Defeating the Multi-Tenant SaaS Concurrency Trap in PostgreSQL

Most backend engineers implement multi-tenant quota checks using a standard "read-then-write" pattern. In production, this pattern is highly unsafe:

SELECT grading_scans_remaining FROM profiles;

If greater than 0, execute the application logic.

UPDATE profiles SET grading_scans_remaining = grading_scans_remaining - 1;

Under high volume or rapid concurrent requests, two independent processes will read the exact same balance before either one deducts usage. This race condition allows multi-tenant users to bypass your billing gates entirely.