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Legacy code refers to existing source code that continues to work but is difficult to maintain, test, or extend. It results from historical choices, missing documentation, and accumulated technical debt. The concept supports assessing risk, prioritizing refactoring or modernization efforts, and choosing strategies for safe migration and long-term maintainabi…
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Legacy code is existing software whose behavior, dependencies, or technical setting make safe change difficult, whether it was originally poorly built or highly successful.
The term belongs to the world of aging software systems whose original requirements, authors, and tests may no longer be fully known. Discussion of legacy systems shifted attention from age alone toward missing knowledge, tight coupling, and business indispensability.
Treat legacy code like an old building without blueprints: it may hold essential rooms, yet every opening could reveal unknown wiring. Before changing it, tests, observation, and small steps create a safer picture.
It records observed behavior before an internal change begins.
Unknown coupling and weak safeguards increase unintended consequences.
The term supports realistic decisions about maintenance, refactoring, and replacement. It explains why a small business change in an established codebase can be disproportionately costly or risky.
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