"A legacy system is an old computer system or application program that continues to be used because the user (typically an organization) does not want to replace or redesign it."
Legacy systems are considered to be potentially problematic by many software engineers (for example, see Bisbal et al., 1999) for several reasons. Legacy systems often run on obsolete (and usually slow) hardware, and sometimes spare parts for such computers become increasingly difficult to obtain. These systems are often hard to maintain, improve, and expand because there is a general lack of understanding of the system. The designers of the system may have left the organization, leaving no one left to explain how it works. Such a lack of understanding can be exacerbated by inadequate documentation or manuals getting lost over the years. Integration with newer systems may also be difficult because new software may use completely different technologies.
Despite these problems, organizations can have compelling reasons for keeping a legacy system, such as:
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