Regression testing is process of finding defects in features of software which were working perfectly prior to bug fixation or inclusion of new features. So most of the test cases will be already present and we have to just re-run them. If new feature has been added, we might have to create new test cases.
Why we need regression testing? The answer for the question lies in the design process. However perfectly we design our system or software, there may be some interdependency among various functions and modules of the software called as coupling. As a result, a small change in one function can have catastrophic effect on the other. So to be assured that, any change done to fix a bug in one module has not affected any other module, we use regression testing.
Qualifying a component for regression testing: When any build comes for regression testing, we have to check whether or not the build worth testing. It may be possible that the build doesn’t satisfy the major requirements, so there is not point putting efforts in testing various smaller requirements. So it is advised that always test critical and difficult parts of the software at the first glance and then move on to the trivial and smaller parts. This process of qualifying software for detailed testing is called as smoke testing.
These are the four types of tests one shall run on the software before qualifying it for further testing.
1) Customer based tests: Find out all scenarios in which software is going to be used and run test cases to test software functionalities in those scenarios. Think at higher level i.e. product level and ask what are the test case critical for your customer and run them on priority. Operation profile of the software may be handy
2) Complex tests: Find out which are the test cases which will test majority of the important features of the software.
3) Expected failure tests: From the past experiences and intuition one can find out what are the ways in which the system can fail and try to test those things first before moving to other features.
4) Big picture test: Test the performance of the software and any other quality attributes. If software doesn’t satisfy these attributes tests then avoid testing other features.
Hence one can now understand that regression testing in not about blindly running already created test cases but there are some other subtle issues to be taken care of.