Layered model of legacy system.

An alternative way of looking at these components of a legacy system is as a series of layers as shown in figure below. Each layer depends on the layer immediately below it and interfaces with that layer. If interfaces are maintained, then we should be able to make changes within a layer without affecting either of the adjacent layers. In practice, however, this simple encapsulation is an over-simplification, and changes to one layer of the system may require consequent changes to layers that are both above and below the changed level. The reasons for this are:

·   Changing one layer in the system may introduce new facilities and higher layers in the system may then be changed to take advantage of these facilities. For example, a new database introduced at the support software layer may include facilities to access the data through a web browser and business processes may be modified to take advantage of this facility.

·   Changing the software may slow the system down so that new hardware is needed to improve the system performance. The increase in performance from the new hardware may then mean that further software changes which were previously impractical become possible.

It is often impossible to maintain hardware interfaces, especially if a radical change to a new type of hardware is proposed. Major changes to the application software may therefore be required.

Fig: Layered model of a legacy system

1. Business processes: These are processes that are used in the business to achieve some business objective. An example of a business process in an insurance company would be issuing an insurance policy; in a manufacturing company, a business process would be accepting an order for products and setting up the associated manufacturing process. Business processes may be designed around a legacy system and constrained by the functionality that it provides.

2. Application software: The application system that provides the business services is usually composed of a number of separate programs which have been developed at different times. Sometime the term legacy system means this application software system rather than the entire system.

3. Support software: The legacy system may rely on a range of support software from the operating system and utilities provided by the hardware manufacturer through to the compilers used for system development. Again, these may be obsolete and no longer supported by their original providers.

4. Hardware: Legacy systems may have been written for hardware that is no longer available, that is expensive to maintain and that may not be compatible with current organizational IT purchasing policies.