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Using Acceptance Tests in Extreme Programming

📄 Contents

  1. Automating the Tests
  2. Timeliness
XP values feedback, and there is no more important feedback than early information on how well the program works. With acceptance tests, the sooner we catch the mistake, the sooner we can make the program work.

Surely you aren't going to assume you're getting what you need. Prove that it works! Acceptance tests allow the customer to know when the system works and tell the programmers what needs to be done.

Customers, remember that you have the right to see progress in the form of a working system, passing repeatable tests that you specify. Well, here's the responsibility part of that: specifying the acceptance tests.

Every system undergoes testing to find out whether it works as needed. Some don't get this testing until they go into use and actual users discover flaws for themselves. This leads to pain; pain leads to hatred; hatred leads to suffering. Don't do that—test before you ship.

Some systems put off overall testing until right before release. They often eat into the testing time with schedule overruns, but even if they allocate the full testing period, this slows things down. The reason is simple: programmers forget what they did. If I write a program today and test it a week from now, my recollection of how I wrote it will have faded, in favor of whatever I'm working on then. So, when I get the defect indication, I won't be able to guess quickly where the problem is. This leads to long debugging sessions, and slows the project down. Don't do that—test right away.

XP values feedback, and there is no more important feedback than early information on how well the program works. If it were possible to have the computer beep at the programmer one second after she made a mistake, there'd be fewer mistakes in the world today. We can't usually test the system's function every second (but see Chapter 13, Unit Testing, for a discussion of how hard we try). With acceptance tests, the sooner we catch the mistake, the sooner we can make the program work. The customer responsibility is to provide those acceptance tests as part of each iteration. If you can get them to the programmers by the middle of each iteration, the project will go faster. You will get more business value by the deadline. That's a promise.

There are many different ways to implement the acceptance testing on your project, and the programmers will pick one. We'll talk about that below. In any case, you need to specify the tests.

What must I test? you're probably asking. The official XP answer is, you only have to test the things that you want to have work. Let's be clear about that: if it's worth having the programmers build it, it's worth knowing that they got it right.

Some projects have a legacy system they are replacing, and they can get their test data from the legacy. In this case, your job will be to select the legacy inputs and outputs you want tested. Some projects use spreadsheets from the customer that provide the inputs and expected outputs. Smart XP programmers will extract the information from the spreadsheet automatically and read it into the system. Some projects use manually calculated values that are typed in by someone on the team.

Some customers give input numbers to the programmers and check the output by just reading it. There is an important issue with this one that has to be mentioned. This is an anti-pattern—a bad idea—called Guru Checks Output. Checking the output of a computer is very error-prone. It's easy to look at the numbers and decide that they look correct. It's also easy to be wrong when you do that. Far better to have the expected answers up front, even if they have to be computed by hand.

One more thing. The rights refer to repeatable tests. All tests in an XP project must be automated. We give you the ability to move very rapidly, and to change your requirements any time you need to. This means that the code will be changing rapidly. The only way to move rapidly with confidence is to have a strong network of tests, both unit and acceptance, that ensure that changes to the system don't break things that already work. The acceptance tests must be automated: insist on it as your right.

We aren't much into dire warnings and predictions, but here's one that's a sure thing: the defects in your system will occur where you don't test. Push your acceptance tests to the limit, in breadth and in depth. You'll be glad you did.

Automating the Tests

The tests must be automated, or you won't get your XP merit badges. However, there are lots of ways this can be done. The specific choice is up to your programmers. Here are some starting ideas:

  • If the program is a batch program, reading inputs and producing outputs, make a standard series of input files, run the program, check the output manually (and carefully) once, and then write simple scripts that compare the test output to the known good output.

  • Use the above trick for reports and lists from any program, batch or not.

  • Build on the xUnit testing framework. Write functional tests as programs. Better yet, make a little scripting language that the programmers can use. Then grow it and make it easier until the customers can use it. Maybe provide a little GUI that displays more detail than the red bar/green bar.

  • Allow the customers to set up tests in their favorite spreadsheet, then read in the test and execute it. Some teams read the spreadsheet data from exported files. Some actually use the "automation" feature of the spreadsheet to read the information. A few actually export test results back to the spreadsheet! This isn't as hard as it sounds—take a look at it and see if it's within your team's ability.

  • Build an input recorder into the product, let the customers exercise it once to define a test. Spill output to files and check them automatically.

  • Use simple file-based tools like grep and diff and Perl to check results. You can get a lot of testing automated very quickly with these tools.

Always build your acceptance tests to be automatic, but build the automation simply and incrementally as you actually need it. It's easy to get sucked into investing in test automation instead of business value. Get the tests automated, but don't go overboard.

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