do the right thing in isolation.

So lets imagine your isolated areas work fine given the requirements. Just because your unit tests pass does not mean you've tested your application. You'll still need to manually test such scenarios you describe. In this scenario it sounds as if stress testing - limiting network resources and so on are required. If your application works as expected - great. If not, you've got missing tests. Unit testing (more in tie with TDD/BDD) is about ensuring small, isolated areas of your application work. You still need integration/manual/regression etc.. testing afterwards. Therefore you should use mocks/stubs to test your small, isolated areas function. Unit testing is more akin to a design process if anything in my opinion.

Answer 3

Integration Testing vs Unit Testing

I should preface this answer by saying I am biased towards integration tests vs unit tests as the primary type of test used in tdd. At work we also have some unit tests mixed in, but only as necessary. The primary reason why we start with an integration test is because we care more about what the application is doing rather than what a particular function does. We also get integration coverage which has been, in my experience, a huge gap for automated testing.

To Mock or Not, Why Not Do Both

Our integration tests can run either fully wired (to unmanaged resource) or with mocks. We have found that helps to cover the gap between real world vs mocks. This also provides us with the option to decide NOT to have a mocked version because the ROI for implementing the mock isn't worth it. You may ask why use mocks at all.

• tests suite runs faster
• guaranteed same response every time (no timeouts, unforeseen degraded network, etc)
• fine-grained control over behavior

Sometimes you shouldn't write a test

Testing, any kind of testing has trade offs. You look at the cost to implement the test, the mock, variant tests, etc and weigh that against the benefits and sometime it doesn't make sense to write the test, the mock, or the variant. This decision is also made within the context of the kind of software your building which really is one of the major factor in deciding how deep and broad your test suite needs to be. To put it another way, I'll write a few tests for the social bacon meetup feature, but I'm not going to write the formal verification test for the bacon-friend algorithm.

Do you simply save sample pages, test against those, and hope that the sites never change?

Testing is not a panacea

Yes, you save samples (as fixtures). You don't hope the page doesn't change, but you can't know how and when it will change. If you have ideas or parameters of how it may change then you can create variants to make sure your code will handle those variants. When and if it does change, and it breaks, you add new samples, fix the problems and move on.

what sort of tests would you write to constantly check those sites live and let you know when the application isn't doing it's job because the site changed something, that now causes your application to crash?

Testing != Monitoring

Tests are tests and part of development (and QA), not for production. MONITORING is what you use in production to make sure your application is working properly. You can write monitors which should alert you when something is broken. That's a whole other topic.

How do you test an application will do its job in the face of a degraded network stack?

Bacon

If it were me I would have a wired and mocked mode for the test (assuming the mock was good enough to be useful). If the mock is difficult to get right, or if it's not worth it then I would just have the wired test. However, I have found that there is almost always a way split the variables in play into different tests. Then each of those tests are targeted to testing that vector of change, while minimizing all the other variability in play. The trick is to write the important variants, not every possible variant.

Further, how about file corruption?

How Much Testing

You mention that checksum being correct, but the file actually being corrupt. The question here is what is the class of software I'm writing. Do I need to be super paranoid about the possibility of a statistically small false positive or not. If I do, then we work to find what how deep and broad to test.

Answer 4

The beauty of mock objects is that you can have more than one. Assume that you are programming against a well-defined interface for a network stack. Then you can have a mock object WellBehavingNetworkStack to test the normal case and another mock object OddlyBehavingNetworkStack that simulates some of the network failures that you expect.

Using unit tests I usually also test argument validation (like ensuring that my code throws NullPointerExceptions), and this is easy in Java, but difficult in C++, since in the latter language you can hit undefined behavior quite easily, and then all bets are off. Therefore you cannot be strictly sure that your unit tests work, even if they seem to. But still you can test for odd situations that do not invoke undefined behavior, which should be quite a lot in well-written code.

Answer 5

Sometimes I'll create two (or more) test suites. One suite uses mocks/stubs and only tests the code I'm writing. The other tests test the database, web sites, network devices, other servers, and whatever else is outside of my control.

Those other tests are really tests of my assumptions about the systems my code interacts with. So if they fail, I know my requirements have changed. I can then update my internal tests to reflect whatever new behavior my code needs to have.

The internal tests include tests that simulate various failures of the external systems. Whenever I observe a new kind of failure, either through my other tests or as a result of a bug report, I have a new internal test to write.

Writing tests that model all the bizarre things that happen in the real world can be challenging, but the result is that you really think about all those cases, and produce robust code.

Answer 6

The proper use of Unit Testing starts from the ground up. That is, you write your unit tests BEFORE you write your production code. The unit tests are then forced to consider error conditions, pre-conditions, post-conditions, etc. Once you write your production code (and the unit tests are able to compile and run successfully), if someone makes a change to the code that changes any of its conditions (even subtly), the unit test will fail and you will learn about it very quickly (either via compiler error or via a failed unit test).

EDIT: Regarding the updated question

What you are trying to test is not really suited well for unit testing. Networking and database connections test better in a simulated integration test. There are far too many things that can break during the initialization of a remote connection to create a useful unit test for it (I'm sure there are some unit-tests-fix-all people that will disagree with me there, but in my experience, trying to unit test network traffic and/or remote database functionality is worse than shoving a square peg though a round hole).