Payment Service – Stripe Integration for Multi-Tenant SaaS

A backend payment microservice designed for integration with multiple SaaS applications. It securely handles Stripe-based billing, multi-tenant authentication, usage tracking, and supports hybrid pricing models (subscription + usage). The service is designed to be modular, allowing multiple payment providers to be plugged in as needed.

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Features

Authentication & Tenant Isolation

• API key per SaaS app (tenant).
• All requests are scoped by app_id.
• Audit logs record all mutating actions per tenant.

Flexible Billing Models

Each SaaS app can define its own pricing structure:

1. One-Time Payments

• Process individual charges.
• Supports refunds.

2. Recurring Subscriptions

• Stripe-managed plans (monthly, yearly, etc).
• Trial support via trial_period_days.

3. Usage-Based Billing

• Track usage metrics (e.g. API calls, messages).
• Bill based on reported usage (Stripe metered pricing).

4. Hybrid Billing

• Combine base subscription + metered overages.

5. Custom Plans per App

• Each SaaS app defines its own Stripe plans/products.

Trials & Grace Periods

• Trial: Supported via Stripe trial period config.
• Grace Periods:
  • Created on over-quota or payment failure.
  • Stored in DB (grace_periods table).
  • Expire or resolve manually.

Audit Logging

• Tracks every mutation (e.g., SET_USAGE, CREATE_SUBSCRIPTION).
• Captures app_id, actor_id, action_type, target_type, and payload_snapshot.
• Queryable by admin.

Stripe Webhooks

• Handles events like:
  • invoice.payment_failed
  • invoice.paid
  • customer.subscription.updated
  • charge.refunded
• Keeps internal DB in sync.

Modular Payment Providers

• Stripe (default)
• PayPal (planned)
• Extensible architecture to support additional providers

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Tech Stack

Layer	Technology	Role
Backend	Node.js + Express	Core API + routing
Payments	Stripe Node SDK	Charges, subscriptions, webhooks
ORM / DB	Prisma + PostgreSQL	Store tenants, payments, subscriptions, etc.
Auth	API Key Middleware	Secure app-to-service access
Scheduler (opt.)	Node-cron / Temporal	Handle retries, usage sync, etc.
Deployment	Docker / Fly.io / Vercel	Portable, scalable deployment

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Requirements

Development

• Node.js 22 LTS
• PostgreSQL
• Stripe account

Environment Variables

DATABASE_URL=postgresql://user:pass@localhost:5432/payment_db
STRIPE_SECRET_KEY=sk_test_...
STRIPE_WEBHOOK_SECRET=whsec_...
APP_SECRET=your_random_secret

Next Steps

• Define DB schema in Prisma
• Implement API routes with middleware
• Setup Stripe webhook receiver
• Build admin CLI/dashboard
• Launch SaaS integration for app1/app2/app3# payment-service