FastAPI Middleware Error Recovery: Preventing One Broken Tenant from Taking Down Your Entire SaaS

Short summary

A practical guide to building tenant-aware middleware in FastAPI that isolates errors per tenant and prevents cascade failures in multi-tenant SaaS. The author shares a hard-won lesson from a 2 AM incident where one tenant's malformed requests nearly took down nine others. Includes concrete code for TenantContextMiddleware and TenantErrorBoundaryMiddleware that catch exceptions, log tenant context, and return safe responses.

• •Middleware in multi-tenant FastAPI apps is your last defense against cascade failures
• •TenantContextMiddleware extracts tenant ID and request context before handlers run
• •TenantErrorBoundaryMiddleware catches all exceptions, logs tenant context, returns safe 5xx without stack traces

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