|
''N''-version programming (NVP), also known as multiversion programming or multiple-version dissimilar software, is a method or process in software engineering where multiple functionally equivalent programs are independently generated from the same initial specifications.〔(N-Version Programming: A Fault-Tolerance Approach to Reliability of Software Operation ), Liming Chen; Avizienis, A., Fault-Tolerant Computing, 1995, ' Highlights from Twenty-Five Years'., Twenty-Fifth International Symposium on, Vol., Iss., 27-30 Jun 1995, Pages:113-〕 The concept of ''N''-version programming was introduced in 1977 by Liming Chen and Algirdas Avizienis with the central conjecture that the "independence of programming efforts will greatly reduce the probability of identical software faults occurring in two or more versions of the program".〔〔 The aim of NVP is to improve the reliability of software operation by building in fault tolerance or redundancy.〔 == NVP approach == The general steps of ''N''-version programming are: # An initial specification of the intended functionality of the software is developed. The specification should unambiguously define: functions, data formats (which include comparison vectors, c-vectors, and comparison status indicators, cs-indicators), cross-check points (cc-points), comparison algorithm, and responses to the comparison algorithm.〔〔A.A. Avizienis, “( The Methodology of N-version Programming )”, Software Fault Tolerance, edited by M. Lyu, John Wiley & Sons, 1995.〕 # From the specifications, two or more versions of the program are independently developed, each by a group that does not interact with the others.〔 The implementations of these functionally equivalent programs use different algorithms and programming languages.〔 At various points of the program, special mechanisms are built into the software which allow the program to be governed by the ''N''-version execution environment (NVX).〔 These special mechanisms include: comparison vectors (c-vectors, a data structure representing the program's state), comparison status indicators (cs-indicators), and synchronization mechanisms.〔 The resulting programs are called ''N''-version software (NVS).〔 # Some ''N''-version execution environment (NVX) is developed which runs the ''N''-version software and makes final decisions of the ''N''-version programs as a whole given the output of each individual ''N''-version program.〔 The implementation of the decision algorithms can vary ranging from simple as accepting the most frequently occurring output (for instance, if a majority of versions agree on some output, then it is likely to be correct) to some more complex algorithm.〔Liburd, Soyini. (An ''N''-version electronic voting system (Thesis) ). Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science, 2004.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「N-version programming」の詳細全文を読む スポンサード リンク
|