# Silo, Pool and Bridge Models — Multi-Tenant SaaS Architecture

Source: https://www.skillbyai.com/en/multi-tenant-saas/mt-silo-pool-bridge

> Describe the silo, pool and bridge tenancy models and compare their isolation, cost and operational effort.

## Three ways to place tenants

Tenancy models describe how much infrastructure tenants share. In the **silo** model each tenant gets dedicated resources: its own database, sometimes its own compute or even its own cloud account. Isolation is strong and per-tenant cost is easy to see, but every tenant adds infrastructure to deploy, patch and monitor. In the **pool** model all tenants share the same application instances and the same database, separated only by a `tenant_id` and code-level or database-level rules. It is the cheapest and simplest to operate, but isolation depends on getting every query right, and noisy neighbours are a constant concern. The **bridge** (or hybrid) model mixes the two layer by layer: for example, shared stateless services in front of a database per tenant, or a shared database with a separate schema per tenant. Real products are rarely pure; they choose per layer.

## From most shared to most isolated

Picture a slider. On the left, pool: everything shared. In the middle, bridge: some layers shared, some dedicated. On the right, silo: everything dedicated per tenant.

![Three stages moving from fully shared resources, to a mix of shared and dedicated layers, to fully dedicated resources per tenant.](assets/figures/multi-tenant-saas/section-2-map.svg) — Figure 2.1: pool (all shared) -> bridge (mixed per layer) -> silo (all dedicated)

## Hostel, PG and independent flat

Pool is a hostel dorm: cheapest, everyone shares rooms and the mess. Bridge is a PG with private rooms but a shared kitchen. Silo is renting your own flat: full privacy and control, but you pay for and maintain everything yourself.

## Isolation is per layer

In interviews, avoid saying a product "is silo" or "is pool" as a whole. Say which layer: "compute is pooled, the database is per tenant, the cache is shared with tenant-prefixed keys." That precision shows you understand where isolation actually comes from.

**Quiz:** A product runs shared API servers, but every customer has a dedicated PostgreSQL database. Which model is this?

- [ ] Pure pool
- [ ] Pure silo
- [x] Bridge (hybrid)
- [ ] Single-tenant on-premises

*Answer:* Bridge (hybrid). Shared compute with dedicated databases mixes pooled and siloed layers, which is the bridge or hybrid model.