by Ed Wasser and Eric Weston

The term "Legacy System" is often used to refer to systems that were built "in an era before evolvability was recognized as a difficult and crucial goal of system design" [Stevens98]. However, even new systems designed to replace legacy systems and built using modern tools and languages often become legacy systems, themselves, soon after they are put into production. This is due to an architecture that does not provide sufficient flexibility to adapt to new and changing business requirements. Legacy system migration efforts often take longer, cost more, or fail altogether due to a lack of effective planning and an unrealistic view of the associated risks. This paper was written to help architects and managers understand many of the risks and issues associated with the migration of legacy systems and to provide practical advice for planning and implementing a legacy system migration effort to minimize the risks and increase the probability of success. The information presented in this paper applies to real-time embedded systems, business and management information systems as well as migration to and from COTS systems.

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