Legacy systems

Because of the time and effort required to develop a complex system, large computer-based systems usually have a long lifetime. For example, military systems are often designed for a 20-year lifetime, and much of the world’s air traffic control still relies on software and operational processes that were originally developed in the 1960s and 1970s. 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.

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 (which may run on expensive mainframe hardware) 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 organisation and cause difficulties with the bank’s auditors.

Risks of replacing legacy systems

Reasons why legacy systems are expensive to maintain

Legacy system components

Layered model of a legacy system

(c) Ian Sommerville 2008