Backend

How to plan database migration rollback in production with compatibility windows, forward fixes, backfill recovery, and clear stop points before destructive schema changes.

How database connection pool exhaustion happens under production load, how to distinguish pool wait from slow SQL, and how to size pools across instances without overloading PostgreSQL.

How API contract testing catches breaking request, response, error, and schema changes before release with OpenAPI diffs, consumer examples, and CI compatibility checks.

How trace sampling affects production debugging, including head sampling, parent-based sampling, tail sampling trade-offs, error traces, rare paths, async work, and metrics that show whether sampling is hiding incidents.

How to choose database transaction boundaries in backend APIs so writes stay atomic without holding locks across external calls, slow work, retries, or post-commit side effects.