Building Multi-Tenant SaaS: Data Isolation Strategies Compared

The Fundamental Multi-Tenancy Question When you build a SaaS, every customer is a tenant. How do you make sure Acme Corp's data never shows up for Globex Corp? There are three architectural patterns, each with different tradeoffs. Strategy 1: Shared Database, Shared Schema All tenants share the same tables. Every row has a tenant_id . CREATE TABLE projects ( id UUID PRIMARY KEY , tenant_id UUID NOT NULL REFERENCES tenants ( id ), name TEXT NOT NULL , created_at TIMESTAMPTZ DEFAULT NOW () ); CREATE INDEX idx_projects_tenant ON projects ( tenant_id ); // Every query MUST include tenant_id async function getProjects ( tenantId : string ) { return db . projects . findMany ({ where : { tenantId }, // NEVER forget this }); } The risk: A missing WHERE tenant_id = ? leaks data across tenants. Mitigation: Middleware that injects tenant context: // Prisma extension that auto-adds tenantId const tenantPrisma = ( tenantId : string ) => prisma . $extends ({ query : { $allModels : { async findMany (