Legacy system
An existing system still in use, with technical and business dependencies accumulated over time.
What is a legacy system?
A legacy system commonly means an older system still in use whose technology, data and interfaces have evolved over time. It may be difficult to change or integrate and still perform important tasks reliably. Age alone makes an application neither unusable nor insecure; concrete characteristics, support and risks need examination.
In a fictional company, an older order application still manages ongoing work and supplies accounting data. Before replacement, business rules, calling systems, authoritative data ownership and operational responsibility are recorded. An individual function may be replaced gradually while the rest is still needed. Whether continued operation, targeted renewal or full replacement fits depends on factors such as separable boundaries, supportability and operational requirements; the “legacy” label does not decide this.
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Related terms
- Technical debtA metaphor for internal software shortcomings that make later changes harder or riskier.
- RefactoringChanging software’s internal structure while preserving its expected observable behaviour.
- Multi-tenancyMulti-tenancy supports separate customer or group scopes in one solution. A tenant is not automatically one person or one database.
- Browser cacheThe browser cache stores suitable HTTP responses for later use. Reuse and revalidation follow the applicable rules.
- Cache invalidationCache invalidation ends reuse of a stored copy under defined rules. The source record remains a separate concern.
- CachingCaching reuses stored data or results. Validity and the permitted user scope belong in the design.
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