Definition and Characteristics of Legacy Systems


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Definition of Legacy Systems

S ome people think "legacy" is synonymous with Cobol, but there are hundreds of legacy languages, many of which (such as assembler and C) are harder to decipher than Cobol. And legacy systems aren't just restricted to the mainframe. Today, they have expanded to include Java, XML, network environments and a host of evolving categories.

A legacy system is any technology that has evolved over the past 50 years and is currently managing information in a production environment. This includes systems built using Assembler, Fortran, Cobol, C, C++, Java, screen-scraping technology, middleware or other coding discipline.

As such, a Legacy System is better defined with its main characteristics:

Large and complex systems (e.g. 10⁷ lines of code)

Geriatric (e.g. more than 10 years old even though not strictly mandatory)

Written in Cobol PL/1, or any other modern language…

Use of legacy database service (e.g. IMS, or no DBMS at all)

But most of all, they have the following characteristics:

Hard to maintain and evolve

The documentation is often missing or unreliable

Mission critical for the organization

Must be operational at all times

Further information:

Read about the architecture of legacy systems.
Refer to the document Problems of legacy systems for a list of problems and consequences of legacy systems.
Please read then the practical claims about such systems, decisions and consequences.

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