• Bi- directional integration is a combination of the top-down & bottom-up integration approaches used together to derive integration steps.

  

• As shown in fig, assume that the software components become available in the order mentioned by the component numbers. The individual components 1, 2, 3, 4 and 5 are tested separately and bi-directional integration performed initially with the use of stubs and drivers.

• Drivers are used to provide upstream connectivity while stubs provide downstream connectivity .A driver is a function which redirects the requests to some other components and stubs simulate the behavior of missing components.

• After the functionality of these integrated components is tested, the drivers and stubs are discarded. Once components 6, 7 and 8 become available, the integration methodology then focus only on those components, as there are the components which need focus and are new. This approach is also called “Sandwich Integration”.

Advantages:

1. Sandwich approach is useful for very large projects having several subprojects.

2. Both Top-down and Bottom-up approach starts at a time as per development schedule.

3. Units are tested and brought together to make a system Integration is done downwards.

Disadvantages:

1. It require very high cost for testing because one part has Top-down approach while another part has bottom-up approach.

2. It cannot be used for smaller system with huge interdependence between different modules. It makes sense when the individual subsystem is as good as complete system.