Agile Glossary

TDD

What is TDD?

“Test-driven development” refers to a style of programming in which three activities are tightly interwoven: coding, testing (in the form of writing unit tests), and design (in the form of refactoring).

It can be succinctly described by the following set of rules:

  • write a “single” unit test describing an aspect of the program
  • run the test, which should fail because the program lacks that feature
  • write “just enough” code, the simplest possible, to make the test pass
  • “refactor” the code until it conforms to the simplicity criteria
  • repeat, “accumulating” unit tests over time

Expected Benefits

  • many teams report significant reductions in defect rates, at the cost of a moderate increase in an initial development effort
  • the same teams tend to report that these overheads are more than offset by a reduction in effort in projects’ final phases
  • although empirical research has so far failed to confirm this, veteran practitioners report that TDD leads to improved design qualities in the code, and more generally a higher degree of “internal” or technical quality, for instance improving the metrics of cohesion and coupling

Common Pitfalls

Typical individual mistakes include:

  • forgetting to run tests frequently
  • writing too many tests at once
  • writing tests that are too large or coarse-grained
  • writing overly trivial tests, for instance omitting assertions
  • writing tests for trivial code, for instance, accessors

Typical team pitfalls include:

  • partial adoption – only a few developers on the team use TDD
  • poor maintenance of the test suite – most commonly leading to a test suite with a prohibitively long running time
  • the abandoned test suite (i.e. seldom or never run) – sometimes as a result of poor maintenance, sometimes as a result of team turnover

Origins

While the idea of having test elaboration precede programming is not original to the Agile community, TDD constitutes a breakthrough insofar as it combines that idea with that of “developer testing”, providing developer testing with renewed respectability.

  • 1976: publication of “Software Reliability” by Glenford Myers, which states as an “axiom” that “a developer should never test their own code” (Dark Ages of Developer Testing)
  • 1990: testing discipline dominated by “black box” techniques, in particular in the form of “capture and replay” testing tools
  • 1991: independent creation of a testing framework at Taligent with striking similarities to SUnit (source)
  • 1994: Kent Beck writes the SUnit testing framework for Smalltalk (source)
  • 1998: article on Extreme Programming mentions that “we usually write the test first” (source)
  • 1998 to 2002: “Test First” is elaborated into “Test Driven”, in particular on the C2.com Wiki
  • 2000: Mock Objects are among the novel techniques developed during that period (source)
  • 2003: publication of “Test Driven Development: By Example” by Kent Beck

By 2006 TDD is a relatively mature discipline which has started encouraging further innovations derived from it, such as ATDD or BDD).

Signs of Use

  • “code coverage” is a common approach to evidencing the use of TDD; while high coverage does not guarantee appropriate use of TDD, coverage below 80% is likely to indicate deficiencies in a team’s mastery of TDD
  • version control logs should show that test code is checked in each time product code is checked in, in roughly comparable amounts

Skill Levels

Beginner

  • able to write a unit test prior to writing the corresponding code
  • able to write code sufficient to make a failing test pass

Intermediate

  • practices “test driven bug fixing”: when a defect is found, write a test exposing the defect before correction
  • able to decompose a compound program feature into a sequence of several unit tests to be written
  • knows and can name a number of tactics to guide the writing of tests (for instance “when testing a recursive algorithm, first write a test for the recursion terminating case”)
  • able to factor out reusable elements from existing unit tests, yielding situation-specific testing tools

Advanced

  • able to formulate a “roadmap” of planned unit tests for macroscopic features (and revise it as necessary)
  • able to “test drive” a variety of design paradigms: object-oriented, functional, event-drive
  • able to “test drive” a variety of technical domains: computation, user interfaces, persistent data access, etc.

Further Reading

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Additional Agile Glossary Terms

An acceptance test is a formal description of the behavior of a software product, generally expressed as an example or a usage scenario. A number of different notations and approaches have been proposed for such examples or scenarios.
The team meets regularly to reflect on the most significant events that occurred since the previous such meeting, and identify opportunities for improvement.
A product backlog is a list of the new features, changes to existing features, bug fixes, infrastructure changes or other activities that a team may deliver in order to achieve a specific outcome.
Backlog grooming is when the product owner and some, or all, of the rest of the team refine the backlog on a regular basis to ensure the backlog contains the appropriate items, that they are prioritized, and that the items at the top of the backlog are ready for delivery.
An acceptance test is a formal description of the behavior of a software product, generally expressed as an example or a usage scenario. A number of different notations and approaches have been proposed for such examples or scenarios.
The team meets regularly to reflect on the most significant events that occurred since the previous such meeting, and identify opportunities for improvement.
A product backlog is a list of the new features, changes to existing features, bug fixes, infrastructure changes or other activities that a team may deliver in order to achieve a specific outcome.

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