2.3.3 Testing of software structure/architecture (structural testing)
The third target of testing is the structure of the system or component. If we are talking about the structure of a system, we may call it the system architecture.
Structural testing is often referred to as “white-box” or “glass-box” because we are interested in what is happening “inside the box”.
Structural testing is most often used as a way of measuring the thoroughness of testing through the coverage of a set of structural elements or coverage items. It can occur at any test level, although is it true to say that it tends to be mostly applied at component and integration and generally is less likely at higher test levels, except for business-process testing. At component integration level it may be based on the architecture of the system, such as a calling hierarchy. A system, system integration or acceptance testing test basis could be a
business model or menu structure.
At component level, and to a lesser extent at component integration testing, there is good tool support to measure code coverage. Coverage measurement tools assess the percentage of executable elements (e.g. statements or decision outcomes) that have been exercised (i.e., covered) by a test suite. If coverage is not 100%, then additional tests may need to be written and run to cover those parts that have not yet been exercised. This of course depends on the exit criteria. (Coverage techniques are covered in Chapter 4.)
The techniques used for structural testing are structure-based techniques, also referred to as white-box techniques. Control flow models are often used to support structural testing.
2.3.4 Testing related to changes (confirmation and regression testing)
The final target of testing is the testing of changes. This category is slightly different to the others because if you have made a change to the software, you will have changed the way it functions, the way it performs (or both) and its structure. However we are looking here at the specific types of tests relating to changes, even though they may include all of the other test types.
Confirmation testing (re-testing)
When a test fails and we determine that the cause of the failure is a software defect, the defect is reported, and we can expect a new version of the software that has had the defect fixed. In this case we will need to execute the test again to confirm that the defect has indeed been fixed. This is known as confirmation testing (also known as re-testing).
When doing confirmation testing, it is important to ensure that the test is executed in exactly the same way as it was the first time, using the same inputs, data and environment. If the test now passes does this mean that the software is now correct? Well, we now know that at least one part of the software is correct – where the defect was. But this is not enough. The fix may have introduced or uncovered a different defect elsewhere in the software. The way to detect these “unexpected side-effects” of fixes is to do regression testing.
Regression testing
Like confirmation testing, regression testing involves executing test cases that have been executed before. The difference is that, for regression testing, the test cases probably passed the last time they were executed (compare this with the test cases executed in confirmation testing – they failed the last time).
The term “regression testing” is something of a misnomer. It would be better if it were called “anti-regression” testing because we are executing tests with the intent of checking that the system has not regressed (that is, it does not now have more defects in it as a result of some change). More specifically, the
purpose of regression testing is to verify that modifications in the software or the environment have not caused unintended adverse side effects and that the system still meets its requirements.
It is common for organizations to have what is usually called a regression test suite or regression test pack. This is a set of test cases that is specifically used for regression testing. They are designed to collectively exercise most functions (certainly the most important ones) in a system but not test any one in detail. It is appropriate to have a regression test suite at every level of testing (component testing, integration testing, system testing, etc.). All of the test cases in a regression test suite would be executed every time a new version of software is produced and this makes them ideal candidates for automation. If the regression test suite is very large it may be more appropriate to select a subset for execution.
Regression tests are executed whenever the software changes, either as a result of fixes or new or changed functionality. It is also a good idea to execute them when some aspect of the environment changes, for example when a new version of a database management system is introduced, or a new version of a source code compiler is used.
Maintenance of a regression test suite should be carried out so it evolves over time in line with the software. As new functionality is added to a system new regression test should be added and as old functionality is changed or removed so too should regression tests be changed or removed. As new tests are added a regression test suite may become very large. If all the tests have to be executed manually it may not be possible to execute them all every time the regression suite is used. In this case a subset of the test cases has to be chosen.
This selection should be made in light of the latest changes that have been made to the software. Sometimes a regression test suite of automated tests can become so large that it is not always possible to execute them all. It may be possible and desirable to eliminate some test cases from a large regression test suite for example if they are repetitive (tests which exercise the same conditions) or can be combined (if they are always run together). Another approach is to eliminate test cases that have not found a defect for a long time (though this approach should be used with some care!).
