Understanding Multi-Tenant SaaS Architecture

A multi-tenant SaaS application serves many customers (tenants) from a single platform. Each tenant feels like they have their own system, but under the hood they share infrastructure and code. Getting this architecture right early is one of the most important decisions in building a SaaS product.

Three common data models

1. Shared database, shared tables: every row carries a tenant ID. It is the cheapest to run and simplest to operate, but it demands strict discipline to keep data isolated.
2. Shared database, separate schemas: each tenant gets its own schema. Isolation is stronger, with moderate operational overhead.
3. Database per tenant: the strongest isolation, and easy to back up or move one customer, but it is the most expensive and complex to manage at scale.

Many products start with the first model and move large or regulated customers to dedicated databases later.

Key considerations

• Data isolation: every query must be scoped to the current tenant, ideally enforced centrally rather than in each feature
• Authentication and roles: users belong to tenants, and permissions apply within a tenant
• Billing: subscriptions, plans, trials and usage limits tied to each tenant
• Customisation: branding, settings and feature flags per tenant without forking the code
• Noisy neighbours: one heavy tenant shouldn't slow down everyone else, so use rate limits and background queues
• Onboarding: self-service sign-up that provisions a new tenant automatically

Plan for growth

Design tenant-aware logging, metrics and backups from day one. It is far easier to build these in at the start than to retrofit them once you have hundreds of customers.