Scaling a Self-Hosted Billing Monitor from 1 to 100 Tenants

When I built BillingWatch , I designed it for a single Stripe account. Then people started asking about multi-tenant support. Here's what the architecture looks like when you scale from 1 to 100 tenants. The Core Multi-Tenancy Pattern The simplest approach: row-level tenant isolation with a tenant_id on every table. BillingWatch uses FastAPI + SQLite (upgradeable to Postgres), with every query scoped to the requesting tenant: from fastapi import FastAPI , Header , HTTPException from sqlalchemy.orm import Session app = FastAPI () def get_tenant_id ( x_tenant_id : str = Header (...)): if not x_tenant_id : raise HTTPException ( status_code = 401 , detail = " Missing tenant header " ) return x_tenant_id @app.get ( " /anomalies " ) def list_anomalies ( tenant_id : str = Depends ( get_tenant_id ), db : Session = Depends ( get_db )): return db . query ( Anomaly ). filter ( Anomaly . tenant_id == tenant_id ). all () Every webhook endpoint, every query, every dashboard call is scoped this way.