I learned in my Database subject about this topic. Legacy systems are considered to be potentially problematic by many software engineers 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 seen by documentation or manuals getting lost over the years. Integration with newer systems may also be difficult because new software may use completely different technologies and may not be compatible.
Despite these problems, business organizations can have good reasons for keeping a legacy system, such as:
>The costs of redesigning the system are prohibitive because it is large, and/or complex.
>The system requires close to 100% availability, so it cannot be taken out of service, and the cost of designing a new system with a similar availability level is high.
>The way the system works is not well understood. A situation like this can occur when the designers of the system have left the organization, and the system has either not been fully documented or such documentation has been lost.
Last reason is: the current system works satisfactorily, and the owner sees no reason for changing it; or in other words, re-learning a new system would have a high cost in lost time and money.
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