Legacy code, as defined by Michael Feathers in Working Effectively with Legacy Code, is code that is without tests and difficult to change. I would like to suggest that there is a new breed of legacy code emerging with the increasing adoption of Test Driven Development and more recently Behaviour Driven Development; Modern legacy code.
These days, if you are lucky, you may encounter code bases with large suites of both unit and functional tests (perhaps I should say examples, specs or scenarios these days!). However, in some cases, although these tests serve as a reliable regression suite guarding against defects being introduced when adding new features, they may fail to clearly describe the desired behaviour of the system clearly.
In my opinion modern legacy code is code that has good functional and unit test coverage, but where the tests (especially functional) fail to clearly communicate the desired behaviour of the application increasing the cost of maintenance. This is a shame as with a little care and focus on readability it is possible to write functional tests that really do serve as documentation describing the behaviour of the application.
Should the concept of modern legacy be used as an excuse to not write functional tests? Absolutely not. Modern legacy should be a term that is used both to encourage teams to search for better ways to express their tests with the goal of improving readability and to seek out approaches that make functional tests easier to write. Modern legacy is a huge improvement over traditional legacy but there is still plenty of room for improvement in how we ensure that our applications are behaving themselves.
In a future post I plan to identify common functional test smells that I have encountered and to provide practical advice on how to spruce them up so that they describe the behaviour of the application more clearly.
In the meantime have a look at the functional tests in your application. Do they clearly describe the behaviour of your application? A good acid test is to ask a developer from another team if they can understand what your application is supposed to do by reading the functional tests for a given functional area. If you observe a puzzled stare followed by a series of WTFs then you may have some modern legacy code.
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