What do you mean by Legacy Software? Discuss in brief about legacy system.

Hundreds of thousands of computer programs fall into one of the seven broad application domains discussed in the preceding subsection. Some of these are state-of-the-art software—just released to individuals, industry, and government. But other programs are older, in some cases much older.

These older programs—often referred to as legacy software—have been the focus of continuous attention and concern since the 1960s. Dayani-Fard and his colleagues [Day99] describe legacy software in the following way:

Legacy software systems . . . were developed decades ago and have been continually modified to meet changes in business requirements and computing platforms. The proliferation of such systems is causing headaches for large organizations who find them costly to maintain and risky to evolve.

Liu and his colleagues [Liu98] extend this description by noting that “many legacy systems remain supportive of core business functions and are ‘indispensable’ to the business.” Hence, legacy software is characterized by longevity and business criticality.

Unfortunately, there is sometimes one additional characteristic that is present in legacy software—poor quality. Legacy systems sometimes have inextensible designs, convoluted code, poor or nonexistent documentation, test cases and results that were never archived, a poorly managed change history—the list can be quite long. And yet, these systems support “core business functions and are indispensable to the business.” What to do?

The only reasonable answer may be: Do nothing, at least until the legacy system must undergo some significant change. If the legacy software meets the needs of its users and runs reliably, it isn’t broken and does not need to be fixed. However, as time passes, legacy systems often evolve for one or more of the following reasons:

•  The software must be adapted to meet the needs of new computing environments or technology.
• The software must be enhanced to implement new business requirements.
• The software must be extended to make it interoperable with other more modern systems or databases.
• The software must be re-architected to make it viable within a network environment.

When these modes of evolution occur, a legacy system must be re-engineered so that it remains viable into the future. The goal of modern software engineering is to “devise methodologies that are founded on the notion of evolution”; that is, the notion that software systems continually change, new software systems are built from the old ones, and . . . all must interoperate and cooperate with each other”