What Is Multi-Tenancy?
Multi-tenancy is an architectural pattern in which a single instance of a software application serves multiple customers, called tenants. Each tenant's data is isolated from other tenants, but the underlying application and infrastructure are shared.
For SaaS products, multi-tenancy enables cost efficiency. Instead of deploying separate infrastructure for each customer, the provider runs one system that scales as more tenants join.
The Three Multi-Tenant Database Strategies
Strategy 1: Shared Database, Shared Schema
All tenants share the same database and the same tables. A tenant_id column on every row identifies which tenant the data belongs to. All queries include a WHERE tenant_id = ? filter.
Advantages:
- Lowest infrastructure cost
- Simplest to implement initially
- Easy to run analytics across all tenants
Disadvantages:
- A bug that forgets the tenant filter can leak data across tenants
- Harder to offer database-level isolation for compliance requirements
- Performance of one large tenant can affect others
Strategy 2: Shared Database, Separate Schemas
All tenants share the same database server, but each tenant has a separate schema. Tables in each schema have identical structure.
Advantages:
- Stronger isolation than shared schema
- Can use schema-level permissions for access control
- Easier to export or migrate a single tenant's data
Disadvantages:
- Schema migrations must be applied to every tenant schema
- More complex to manage as the number of tenants grows
Strategy 3: Separate Databases Per Tenant
Each tenant has a dedicated database instance. The application routes each request to the correct database based on the tenant identifier.
Advantages:
- Maximum isolation
- Tenant data can be stored in different regions for compliance
- Easy to offer database backups and restores at the tenant level
Disadvantages:
- Highest infrastructure cost
- Complex connection management across many database instances
Identifying the Tenant
The application needs to determine which tenant is making each request. Common approaches include:
Subdomain-Based Routing
Each tenant has a unique subdomain: acme.yourapp.com. The application reads the subdomain and maps it to a tenant record.
Path-Based Routing
The tenant identifier appears in the URL path: yourapp.com/app/acme. This is simpler to implement.
Custom Domains
Enterprise tenants may want their own domain pointing to your SaaS product. This requires reverse proxy configuration and certificate provisioning per tenant.
Implementing Tenant Isolation
Regardless of the database strategy, application code must enforce tenant boundaries:
- Extract the tenant ID from the request.
- Validate that the authenticated user belongs to the tenant.
- Scope every database query to the tenant ID.
- Test isolation explicitly. Write automated tests that confirm a user from tenant A cannot access data belonging to tenant B.
Conclusion
Multi-tenancy is a defining characteristic of SaaS products. Most SaaS products start with a shared database and shared schema for simplicity, and migrate toward greater isolation as enterprise requirements demand it.