Combinatorial Testing: A Powerful Techniques for Software Testers

As a software tester for many years, I have come across various techniques and methods of software testing. One approach that has always delivered remarkable results is Combinatorial Testing. In this post, I am going to explain what combinatorial testing is all about, why it is so useful and how you can apply it in your own testing work.

What is Combinatorial Testing?

In simple terms, combinatorial testing focuses on testing different combinations of input parameters, instead of testing every single combination which would take too much time. The main idea is that most software bugs happen because of interactions between a small number of parameters, not because of one specific isolated value. Let us look at an example to understand this better.

Imagine you're responsible for testing a web application that allows users to book flights. The application has several parameters that users can select:

? Departure City: New York, London, Paris, Tokyo

? Arrival City: New York, London, Paris, Tokyo

? Class: Economy, Premium Economy, Business, First

? Passengers: 1, 2, 3, 4

? Payment Method: Credit Card, Debit Card, Bank Transfer

To exhaustively evaluate every combination, you would need 4 x 4 x 4 x 4 x 3 = 768 test cases. That's a significant amount of testing, and it would likely take a considerable amount of time and resources. The most popular method is pairwise testing, where the aim is to test each value of each parameter with each value of every other parameter at least once.

Combinatorial Testing Tools

One great thing about combinatorial testing is that there are tools available to automatically generate the test cases for you. Two tools I use often and recommend are AllPairs and Hexawise. AllPairs is an open-source tool made by James Bach, a famous name in the software testing world. I love AllPairs because it is simple and flexible to use. You create a text file listing your parameters and their values, like this:

Departure City: New York, London, Paris, Tokyo
Arrival City: New York, London, Paris, Tokyo
Class: Economy, Premium Economy, Business, First
Passengers: 1, 2, 3, 4
Payment Method: Credit Card, PayPal, Bank Transfer        

Then you run this file through AllPairs, and it generates a set of test cases covering all the pairwise combinations. You can also use AllPairs for higher-order combinations, like testing all triples or quadruples.

Hexawise is another tool I often use, especially for more complex testing scenarios. With Hexawise, you build a model of the system you are testing, specifying the parameters, values, and any constraints or dependencies between them.

What I really like about Hexawise is that it can generate an optimized set of test cases based on the coverage level you want. You can fine-tune exactly how thorough you want to be, from pairwise coverage all the way to full combinatorial coverage.

Using these tools has made my testing process much more efficient and effective. Instead of manually creating test cases which can take hours or days, I can let AllPairs or Hexawise do the challenging work. This lets me focus on more important parts of testing, like analyzing results and tracking down those elusive bugs.

Benefits of Combinatorial Testing

I have used combinatorial testing in many projects, and I can vouch for its multiple advantages:

Efficiency: By focusing only on the key combinations, combinatorial testing helps you get high coverage with few test cases. This saves a lot of time and effort.

Effectiveness: Even with fewer test cases, combinatorial testing is exceptionally good at finding defects. Research shows that pairwise testing can usually find 50% to 97% of all bugs.

Scalability: As software gets more complex, the possible combinations can quickly become too many to handle. Combinatorial testing helps you scale your testing to match this complexity.

Versatility: You can use combinatorial testing at different testing levels, from unit testing to system and acceptance testing. It is a versatile technique that adds value through the entire software development process.

Limitations of Combinatorial Testing

While combinatorial testing is powerful, it does have limitations:

Not exhaustive: By design, combinatorial testing does not check every combination. There is a small chance a bug might slip through, especially if it happens only for a very unusual combination of inputs.

Needs careful parameter selection: For combinatorial testing to work well, you need to choose the right parameters and values to test. This needs a solid understanding of the system under test and where defects are likely to occur.

May miss certain defect types: Combinatorial testing is particularly good at finding interaction defects but may not be as effective for other issues like performance problems or security holes.

Despite these limitations, I have found that the benefits of combinatorial testing are much greater than the drawbacks in most testing situations.

?Conclusion

In summary, combinatorial testing is a valuable technique that every software tester should know and use. By focusing on the key interactions between parameters, it enables you to get high coverage with a manageable number of test cases.

Tools like AllPairs and Hexawise make combinatorial testing even easier and more efficient. They automate the test case creation process, allowing you to concentrate on more important aspects of testing.

Of course, combinatorial testing is not a magic solution. It has limitations and cannot replace other types of testing techniques. However, when used properly, it can be incredibly effective at finding defects.

So, if you have not tried combinatorial testing yet, I highly encourage you to give it a shot. Start with a simple pairwise approach, and then try higher-order combinations as you get more comfortable. And try out tools like AllPairs and Hexawise - they have been game-changers for me, and I am sure they can be for you too.