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20060126 Thursday January 26, 2006

Test Dependencies versus Short Test Cycles

A lightweight build system should be able to run a project's test harness quickly so that developers can validate their work and promptly move on to the next thing. Each test should, in theory, stand alone and not require the outcome of prior tests. But if testing the application requires setting up a lot of data to run against, the can run into a fundamental conflict with the practical. How does it go? "The difference between theory and practice is different in theory and in practice."

Recently I've been developing a caching subsystem that should support fixed size LRU's, expiration and so forth. I'd rather re-use the data that I already have in the other test's data set -- there are existing tests that exercise the data writer and reader classes. For my cache manager class, I started off the testing with a simple test case that creates a synthetic entity, a mock object, and validates that it can store and fetch as well as store and lazily expire the object. Great, that was easy!

What about the case of putting a lot of objects in the cache and expiring the oldest entries? What about putting a lot of objects in the cache and test fetching them while the expiration thread is concurrently removing expired entries? Testing the multi-threaded behavior is already a sufficient PITA, having to synthesize a legion of mock objects means more code to maintain -- elsewhere in the build system I have classes that the tests verify can access legions of objects, why not use that? The best code is the code that you don't have to maintain.

<sigh />
I want to be agile, I want to reuse and maintain less code and I want the test harness to run quickly. Is that too much to ask?

My take on this is that agile methodologies are composed of a set practices and principles that promote (among other things) flexible, confident and collaborative development. Working in a small startup, as I do at Technorati, all three are vital to our technical execution. I have a dogma about confidence:

Testing is where the rubber meets the road. Making a change and then putting your finger to the wind will not suffice. Internal improvement of the implementations, refactoring, are a thrice-hourly activity. Test driven development (TDD) is essential to confidently refactoring (internal implementations) and making alterations to the external interface. Serving this principle is a vital practice: automation. If testing consists of a finger to the wind, a manual process of walking through different client scenarios, it won't get done or it won't get done with sufficient thoroughness and the quality is guaranteed to lapse. Testing should come first (or, failing that, as early as possible), be automated and performed often, continuously. If tests are laborious and time consuming, guess what? People won't run them. And then you're back in wing-and-a-prayers-ville.

Lately I've been favoring maven for build management (complete with all of it's project lifecycle goodies). Maven gives me less build code to maintain (less build.xml stuff). However, one thing that's really jumped in my way is that in the project.xml file, there's only one way and one place to define how to run the tests. This is a problem that highlights one of the key tensions with TDD: from a purist standpoint, that's correct; there should be one test harness that runs each test case in isolation of the rest. But in my experience, projects usually have different levels of capability and integration that require a choice, either:

  1. the tests require their own data setup and teardown cycles, which my require a lot of time consuming data re-initialization ("drop the database tables if they exist", "create the database tables", "populate the database tables", "do stuff with the data")

  2. OR
  3. tests can be run in a predictable order with different known data states along the way
The latter is widely frowned upon by the TDD purists. But purity ain't gonna pay the bills. I reached a point where I had to kick the proverbial dogma under the proverbial karma.

I ended up writing an ant test runner that maven invokes after the database is setup. Each set of tests that transitions the data to a known state lays the ground work for the next set of tests. Perhaps I'd feel differently about it if I had more success with DBUnit or had a mock-object generator that could materialize classes pre-populated with desired data states. In the meantime, my test harness runs three times faster and there's less build plumbing (which is code) to maintain had I adhered to the TDD dogma.

( Jan 26 2006, 06:58:22 PM PST ) Permalink


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