Legacy System

What do you mean by legacy system? Define about the layered model of legacy system.

            Because of the time and effort required to develop a complex system, large computer based systems usually have a long lifetime. It is sometimes too expensive and too risky to discard such business critical systems after a few years of use. Their development continues throughout their life with changes to accommodate new requirements, new operating platforms, and so forth. Such systems are called Legacy Systems. Legacy system is obsolete computer system that may still be in use because its data cannot be changed to newer or standard formats, or its application programs cannot be upgraded. s an old method, technology, computer system, or application program that continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient methods of performing a task are now available. 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. Legacy systems utilize outmoded programming languages, software and/or hardware that typically are no longer supported by the respective vendors.

            Legacy systems are socio-technical computer-based systems that have been developed in the past, often using older or obsolete technology. These systems include not only hardware and software but also legacy processes and procedures- old ways of doing things that are difficult to change because they rely on legacy software. Changes to one part of the system inevitably involve changes to other components.

            Legacy systems are often business-critical systems. They are maintained because it is too risky to replace them. For example, for most banks the customer accounting system was one of their earliest systems. Organisational policies and procedures may rely on this system. If the bank were to scrap and replace the customer accounting software then there would be a serious business risk if the replacement system didn't work properly. Furthermore, existing procedures would have to change, and this may upset the people in the organization and cause with the bank's auditors.