The importance of writing and maintaining strong automated tests for software is well established in the modern software engineering community (and, in fact, tests will be written before the code when using test-driven development, TDD). But does it always pay off to write automated tests for some piece of code? Are there situations where one is better off not writing automated tests? Will you be considered to be a bad developer and chastised by your peers, users, stakeholders, or your manager if every line of code that you write is not under strong automated testing?
I have been wrestling with this question since 2007 when I first read the book Working Effectively with Legacy Code1 and many other books on software engineering3 and radically changed my thinking about automated testing. In the 10+ years of my professional career prior to 2007, I wrote a lot of computational science and engineering (CSE) and applied math software with little strong automated testing. I used the validation-centric approach2 to test the software that I was writing: that is, test/validate only the software in the downstream customer code and then perhaps only manually. As a result, I had personally written O(100k) lines of code that were being used by other people and that I was in the position to have to maintain. When I realized that almost all of this software was considered "legacy code" according to Working Effectively with Legacy Code (i.e., "code without tests == legacy code"), I felt ashamed and horrified at the mass of useful but buggy and difficult-to-maintain software that I had created and now would have to deal with for years to come. (Just recently a nasty bug came up in software I wrote around 2006 without strong automated testing that caused a good deal of harm and was an embarrassment.)
I resolved to turn over a new leaf and become a born-again Agile software engineer who would use TDD to write all new code and religiously apply the Legacy Code Change Algorithm1 to modify all legacy code. And over the years since my conversion, I often rigorously wrote automated tests for nearly every piece of software I touched. Admittedly, under schedule pressures I sometimes slipped, reverted back to my old ways, failed to write sufficient automated tests -- and came to regret it. However, there were also times where my overzealous drive to religiously test everything (which usually came after the guilt of a recent slip of not testing enough) caused me to waste time writing automated tests that were not worth the effort. (One example is for some code in Trilinos that did not end up getting a lot of usage and is now actually deprecated to be removed.) And having followed this road from 2007 until now (having written and maintained thousands of automated tests over that time period), I have come to realize that while in the majority of cases writing automated tests is necessary or overwhelmingly beneficial, alternatively there are cases when it is actually better (all things considered) not to write automated tests for some pieces of software (or to hold off in writing automated tests for the moment).
Criteria for when writing automated tests may not pay off
In deciding whether to write automated tests for a piece of software, one should consider the following questions/criteria:
- Will the damage done by manifested defects be minor?
- Will the majority of manifested defects be obvious?
- Will fixing the majority of the manifested defects be easy?
- Will manually testing the software after a change be easy?
- Will writing automated tests be hard, or will the tests be hard to maintain?
If the answer to all five of these questions is "yes", then it is often better not to write automated tests for a piece of software. The time spent writing, maintaining, and running the automated tests likely will not have enough benefit to outweigh the cost.
For example, let's apply the criteria to the case of some bash scripts used locally that load some modules and then configure, build, and run tests for some software.
- The damage done by not having everything complete correctly will be minor because you can just run the scripts again.
- A failure will be obvious because the software will not even build and no tests will be run.
- The scripts do fairly basic stuff and will be easy to fix if they fail.
- It would be difficult to mock up the various commands run by the scripts.
For locally run scripts of this type, it is often not worth the investment to write automated tests.
An example of where automated tests should have been added
A critical point worth remembering is that if some scripts are being used to automate an important process such as deploying the software to users, where not performing the task correctly would cause nontrivial harm, then one should write some automated tests to protect the critical functionality of the scripts. Or, if other people are running the scripts and it will not be obvious to them that a failure has occurred or they will not know how to fix it quickly, then one should likely write some automated tests for such scripts.
For example, there was a bash test-driver script in a CSE project where the script (run as a cron job) failed to detect that the code's tests were failing and instead sent out emails that everything was passing. As a result, the code was broken for weeks with no one noticing. A release of the software went out (to hundreds of internal users), and it was the users who noticed the new defects. This wasted user's time, may have resulted in incorrect results, and damaged the reputation of the project releasing the software. The lesson is that many scripts may actually need to be considered software in their own right and need to have strong automated testing just like any piece of nontrivial software. Just because some piece of software is written in bash or Python does not mean it can be dismissed as "scripts" and avoid any automated testing. And just because it is harder to write automated tests in some languages like bash than in others like Java, that is not an excuse for not writing automated tests. One can write automated tests in any Turing-complete language.
If the first four criteria above are satisfied but it is not too hard to write automated tests, then often one should write them anyway because it will make the code easier extend and to maintain. Adjectives like "minor," "obvious," "easy," and "hard" are all subjective and do not have precise definitions. For example, while one person might consider these five criteria as "minor," "obvious," "easy," "easy," and "hard," another person might consider them "significant," "nonobvious," "not easy," "painful," and "tractable." In particular, if one does not know the sensing, separation, and fake collaborators strategies for unit testing described in Working Effectively with Legacy Code, then one might think that adding automated tests for a piece of software is "hard" while another more knowledgeable or experienced developer might consider adding tests for that piece of software to be quite tractable.
While writing a high-quality automated test suite for a piece of software often pays off (i.e., reducing initial development costs and improving long-term maintenance), in some situations it does not. If the five criteria listed here are satisfied, then it is often better not to write automated tests and instead do manual testing when any changes are made to the code.
Please do not use this blog article as an excuse for not writing automated tests for some piece of software by trying to convince yourself that the damage done by defects will be "minor," any failures will be "obvious," or the defects will be "easy" for anyone to fix. You might think that; but your users, stakeholders, and other developers (who will need to maintain this software) may not feel the same way. So please use discretion when applying these criteria to some piece of software when deciding whether or not to write automated tests (and how much testing is reasonable).
Roscoe A. Bartlett is a computational scientist and engineer in the Center for Computing Research at Sandia National Laboratories. Roscoe is a long-time contributor to the Trilinos project and is currently working on various subprojects in the Exascale Computing Project including IDEAS-ECP.