Martin fowler – eradicating non-determinism in tests

An automated regression suite can play a vital role on a software project, valuable both for reducing defects in production and essential for evolutionary design. In talking with development teams I’ve often heard about the problem of non-deterministic tests – tests that sometimes pass and sometimes fail. Left uncontrolled, non-deterministic tests can completely destroy the value of an automated regression suite. In this article I outline how to deal with non-deterministic tests. Initially quarantine helps to reduce their damage to other tests, but you still have to fix them soon. Therefore I discuss treatments for the common causes for non-determinism: lack of isolation, asynchronous behavior, remote services, time, and resource leaks.

14 April 2011

Martin Fowler


Why non-deterministic tests are a problem
Lack of Isolation
Asynchronous Behavior
Remote Services
Resource Leaks


enjoyed watching ThoughtWorks tackle many difficult enterprise applications, bringing successful deliveries to many clients who have rarely seen success. Our experiences have been a great demonstration that agile methods, deeply controversial and distrusted when we wrote the manifesto a decade ago, can be used successfully.

There are many flavors of agile development out there, but in what we do there is a central role for automated testing. Automated testing was a core approach to Extreme Programming from the beginning, and that philosophy has been the biggest inspiration to our agile work. So we’ve gained a lot of experience in using automated testing as a core part of software development.

Automated testing can look easy when presented in a text book. And indeed the basic ideas are really quite simple. But in the pressure-cooker of a delivery project, trials come up that are often not given much attention in texts. As I know too well, authors have a habit of skimming over many details in order to get a core point across. In my conversations with our delivery teams, one recurring problem that we’ve run into is tests which have become unreliable, so unreliable that people don’t pay much attention to whether they pass or fail. A primary cause of this unreliability is that some tests have become non-deterministic.

A test is non-deterministic when it passes sometimes and fails sometimes, without any noticeable change in the code, tests, or environment. Such tests fail, then you re-run them and they pass. Test failures for such tests are seemingly random.

Non-determinism can plague any kind of test, but it’s particularly prone to affect tests with a broad scope, such as acceptance or functional tests.

Why non-deterministic tests are a problem

Non-deterministic tests have two problems, firstly they are useless, secondly they are a virulent infection that can completely ruin your entire test suite. As a result they need to be dealt with as soon as you can, before your entire deployment pipeline is compromised.

I’ll start with expanding on their uselessness. The primary benefit of having automated tests is that they provide bug detection mechanism by acting as regression tests[1]. When a regression test goes red, you know you’ve got an immediate problem, often because a bug has crept into the system without you realizing.

Having such a bug detector has huge benefits. Most obviously it means that you can find and fix bugs just after they are introduced.

Martin fowler – eradicating non-determinism in tests