Which edge cases and tests matter most?

Short explanation:

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.

Key points:

• Missing records, invalid input, denied access and duplicate requests.

• Concurrent updates, transaction rollback and dependency failures.

• Test final behavior and database rules, not just whether a mocked method was called.

• Test the database-backed invariant when the bug depends on concurrency; two mocked calls cannot prove a unique constraint works.

Interview point:

• Connect each requested test to a real failure it would catch.