How do you design and implement multi-tenant SaaS applications?

Multi-tenant SaaS architecture serves multiple customers from a shared infrastructure while maintaining data isolation. Tenancy models: shared database with tenantid column (simplest, use RLS for isolation), schema-per-tenant (moderate isolation, shared database), database-per-tenant (maximum isolation, highest operational overhead). For most SaaS applications, shared database with RLS provides the best balance of simplicity and security. Implementation: every table includes a tenantid column referencing the tenants table, RLS policies filter all queries by the authenticated user's tenantid (stored in JWT claims), application middleware injects tenant context into every request, and API routes are tenant-scoped. Data isolation verification: write integration tests that attempt cross-tenant data access and verify denial, implement database-level constraints preventing orphaned records, and audit RLS policies regularly. Customization: tenant-specific configuration stored in a JSON column (branding, feature flags, limits), custom domain support via wildcard SSL certificates and reverse proxy routing, and white-labeling capabilities for B2B2C models. Scaling: implement tenant-aware rate limiting, noisy neighbor detection (throttle tenants consuming disproportionate resources), and tenant-based routing for large customers to dedicated infrastructure. Billing: track resource usage per tenant (API calls, storage, users), implement tiered pricing with soft and hard limits, and provide usage dashboards for tenant administrators.