This series is written in the open, from a real production system. This chapter is the vault: identity, isolation, authorization, and the tests that prove it holds. [All chapters and diagrams live in the public repo.] Multi-tenant means many unrelated companies share one running system and one database, each seeing only its own data. Get it right and hosting stays cheap forever. Get it wrong and Business A reads Business B's invoices — for a billing product, that's the ballgame. This chap...