Legacy System - Overview

Overview

Organizations can have compelling reasons for keeping a legacy system, such as:

• The system works satisfactorily, and the owner sees no reason for changing it.
• The costs of redesigning or replacing the system are prohibitive because it is large, monolithic, and/or complex.
• Retraining on a new system would be costly in lost time and money, compared to the anticipated appreciable benefits of replacing it (which may be zero).
• The system requires near-constant availability, so it cannot be taken out of service, and the cost of designing a new system with a similar availability level is high. Examples include systems to handle customers' accounts in banks, computer reservation systems, air traffic control, energy distribution (power grids), nuclear power plants, military defense installations, and systems such as the TOPS database.
• The way that the system works is not well understood. Such a situation can occur when the designers of the system have left the organization, and the system has either not been fully documented or documentation has been lost.
• The user expects that the system can easily be replaced when this becomes necessary.