What are legacy systems?

“A legacy system is old hard- or software that continues to be used, typically because it still functions for the users' needs, even though newer technology is available. "Legacy" may have little to do with the size or age of the system — mainframes run 64-bit Linux and Java alongside 1960s vintage code.”

Generally, legacy enterprise software is an application that processes large or complex data. The application is becoming cumbersome and hard to maintain. Legacy systems are by definition hard to modify or update.

Enterprise IT & Legacy -- The dangers:

o Increasing Complexity

While enterprise IT has obviously accelerated the pace, reach and agility of business, it has also introduced a huge amount of complexity into enterprise fabrics. Typically, roughly 80% of an IT budget is spent on maintenance and development of existing enterprise systems while 10% percent of the entire workforce is devoted to IT operations

According to Gartner: 'Many enterprises have little desire to open a 'Pandora's Box' of legacy systems'. Current trends indicate that most prefer non-invasive extension approaches such as service-oriented architectures. It provides immediate, short term and low risk resolution to e-business demands'. However, as Gartner point out, this approach is tactically useful but strategically dangerous, as it increases the complexity of an already complex system!

Reasons why Legacy systems exist:

Organizations apparently have compelling reasons for keeping a legacy system, because:

• The system works satisfactorily, and the owner sees no reason for changing it.

• The costs of redesigning or replacing the system are prohibitive because it is large, monolithic, and/or complex.

• Retraining on a new system would be costly in lost time and money, compared to the anticipated appreciable benefits of replacing it (which may be zero).

• The system requires close to 100 percent availability, so it cannot be taken out of service, and the cost of designing a new system with a similar availability level is high. Examples include systems to handle customers' accounts in banks, computer reservation systems, air traffic control, energy distribution (power grids), nuclear power plants, military defense installations, and systems such as the TOPS database.

• The way that the system works is not well understood. Such a situation can occur when the designers of the system have left the organization, and the system has either not been fully documented or documentation has been lost.

• The user expects that the system can easily be replaced when this becomes necessary.

Useful references: http://www.acm.org/ubiquity/views/k_delic_1.html http://en.wikipedia.org/wiki/Legacy_system http://www.nccmembership.co.uk/pooled/articles/BF_WEBART/view.asp?Q=BF_WEBART_218555