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What is top – down integration testing?

– Integration testing is forming or integrating various individual units in to one and testing them as one unit.
– Integration testing lies between unit testing and system testing.
– It should be carried out before validation testing.
– The units or the modules that have passed the unit tests successfully can only be subjected to integration testing. – The tests that have to be applied to the integrated units are defined in the integration testing test plan.
– The integration testing is carried out with the purpose of verifying the functionality, reliability requirements and performance of the units under testing.

There are two types of integration testing namely:

  • The top – down integration testing and
  • The bottom – up integration testing
  • About Top-Down Integration Testing

    – In this approach to integration testing, the integrated modules at the top most level are first tested.
    – The module branches are tested one by one in a defined procedure until all the related modules have been covered.
    – Top down integration testing is often categorized as the incremental integration testing technique.
    – The testing starts by testing the modules at the top most level and the modules at the lower levels are progressively added to this one by one.
    – It is required to simulate the lower level modules.
    – This simulation is achieved with the help of the stubs.
    – These stubs are capable of mimicking the functionality of the modules at the lower levels.
    – When the lower level code is added to the assemblage or the design items, the actual components are said to replace those stubs.

    Top – down integration testing can be carried out in either of the two ways:

  • Breadth first manner
  • Depth first manner
  • Below mentioned are some of the advantages of the top – down integration testing:

  • Top – down integration testing does not requires you to write any drivers.
  • With top – down integration testing early working modules of the application can be achieved. This makes the early detection and the correction of the program easy.
  • Now here are some of the disadvantages of top – down approach:

  • Utmost care is required for writing the stubs since they are only used for the simulation of the setting of the output parameters.
  • It is difficult to get this testing done by the third parties or the other people and therefore they are the developers who have to spend most of the time on integration testing.
  • Consider the following example for having a clearer view of the top down approach.
    – Suppose at the top most we have the component A and at the first lower level we have components B and C.
    – Using the top down approach, we first test A individually and for assisting it we use the B and C stubs.
    – The component A should be tested such that it calls C which is the stub for the component B and next again component A should be tested such that it calls B which is the stub for C.
    – In the first case if any error occurs, it is either in the A and B interface or in B only.
    – Similarly, in the second case if any error occurs, it is either in A and C interface or in C only.
    – Since stubs are require for the simulation of the components’ activities, a number of stubs might be required.
    – Once the whole system has been tested from top to bottom, it can then be fully assembled and subjected to system testing.
    – However, it is not possible to figure out where the fault has occurred unless error message is given by the specific component. Some other approaches are:

  • The Sandwich approach
  • The big – bang approach
  • The Modified sandwich approach
  • The Modified top – down approach
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