Editing Legacy system (section)

==Overview==
[[File:Windows XP sighted 'in the wild' on a cash point, 3 August 2018.jpg|thumb|right|Although it has been unsupported since April 2014, [[Windows XP]] has endured continued use in fields such as [[automated teller machine|ATM]] [[operating system]] software.]]

The first use of the term ''legacy'' to describe computer systems probably occurred in the 1960s.<ref>{{Cite web|last=Tawde|first=Swati|title=Legacy System|url=https://www.educba.com/legacy-system/|website=educba|date=4 December 2020 }}</ref>  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While this term may indicate that some engineers may feel that a system is out of date, a legacy system can continue to be used for a variety of reasons.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 format]]s and [[character encoding]]s) is a goal that [[software developer]]s often include in their work.

Even if a legacy system is no longer used, it 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 well, and the owner sees no reason to change it.
* The costs of redesigning or replacing the system are prohibitive because it is large, [[monolithic system|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 [[bank]]s, [[computer reservations system]]s, [[air traffic control]], energy distribution ([[power grid]]s), [[nuclear power plant]]s, 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 "[[Backdoor (computing)|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.