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({ args, query }) { args.wh