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In computing, a legacy system is an old method, technology, computer system, or application program, "of, relating to, or being a previous or outdated computer system."〔(【引用サイトリンク】url=http://www.merriam-webster.com/dictionary/legacy )〕 Often a pejorative term, referencing a system as "legacy" often implies that the system is out of date or in need of replacement. == Overview == The first use of the term legacy to describe computer systems probably occurred in the 1970s. By the 1980s it was commonly used to refer to existing computer systems to distinguish them from the design and implementation of new systems. Legacy was often heard during a conversion process, for example, when moving data from the legacy system to a new database. While this term may indicate that some engineers may feel that a system is out of date, a legacy system may continue to be used for a variety of reasons. It may simply be that the system still provides for the users' needs. In addition, the decision to keep an old system may be influenced by economic reasons such as return on investment challenges or vendor lock-in, the inherent challenges of change management, or a variety of other reasons other than functionality. Backward compatibility (such as the ability of newer systems to handle legacy file formats and character encodings) is a goal that software developers often include in their work. Even if it is no longer used, a legacy system may continue to impact the organization due to its historical role. Historic data may not have been converted into the new system format and may exist within the new system with the use of a customized schema crosswalk, or may exist only in a data warehouse. In either case, the effect on business intelligence and operational reporting can be significant. 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. Organizations can have compelling reasons for keeping a legacy system, such as: * The system works satisfactorily, and the owner sees no reason to change 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 reservations 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. * Newer systems perform undesirable (especially for individual or non-institutional users) secondary functions such as ''a'') tracking and reporting of user activity and/or ''b'') automatic updating that creates "back-door" security vulnerabilities and leaves end users dependent on the good faith and honesty of the vendor providing the updates. This problem is especially acute when these secondary functions of a newer system cannot be disabled. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Legacy system」の詳細全文を読む スポンサード リンク
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