We start testing activities from the first phase of the software development life cycle. We may generate test cases from the SRS and SDD documents and use them during system and acceptance testing. Hence, development and testing activities are carried out simultaneously in order to produce good quality maintainable software in time and within budget. We may carry out testing at many levels and may also take help of a software testing tool. Whenever we experience a failure, we debug the source code to find reasons for such a failure. Finding the reasons for a failure is a very significant testing activity and consumes a huge amount of resources and may also delay the release of the software.

LEVELS OF TESTING

Software testing is generally carried out at different levels. There are four such levels namely unit testing, integration testing, system testing and acceptance testing as shown in Figure 8.1. The first three levels of testing activities are done by the testers and the last level of testing (acceptance) is done by the customer(s)/user(s). Each level has specific testing objectives. For example, at the unit testing level, independent units are tested using functional and/or structural testing techniques. At the integration testing level, two or more units are combined and testing is carried out to test the integration related issues of various units. At the system testing level, the system is tested as a whole and primarily functional testing techniques are used to test the system.