Connection Per Tenant in EF Core: Design & Pitfalls for SQL Server
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: