Performance

Why read replicas often fail to reduce primary database load when reads are coupled to writes, replica lag triggers fallback, and query cost is misread.

Why a database index can exist and still fail to speed up a query: selectivity, row estimates, composite index order, and ORDER BY LIMIT behavior.

Why production caches return stale or contradictory data: cache-aside races, invalidation gaps, TTL drift, local cache divergence, and safer rollouts.