Legacy system

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A legacy system is an old method, technology, computer system, or application program that continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient methods of performing a task are now available. A legacy system may include procedures or terminology which are no longer relevant in the current context, and may hinder or confuse understanding of the methods or technologies used.

The term "legacy" may have little to do with the size or age of the system — mainframes run 64-bit Linux and Java alongside 1960s vintage code.

Although the term is most commonly used to describe computers and software, it may also be used to describe human behaviors, methods, and tools. For example, timber framing using wattle and daub is a legacy building construction method.

Contents

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 close to 100 percent 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.

NASA example

NASA's Space Shuttle program still uses a large amount of 1970s-era technology. Replacement is cost-prohibitive because of the expensive requirement for flight certification; the legacy hardware currently being used has completed the expensive integration and certification requirement for flight, but any new equipment would have to go through that entire process – requiring extensive tests of the new components in their new configurations – before a single unit could be used in the Space Shuttle program. This would make any new system that started the certification process a de facto legacy system by the time of completion.

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