Legacy system

A Legacy System (or historic system) is a computer system or application program which continues to be used because of the cost of replacing or redesigning it and often despite its poor competitiveness and compatibility with modern equivalents. The implication is that the system is large, monolithic, difficult and expensive to modify.

If legacy software only runs on antiquated hardware the cost of maintaining this may eventually outweigh the cost of replacing both the software and hardware unless some form of emulation or backward compatibility allows the software to run on new hardware.

Many of these systems do still meet the basic needs of the organisation - the systems to handle customers' accounts in our banks are a good example. Therefore the organisation cannot afford to stop them and yet some cannot afford to update them.

The change being undertaken in some organistions is to switch to Automated Business Process (ABP) software, which generates complete systems - these systems can then interface to the organisations legacy systems and use them as data repositories.

This approach can provide a number of significant benefits:

The users are insulated from the inefficiencies of their legacy systems.
The legacy systems can be left to 'crunch' the data (their real strength).
Business changes can be incorporated quickly and easily in the ABP software.

Based on a FOLDOC entry