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SOFTWARE MAINTENANCE SOFTWARE MAINTENANCE ¡¡¡¡The term¡°software maintenance¡±is used to describe the software engineering activities that occur following delivery of a software product to the customer£®The maintenance phase of the software life cycle is the time period in which a software product performs useful work£®Typically£¬the development cycle for a software product spans 1 or 2 years£¬while the maintenance phase spans 5 to 10 years£® ¡¡¡¡Maintenance activities involve making enhancements[1] to software products£¬adapting products to new environments£¬and correcting problems£®Software product enhancement may involve providing new functional capabilities£¬improving user displays and modes of interaction£¬upgrading external documents and internal documentation£¬or upgrading the performance characteristics of a system£®Adaptation of software to a new environment may involve moving the software to a different machine£¬or for instance£¬modifying the software to accommodate a new telecommunications protocol or an additional disk drive£®Problem correction involves modification and revalidation of software to correct errors£®Some errors require immediate attention£¬some can be corrected on a scheduled£¬periodic[2 ]basis£¬and others are known but never corrected£® ¡¡¡¡It is well established that maintenance activities consume a large portion of the total life-cycle budget[3]£®It is not uncommon for software maintenance to account for 70 percent of total software life-cycle costs£¨with development requiring 30 percent£©£®As a general rule of thumb[4]£¬the distribution of effort for software maintenance includes 60 percent of the maintenance budget for enhancement£¬and 20 percent each for adaptation and correction£® ¡¡¡¡If maintenance consumes 70 percent of the total life-cycle effort devoted to a particular software product£¬and if 60 percent of maintenance goes to enhancing the product£¬then 42 percent of the total life-cycle effort for that product is dedicated to product enhancement£®Given this perspective£¬it is apparent that the product delivered to the customer at the end of the development cycle is only the initial version of the system£®Some authors have suggested that the appropriate life-cycle model for software is development¡úevolution¡úevolution¡úevolution¡­ ¡¡¡¡This perspective makes it apparent that the primary goal of software development should be production of maintainable software systems£®Maintainability£¬like all high-level quality attributes£¬can be expressed in terms of attributes that are built into[5] the product£®The primary product attributes that contribute to software maintainability are clarity£¬modularity£¬and good internal documentation of the source code£¬as well as appropriate supporting documents£® It should also be observed that software maintenance is a microcosm [6] of the software development cycle£®Enhancement and adaptation of software reinitiate development in the analysis phase£¬while correction of a software problem may reinitiate the development cycle in the analysis phase£¬the design phase£¬or the implementation phase[7]£®Thus£¬all of the tools and techniques used to develop software are potentially useful for software maintenance£® ¡¡¡¡Analysis activities during software maintenance involve understanding the scope and effect of a desired change£¬as well as the constraints on making the change£®Design during maintenance involves redesigning the product to incorporate the desired changes£®The changes must then be implemented£¬internal documentation of the code must be updated£¬and new test cases must be designed to assess the adequacy of the modification£®Also£¬the supporting documents£¨requirements£¬design specifications£¬test plan£¬principles of operation£¬user¡¯s manual£¬crossreference directories£¬etc.£©must be updated to reflect the changes£®Updated versions of the software£¨code and supporting documents£©must then be distributed to various customer sites£¬and configuration control records for each site must be updated£® ¡¡¡¡All of these tasks must be accomplished using a systematic£¬orderly approach to tracking and analysis of change requests£¬and careful redesign£¬reimplementation£¬revalidation£¬and redocumentation of the changes£®Otherwise£¬the software product will quickly degrade as a result of the maintenance process£®It is not unusual for a well designed£¬properly implemented£¬and adequately documented initial version of a software product to become unmaintainable due to inadequate maintenance procedures£®This can result in situations in which it becomes easier and less expensive to reimplement a module or subsystem than to modify the existing version£®Software maintenance activities must not destroy the maintainability of software£®A small change in the source code often requires extensive changes to the test suite and the supporting documents£®Failure to recognize the true cost of a¡°small change¡±in the source code is one of the most significant problems in software maintenance£®    Êղؠ  ·ÖÏí   ¶¥(0)

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