What is Legacy System

In the context of computing, the term legacy system refers to computer systems, software, programming language, application programs or any other technology that is either out of date, obsolete or might be still in use because its application programs cannot be upgraded. Sometimes the legacy system can be used as a disparaging term for technology that is still up-to-date but he data cannot be converted into newer or standard formats.

These are usually software systems developed for an organization, years back using technologies that are now obsolete. Since these systems remain to be business critical even on the present date, they have been given the name legacy systems.

Legacy System Structures

More than simple software systems, legacy systems can be considered to be socio-technical systems. The way in which legacy systems are mostly structured is as follows:

• System Hardware or Mainframe Hardware
• Support software comprising of operating systems and utilities
• Application software consisting of multiple different programmes
• Application data which is the data used by these programs, often crucial business information
• Business Processes directed towards a specific business objective, which rely on the legacy software and hardware
• Business policies and rules that are constraints on business operations

Legacy Systems Categorized 

Legacy systems can be categorized according to their limitations of usage, as:

• Low quality, low business value— Systems that need to be scrapped right away
• Low-quality, high-business value— Systems that creates an important business contribution but are expensive to maintain. Hence, these systems are required to be re-engineered or replaced if a suitable system is available
• High-quality, low-business value— This category of legacy systems need to be replaced with COTS (commercial off-the-shelf) product, scrapped completely or maintained.
• High-quality, high business value— Systems that continues to be operated using normal system maintenance

Data & Design

Legacy systems generally use obsolete Data Base Management Systems (DBMS) which in most cases are incompatible with the other DBMSs used by the organization. To transform into an entirely new database and mainframe the organization might need to design a teleprocessing monitor that monitors the transfer of data between multiple local and remote terminals. Sometimes, the system may be file-based with incompatible files which might eventually be required to move into an upgraded database-management system.

Rather than being organised as a set of interacting objects, most of the legacy systems had been designed long back using a function-oriented design strategy. To support function-oriented design, several methods and CASE (Computer Aided Software Engineering) tools are available and this approach is still used for many business applications.

Modification & Replacement

It is expensive and risky, at the same time, to either replace or maintain legacy systems. With the aid of re-engineering, businesses may choose to extend the system lifetime, but that involves weighing up of both cost and risks.

In order to remain useful, systems have to be upgraded with time. It should be possible, principally, to replace a layer in the system leaving the other layers unchanged but in practice, this is near to impossible. The reasons behind are:

• Changing one layer introduces newer facilities and higher level layers must consecutively be changed to make use of the same
• Changing the core software may slow it down so hardware changes are eventually required
• It is often impossible to maintain hardware interfaces because of the wide gap between mainframes and client-server systems

Being much more than mere application software, legacy systems also include business processes and supporting software and hardware. And hence, the business value of a system and its quality should be used to choose an effective evolution strategy.