Multi-Tenant SaaS System Design: Silo vs Pool, and the Hybrid I Recommend
Tenant isolation strategies, Postgres row-level security, per-tenant rate limits and billing guardrails from a platform serving 1.2M users.
The isolation spectrum
Silo (DB per tenant) is safest and most expensive. Pool (shared tables with tenant_id) is cheapest and riskiest. I ship the hybrid: shared pool for standard tenants with Postgres Row-Level Security, dedicated schemas or databases for enterprise tiers with data-residency needs.
RLS policies enforce tenant_id on every query — even if app code forgets a where clause, Postgres refuses to leak rows. Combine with per-tenant Redis prefixes and queue routing.
What breaks at scale
Noisy neighbors (solved with tiered queues + rate limits), runaway billing jobs (idempotency + per-tenant budgets), and schema migrations locking everyone (expand-contract + online DDL). Design for these on day one; retrofitting multi-tenancy is 10x harder.
verifiedKey takeaways
- arrow_rightHybrid tenancy: pool by default, silo for enterprise.
- arrow_rightRLS is your last line of defense — enable it.
- arrow_rightPer-tenant quotas protect both UX and margins.