What is a defect, and how is it different from a failure?

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A defect is an error, flaw, or bug in a software product that causes it to behave unexpectedly or incorrectly. It typically originates from mistakes made during the design, coding, or requirements phases of software development. Defects are identified during development or testing before the product is released.

In contrast, a failure is the actual manifestation of a defect when the software does not perform as expected during execution—either in testing or, more critically, in a live environment. A failure is the visible outcome of one or more underlying defects.

Key Differences:

1. Stage of Detection:

   • Defect is usually found during development or testing.

   • Failure occurs during execution, often after deployment.

2. Visibility:

   • Defect may not always cause immediate issues but can lead to problems later.

   • Failure is observable and impacts functionality or user experience.

3. Impact:

   • A defect might be harmless if never executed.

   • A failure always indicates something has gone wrong in the system.

Example:
If a developer writes incorrect logic in a function that calculates discounts, that’s a defect. If a customer later gets the wrong discount during checkout because of this logic, that’s a failure.

In short, every failure is caused by one or more defects, but not all defects necessarily lead to failures.

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