SaaS

Building multi-tenant applications, a common pattern for Software as a Service (SaaS) products, introduces a fundamental challenge: how do you know which tenant is making the current request? Every incoming call to your API or web app must be correctly associated with a specific tenant to ensure data isolation and deliver a customized experience. The answer lies in a clean, powerful pattern: per-request tenant resolution using custom middleware.

In this guide, we’ll walk through creating a robust tenant resolution strategy in ASP.NET Core. We’ll build custom middleware that identifies the tenant from the incoming request and makes that information available to the rest of the application through dependency injection.

The Goal: What is Tenant Resolution?

At its core, tenant resolution is the process of inspecting an incoming HttpContext to extract a unique tenant identifier. Once you have this identifier, you can use it to look up the tenant’s details (like their database connection string, theme information, or feature flags) and load them into a request-scoped context.

There are several common strategies for identifying a tenant:

• Hostname/Subdomain: tenant1.yourapp.com, tenant2.yourapp.com
• URL Path: yourapp.com/tenants/tenant1/
• HTTP Header: X-Tenant-Id: tenant1
• Query String: yourapp.com/products?tenantId=tenant1

The beauty of the middleware approach is that it centralizes this logic, making your application cleaner and easier to maintain, regardless of …

Multi-tenant SaaS applications face a critical decision: how to isolate tenant data while maintaining performance and security. One wrong move and Tenant A sees Tenant B’s customer records. I’ve seen this exact scenario destroy customer trust overnight.

The connection-per-tenant approach offers strong isolation but introduces complexity that can sink your architecture if not handled correctly. Let’s look at the real-world patterns, pitfalls, and production-ready solutions.

The Multi-Tenant Connection Problem

Most SaaS applications start simple: one database, one connection string, everything shared. This works until you need compliance certifications, enterprise customers demand data isolation, or you face your first security audit.

Common Early Mistakes

Shared Connection String Chaos: Using the same connection string for all tenants with TenantId filtering sounds efficient. Until someone forgets a WHERE clause and exposes everything.

Per-Tenant Schema Confusion: Mapping different schemas in EF Core without proper context switching leads to runtime errors that are painful to debug.

Security Breach Example: I’ve consulted on incidents where a missing global query filter resulted in tenant data leakage. The fix took hours, but the damage to customer relationships lasted months.

Multi-Tenant Database Design Patterns

Here are three proven approaches for handling tenant connections, each with distinct trade-offs:

Database Per Tenant (Full Isolation) …