How to rewrite a complex test

An Integrated Test is suboptimal for asserting basic functionality and basic sub-component collaboration behavior.

So if you have a massive Integrated Test, how is it possible to rewrite that test into some number of the following kinds of tests?

• focused unit test
• focused collaboration test (how one class collaborates with another)
  
• systems-level integration test (load balancer behavior, queuing system behavior)
  

I think it comes down to the following activities:

• enumerate the different permutations of state
• enumerate the different permutations of flow
• for each permutation of state: create one focused unit test
• for each permutation of flow: decide whether 1) the permutation of flow devolves into a sub-component collaboration test, or 2) into a systems-level integration test
  
• create the required focused collaboration tests
  
• create any required systems-level integration tests (usually very rare)

There is an interesting smell that comes from the activity of creating tests. There may be an existing test that is responsible for asserting the focused behavior, but it isn't in the right place, so it is hard to find out whether it exists. In this case, the act of "create focused test" implies the act of "move focused test into its rightful home" (so others, including yourself, can find it later).

In a meeting in Feb. 2010, I wrote the following about problems I've experienced with a test suite at work:

• Inability to run a test context-free => high re-run costs and downstream delays
• Too much custom test infrastructure => high maintenance costs
• Risk of centralized integration => waiting on central integration before shipping

The approach I suggested was to find the top 20% costliest tests, and focus on those.

I suggested measuring "costliest tests" using a combination of the following criteria:

• How many superfluous assertions in this test?
• How many superfluous historical failures has this test generated in the last 6 months?
• How long does it take to run this test?
• How many "permutations of state" is this test trying to cover?
• How many "permutations of flow" is this test trying to cover?
• How far away from the code is this test?
• Is there a place closer to the code where those "permutations of state and flow" can be adequately tested?
• Are there ways to ensure all the "permutations of flow" can be covered without having to mix the test with trying to test all the "permutations of state" at the same time?

The whole idea is to simulate expensive parts of our tests in a way that still gives us the confidence that the test is valid and covers the desired integration case.