SaaS Backend: MVP to Scale

A B2B SaaS product needed a backend that could ship fast for early customers but scale as usage and multi-tenancy grew—without a full rewrite.

Challenge

• MVP had to support auth, subscriptions, and core product data in months.
• Cost had to stay predictable as tenant count and usage grew.
• Data isolation and security were non-negotiable for enterprise deals.

Architecture approach

• Firestore as core: Document model designed around tenant-scoped collections and access patterns. Security rules enforced tenant isolation; no cross-tenant reads.
• Auth: Firebase Auth with custom claims for role and tenant. Claims updated via Cloud Functions on sign-up and admin actions.
• Billing: Stripe for subscriptions; webhooks processed with idempotency. Subscription state stored in Firestore and mirrored to Stripe so product logic had a single source of truth.
• Scaling levers: Read-heavy paths used local cache where appropriate. Write-heavy or reporting workloads moved to Cloud Functions + BigQuery for analytics, keeping Firestore for transactional reads/writes.

Outcomes

• Launched on time with a clean path to add tenants and features.
• Cost scaled with revenue; no surprise bills from unbounded reads.
• Later phases (e.g. AI features) plugged into the same event and data model.

This is SaaS backend engineering and cloud & serverless architecture in practice. Request an architecture review to align your MVP with scale.