The "second customer signed up" moment is such a universal inflection point for SaaS backends. Multi-tenancy on Lambda with Fastify is a solid combo — Fastify's plugin encapsulation model maps really well to tenant isolation since you can scope decorators and hooks per-tenant context. One thing I'd flag for anyone following this pattern: make sure your tenant resolution happens in a preHandler hook before any business logic touches the request, and consider using Lambda's built-in request context (not global state) for tenant data since cold starts can share execution environments across invocations. The auth layer is the one place where a data leak between tenants is catastrophic, so row-level security at the database layer as a second guard is worth the overhead even if your middleware handles it correctly.
AM
Archit Mittal
I Automate Chaos — AI workflows, n8n, Claude, and open-source automation for businesses. Turning repetitive work into one-click systems.
The "second customer signed up" moment is such a universal inflection point for SaaS backends. Multi-tenancy on Lambda with Fastify is a solid combo — Fastify's plugin encapsulation model maps really well to tenant isolation since you can scope decorators and hooks per-tenant context. One thing I'd flag for anyone following this pattern: make sure your tenant resolution happens in a preHandler hook before any business logic touches the request, and consider using Lambda's built-in request context (not global state) for tenant data since cold starts can share execution environments across invocations. The auth layer is the one place where a data leak between tenants is catastrophic, so row-level security at the database layer as a second guard is worth the overhead even if your middleware handles it correctly.