Legacy code
Encyclopedia
Legacy code is source code
that related to a no-longer supported or manufactured operating system
or other computer
technology. The term can also mean code inserted into modern software for the purpose of maintaining an older or previously supported feature — for example supporting a serial interface even though many modern systems do not have a serial port
. It may also be in the form of supporting older file formats that may have been encoding in non-ASCII
characters
, such as EBCDIC
In practice, most source code has some dependency on the platform for which it is designed— even if a programmer uses a platform-independent programming language like Java
, it is hard to write a large, useful program that is totally independent of its environment. When the manufacturer upgrades a platform (or the platform is superseded), the code may no longer work without changes, and becomes legacy code. A large part of the task of a software engineer
is to continually alter code to prevent this.
While the term usually refers to source code, it can also apply to executable
code that no longer runs on a later version of a system, or requires a compatibility layer
to do so. An example would be a classic Macintosh application
which will not run natively on Mac OS X
, but runs inside the Classic environment
, or a Win16 application running on Windows XP
using the Windows on Windows
feature in XP.
community has developed other interpretations for the term legacy code. Among the most prevalent are source code inherited from someone else and source code inherited from an older version of the software. Michael Feathers' Working Effectively with Legacy Code (ISBN 0-13-117705-2) introduced a definition of legacy code as code without tests, which reflects the perspective of legacy code being difficult to work with in part due to a lack of automated regression tests
. He also defined Characterization Test
s to start putting legacy code under test.
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
that related to a no-longer supported or manufactured operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
or other computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
technology. The term can also mean code inserted into modern software for the purpose of maintaining an older or previously supported feature — for example supporting a serial interface even though many modern systems do not have a serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
. It may also be in the form of supporting older file formats that may have been encoding in non-ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
characters
Character encoding
A character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
, such as EBCDIC
EBCDIC
Extended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....
In practice, most source code has some dependency on the platform for which it is designed— even if a programmer uses a platform-independent programming language like Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, it is hard to write a large, useful program that is totally independent of its environment. When the manufacturer upgrades a platform (or the platform is superseded), the code may no longer work without changes, and becomes legacy code. A large part of the task of a software engineer
Software engineer
A software engineer is an engineer who applies the principles of software engineering to the design, development, testing, and evaluation of the software and systems that make computers or anything containing software, such as computer chips, work.- Overview :...
is to continually alter code to prevent this.
While the term usually refers to source code, it can also apply to executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
code that no longer runs on a later version of a system, or requires a compatibility layer
Compatibility layer
A compatibility layer is a term that refers to components that allow for non-native support of components.In software engineering, a compatibility layer allows binaries for a legacy or foreign system to run on a host system. This translates system calls for the foreign system into native system...
to do so. An example would be a classic Macintosh application
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
which will not run natively on Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, but runs inside the Classic environment
Classic (Mac OS X)
Classic, or Classic Environment, was a hardware and software abstraction layer in Mac OS X that allowed applications compatible with Mac OS 9 to run on the Mac OS X operating system...
, or a Win16 application running on Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
using the Windows on Windows
Windows on Windows
In computing, Windows on Windows - commonly referred to by its acronym WOW or WoW - is a software component of 32-bit versions of the Microsoft Windows NT family of operating systems that provides limited support for running legacy Win16 applications - applications written for Windows 3.x...
feature in XP.
Modern interpretations
More recently, the software engineeringSoftware engineering
Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
community has developed other interpretations for the term legacy code. Among the most prevalent are source code inherited from someone else and source code inherited from an older version of the software. Michael Feathers' Working Effectively with Legacy Code (ISBN 0-13-117705-2) introduced a definition of legacy code as code without tests, which reflects the perspective of legacy code being difficult to work with in part due to a lack of automated regression tests
Regression testing
Regression testing is any type of software testing that seeks to uncover new errors, or regressions, in existing functionality after changes have been made to a system, such as functional enhancements, patches or configuration changes....
. He also defined Characterization Test
Characterization test
In computer programming, a characterization test is a means to describe the actual behaviour of an existing piece of software, and therefore protect existing behaviour of legacy code against unintended changes via automated testing...
s to start putting legacy code under test.