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'''Shared Variables''' are a feature of the [[APL programming language|APL]] language which allowed mainframe APL programs to communicate with other facilities on a computer, which could include external files, DBMS, or other users. Shared Variables were first introduced by [[International Business Machines Corporation|IBM]] in their APL.SV program product in 1973 and continue to be available today in IBM and Dyalog APL for [[Linux]] and Windows.
The Shared Variable facility is roughly analagous to a Windows out-of-process server today.
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Shared Variables were one technique used by APL implementors and vendors to increase the richness of the APL language, and without touching the core implementation. With the advent of more powerful personal computing, the exodus of the APL user community to smaller computers was inevitable. APL was first available on Z80 based hardware, later the original IBM PC, and today on the 32- and 64-bit Linux and Windows workstations. Although Dyalog APL included an implementation of shared variables for communication with the now-deprecated Microsoft Windows [[Dynamic Data Exchange|DDE]], it is interesting to note that only IBM continued to use Shared Variables as a means to supply new features to their versions of the APL2 language for non-mainframe computers.
Nearly all other APL vendors chose to implement new functionality, such as access to [[Linux]] and Windows native features, graphical user interface, presentation graphics, database management system interfaces, and so on, more directly in their respective versions of the APL language. In modern non-IBM APL implementations, the Shared Variable interface has been largely supplanted by COM, ActiveX, and .Net.
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