What is multi-tenancy in a MERN SaaS?

Multi-tenancy means serving multiple customers (tenants) from a single MERN application while keeping their data, users, and configuration isolated. We use three patterns: shared database with a tenant field (fastest to ship, best for early-stage), separate database per tenant on a shared Atlas cluster (for mid-stage products with compliance requirements), and isolated Atlas cluster per tenant (for enterprise tier, SOC 2, HIPAA, data residency). We lock the tenancy pattern in week one because changing it post-launch is one of the most expensive refactors in SaaS engineering.