Legacy system

A legacy system is an existing computer system or application program which continues to be used because the user (typically an organization) does not want to replace or redesign it. Many people use this term to refer to "antiquated" systems.

Related Topics:
Computer system - Application program

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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 hard 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, so there is 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.

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Despite these problems, organizations can have compelling reasons for keeping a legacy system, such as:

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• The costs of redesigning the system are prohibitive because it is large, monolithic, 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 are high.
• The way 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 such documentation has been lost over the years.
• The user expects that the system can easily be replaced when this becomes necessary.
• The system works satisfactorily, and the owner sees no reason for changing it.

If legacy software only runs on antiquated hardware, the cost of maintaining the system may eventually outweigh the cost of replacing both the software and hardware unless some form of emulation or backward compatibility allows the software to run on new hardware. However, many of these systems do still meet the basic needs of the organization. The systems to handle customers' accounts in banks are one example. Therefore the organization cannot afford to stop them and yet some cannot afford to update them.

Related Topics:
Software - Hardware - Emulation - Backward compatibility - Bank

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A demand of extremely high availability is commonly the case in computer reservation systems, air traffic control, energy distribution (power grids), nuclear power plants, military defense installations, and other systems critical to safety, security, traffic throughput, and/or economic profits. For example see the TOPS database system.

Related Topics:
Computer reservation system - Air traffic control - Power grid - Nuclear power plant - TOPS

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The change being undertaken in some organizations is to switch to Automated Business Process (ABP) software which generates complete systems. These systems can then interface to the organizations' legacy systems and use them as data repositories. This approach can provide a number of significant benefits: the users are insulated from the inefficiencies of their legacy systems, and the changes can be incorporated quickly and easily in the ABP software. (At least, that's the intention.)

Related Topics:
Automated Business Process - Data repositories

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Note that "legacy" has little to do with the size or even age of the system — mainframes run 64-bit Linux and Java, after all, right alongside 1960s vintage code. In fact, some of the thorniest legacy problems organizations now face is in trying to leverage or replace existing "fat client" Visual BASIC code as customers demand reliable Web access.

Related Topics:
Mainframe - Linux - Java - Fat client - Visual BASIC

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