Introduction

The process in which all the different software modules that constitute the product are put together in order to form the system.
However, integration testing cannot stand alone. Module testing precedes the integration phase.
Disadvantages:
1. An error-prone process.
2. Relationships between interfaces are often misunderstood.
3. A software module behaves correctly when tested alone, but may behave incorrectly when combined with other modules.
4. Errors are difficult to be found.
5. Not a predictable process.
If integration is not controlled right, the correction of an error may bring another error to surface, probably located in a second module! (and so on...)
It can put the whole production plan out of schedule! For this reason, there has to be some accepted strategy in the development team referencing the Integration steps and mechanisms.

Big-bang Approach

Possibly the simplest integration approach and based on postponing the integration until all software modules are developed and then throwing everything together in one step.
A non-incremental integration because of this single step...
Disadvantages:
1. The time leading up to the Big-bang is lost time that could have been spent testing!
2. All the failures will occur at the same time.
3. Very difficult and time-consuming to localize and correct defects.
Impossible to get the system to run at all and this approach should be avoided!

The test starts with the top level component of the system that calls other components but is not called itself.
Stubs replace all subordinate components and integration proceeds with lower level components.
The higher level that has already been tested serves as test driver.
Advantage:
1. Test drivers are not needed (or only simple ones are required) because the higher level components serve as main part of the test environment.
Disadvantage:
1. Lower level components not yet integrated must be replaced by stubs.

The test starts with the lower level modules.
Drivers are needed in order to simulate the function of higher level modules.
Successively, drivers are substituted by the actual modules and this continues until the whole system is tested.
Advantages:
1. Lower levels in the hierarchy facilitate the higher ones.
2. The progress of our project can be easily presented in form of percentage (coverage).
Disadvantages:
1. Top level test is the last to be done.
2. This approach is difficult, if not impossible, to be applied in synchronized systems.

Considers that the system is composed of three layers.
Top-Down approach is used to test the upper layer and Bottom-Up to test the lower.
When the middle layer is reached, its modules are selected according to the structure of the system’s hierarchy.
Advantages:
1. Combines the advantages of Bottom-Up and Top-Down approaches!
2. Minimizes the need for stubs and drivers.
Disadvantage:
1. Lack of a detailed Module Testing before the integration phase.

The components are integrated in the order in which they are finished in the development cycle.
Advantages:
1. Time saving.
2. Every component is integrated as early as possible into its environment.
Disadvantage:
1. Stubs as well as test drivers are required...

A skeleton is built into which components are gradually integrated.
Skeleton choice?
Various criteria! For example, build the skeleton based on the number of different packages in your project.
Advantage:
1. Components can be integrated in any order.
Disadvantage:
1. Labour intensive since criteria are usually hard to be decided!

Method	Advantages	Disadvantages	Comments
Top-Down	No drivers needed Interface errors found on time	Stubs needed Errors exist in basic modules Limited testers at first	Presentational purposes Hard to maintain Focuses on interface testing
Bottom-Up	No stubs needed Module errors found on time	Drivers needed Interface errors discovered late	More coding on each step More widely used approach Fast module testing Modules integrated in groups