Legacy system
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A legacy system is an old computer system or application program that continues to be used because the user (typically an organization) does not want to replace or redesign it.
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[edit] Overview
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Legacy systems are considered to be potentially problematic by many software engineers (for example, see Bisbal et al., 1999) for several reasons. Legacy systems often run on obsolete (and usually slow) hardware, and sometimes spare parts for such computers become increasingly difficult to obtain. These systems are often hard to maintain, improve, and expand because there is a general lack of understanding of the system. The designers of the system may have left the organization, leaving no one left to explain how it works. Such a lack of understanding can be exacerbated by inadequate documentation or manuals getting lost over the years. Integration with newer systems may also be difficult because new software may use completely different technologies.
Despite these problems, organizations can have compelling reasons for keeping a legacy system, such as:
- The costs of redesigning the system are prohibitive because it is large, monolithic, and/or complex.
- The system requires close to 100% availability, so it cannot be taken out of service, and the cost of designing a new system with a similar availability level is high.
- The way 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 such documentation has been lost.
- The user expects that the system can easily be replaced when this becomes necessary.
- The system works satisfactorily, and the owner sees no reason for changing it; or in other words, re-learning a new system would have a prohibitive attendant cost in lost time and money.
If legacy software runs on only antiquated hardware, the cost of maintaining the system may eventually outweigh the cost of replacing both the software and hardware unless some form of emulation or backward compatibility allows the software to run on new hardware. However, many of these systems do still meet the basic needs of the organization. The systems to handle customers' accounts in banks are one example. Therefore the organization cannot afford to stop them and yet some cannot afford to update them.
A demand of extremely high availability is commonly the case in computer reservation systems, air traffic control, energy distribution (power grids), nuclear power plants, military defense installations, and other systems critical to safety, security, traffic throughput, and/or economic profits. For example see the TOPS database system.
The change being undertaken in some organizations is to switch to Automated Business Process (ABP) software which generates complete systems. These systems can then interface to the organizations' legacy systems and use them as data repositories. This approach can provide a number of significant benefits: the users are insulated from the inefficiencies of their legacy systems, and the changes can be incorporated quickly and easily in the ABP software. (At least, that's the intention.)
Note that "legacy" has little to do with the size or even age of the system — mainframes run 64-bit Linux and Java, after all, right alongside 1960s vintage code. In fact, some of the thorniest legacy problems organisations now face are in trying to leverage or replace existing "fat client" Visual Basic code as customers demand reliable Web access.[citation needed]
[edit] Legacy support
The term legacy support is often used with reference to obsolete or "legacy" computer hardware, whether peripherals or core components. Operating systems with "legacy support" can detect and use legacy hardware.
It is also used as a verb for what vendors do for products in legacy mode - they "support", or provide software maintenance, for obsolete or "legacy" products.
In some cases, "legacy mode" refers more specifically to backward compatibility.
The computer mainframe era saw many applications running in legacy mode. In the modern business computing environment, n-tier, or 3-tier architectures are more difficult to place into legacy mode as they include many components making up a single system. Government regulatory changes must also be considered in a system running in legacy mode.
Virtualization technology allows for a resurgence of modern software applications entering legacy mode. As system complexity and software costs increase, many computing users are keeping their current systems permanently in legacy mode.
[edit] Brownfield architecture
IT has borrowed the term brownfield from the building industry, where undeveloped land (and especially unpolluted land) is described as greenfield and previously developed land – which is often polluted and abandoned – is described as brownfield.[1]
- A brownfield architecture is an IT network design that incorporates legacy systems.
- A brownfield deployment is an upgrade or addition to an existing IT network and uses some legacy components.
[edit] Alternative view
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There is an alternate point of view — growing since the "Dot Com" bubble burst in 1999 — that legacy systems are simply (and only) computer systems that are both installed and working. In other words, the term is not at all pejorative — quite the opposite. Perhaps the term "legacy" is only an effort by computer industry salesmen to generate artificial churn in order to encourage purchase of unneeded technology. Bjarne Stroustrup, creator of the C++ language, addressed this issue succinctly:
"Legacy code" often differs from its suggested alternative by actually working and scaling.
IT analysts estimate that the cost to replace business logic is about five times that of reuse, and that's not counting the risks involved in wholesale replacement. Shareholders and managers are increasingly asking, "Why are we spending so much money on new technology with so little to show for it?" Ideally businesses would never have to rewrite most core business logic. After all, debits must equal credits — they always have, and they always will. Businesses and governments are also recoiling at well-publicized system failures and security breaches that all too commonly arrive with new software — failures which are utterly catastrophic in many cases. (A regional airline fired its CEO due to the failure of an antiquated legacy crew scheduling system during Christmas, 2004, for example.[2]) There's also a growing backlash against large, packaged software products (SAP, Oracle, PeopleSoft, and others) which were oversold and in some cases have proven too costly, inflexible, and poorly matched to business needs.
Increasingly the IT industry is responding to these understandable business concerns. "Legacy modernization" and "legacy transformation" are now popular terms, and they mean reusing and refactoring existing, core business logic by providing new user interfaces (typically Web interfaces) sometimes through the use of techniques such as screen scraping and service-enabled access (e.g., through Web services). These techniques allow organisations to understand their existing code assets (using discovery tools), provide new user and application interfaces to existing code, improve workflow, contain costs, minimize risk, and enjoy classic qualities of service (near 100% uptime, security, scalability, etc.).[citation needed] Technology companies involved in "enterprise transformation" are growing and profiting by what many people feel is a more rational approach toward legacy systems.[citation needed]
The reexamination of attitudes toward legacy systems is also inviting more reflection on what makes legacy systems as durable as they are. Technologists are relearning the fact that sound architecture, practiced up front, helps businesses avoid costly and risky rewrites in the first place. The most common legacy systems tend to be those which embraced well-known IT architectural principles, with careful planning and strict methodology during implementation. Poorly designed systems often don't last. Thus, many organisations are rediscovering not only the value in the legacy systems themselves but also their philosophical underpinnings.
[edit] See also
[edit] References
[edit] Further reading
- "Failure of a relatively new crew scheduling system during Christmas" by Stephanie Overby, CIO Magazine, May 1, 2005
- Bisbal, J., Lawless, D., Wu, B. & Grimson, J. (1999). Legacy Information Systems: Issues and Directions. IEEE Software, 16, 103-111.
- Jim McGee. "Legacy Systems: Why History Matters", Enterprise Systems Journal, 2005-11-10.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
