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Edge cases are valid boundary conditions and failure situations that ordinary examples may not exercise. Useful tests prove important rules under those conditions, including concurrent and repeated requests. The highest-value case is often where a missing record, denied access or partial failure changes control flow.

The example performs a read, checks stock in application code, and later writes a modified value. Those steps are not one atomic operation, so another request can change the record between them. It also trusts the quantity without showing validation, making both business-rule and concurrency bugs possible.

A conditional atomic update makes the database check availability and reduce stock as one write operation. This avoids the application-level read/check/write gap. It protects the stock row, but related reservation records and repeated client submissions still need their own transaction and idempotency design.

Edge cases are valid boundary conditions and failure situations that ordinary examples may not exercise. Useful tests prove important rules under those conditions, including concurrent and repeated requests. The highest-value case is often where a missing record, denied access or partial failure changes control flow.

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