Legacy system
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A legacy system is typically a computer system or application program which continues to be used because the cost of replacing or redesigning it is prohibitive. The implication is that the system is large, monolithic, and difficult (thus expensive) to modify, and/or demands ~100% availability.
If legacy software only runs on antiquated hardware, the cost of maintaining the system 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. However, many of these systems do still meet the basic needs of the organisation; the systems to handle customers' accounts in banks are one example. Therefore the organisation cannot afford to stop them and yet some cannot afford to update them.
A demand of extremely high availability is commonly the case in computer reservation systems, air traffic control, energy distribution (power grids), nuclear power plants, military defence installations, and other systems critical to safety, security, traffic throughput, and/or economic profits.
The change being undertaken in some organisations 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, and the changes can be incorporated quickly and easily in the ABP software (at least, that's the intention).
There's an alternative point of view that legacy systems are simply computer systems that are both installed and working. In other words, the term is not at all pejorative -- quite the opposite. Perhaps the term is only an effort by computer industry salesmen to generate artificial churn in order to encourage purchase of unneeded technology.
This article was originally based on material from the Free On-line Dictionary of Computing, which is used under the GFDL.
