Multi-tenancy is the economic engine of SaaS. Sharing infrastructure across customers reduces cost and simplifies operations. But it introduces a risk that can end your business overnight: tenant data leakage. When one customer can see another customer's data — even accidentally — the consequences are severe. Regulatory fines, contract termination, public disclosure requirements, and irreparable trust damage. I have worked on multi-tenant platforms serving hundreds of organisations, and I have learned that data isolation is not something you bolt on later. It is a foundational architectural decision that shapes everything from database design to query patterns to testing strategy. This article covers the three main tenancy models, practical implementation patterns in Laravel and Node.js, and the security controls that prevent data leakage in production. The Three Tenancy Models 1. Database-per-Tenant Each tenant gets a completely separate database. Maximum isolation, maximum operational complexity.…