Inventory, Sales & Billing SaaS for Small Businesses

A multi-tenant, production-ready SaaS application that enables small shop owners to manage inventory, sales, billing, customers, credit (Udhar), staff roles, and business analytics all from a single dashboard.

Product Vision

Small retail businesses (kirana stores, mobile shops, medical stores) often rely on manual registers or fragmented apps for inventory, billing, and credit tracking.

This product provides:

• End-to-end shop management
• POS-style billing
• Real-time inventory updates
• Customer credit (Udhar) ledger
• Actionable business insights

Built with scalability, security, and real-world workflows in mind.

Core Principles

• Multi-tenant by design (each business is fully isolated)
• Transaction-safe operations (no inconsistent stock)
• Role-based access control
• Backend-driven business logic
• Production-grade architecture

High-Level Architecture

                ┌──────────────┐
                │   Frontend   │
                │ React + TS   │
                │ (Vite)       │
                └──────┬───────┘
                       │ HTTPS (JWT)
                       ▼
               ┌──────────────────┐
               │   Backend API    │
               │ Express + TS     │
               │ (REST)           │
               └──────┬───────────┘
                      │
        ┌─────────────┴─────────────┐
        ▼                           ▼
┌────────────────┐         ┌─────────────────┐
│ MongoDB Atlas  │         │ PDF Generator   │
│ (Multi-tenant) │         │ (Invoices)      │
└────────────────┘         └─────────────────┘

Key Architectural Decisions

1️ Multi-Tenancy via Business Isolation

• Every user belongs to exactly one business
• All entities (products, sales, customers, staff) are scoped by businessId
• businessId is embedded inside JWT

Why?

• Prevents data leaks
• Enables SaaS scalability
• Allows future subscription plans

2️ JWT-Based Authentication

• Stateless authentication

• Token includes:

  {
    "userId": "...",
    "businessId": "...",
    "role": "OWNER | MANAGER | CASHIER"
  }

Benefits

• Horizontal scalability
• Easy frontend integration
• Clear authorization boundaries

3️ Role-Based Access Control (RBAC)

Role	Capabilities
OWNER	Full access
MANAGER	Inventory + reports
CASHIER	Billing only

Why?

• Reflects real shop hierarchy
• Prevents accidental destructive actions
• Enterprise-grade authorization model

4️ Transaction-Safe Billing (Critical)

Billing uses MongoDB transactions:

Create Sale
 ├─ Validate products
 ├─ Check stock
 ├─ Deduct stock
 ├─ Create sale record
 └─ Commit transaction

If any step fails → rollback

Why this matters

• No overselling
• No partial updates
• Production reliability

Data Model (Simplified)

User
 ├─ role
 └─ businessId

Business
 └─ ownerId

Product
 └─ businessId

Sale
 ├─ businessId
 ├─ items[]
 └─ paymentStatus

Customer
 ├─ businessId
 └─ balance (Udhar)

Staff (User)
 └─ role-based access

Billing & Invoice System

Features

• POS-style billing
• Paid / Unpaid (Udhar)
• Auto stock deduction
• Auto invoice number
• PDF invoice generation (server-side)

Why server-side PDFs?

• Tamper-proof
• Consistent formatting
• Audit-friendly

Udhar (Credit Ledger) System

A key India-specific differentiator.

How it works

• Unpaid sales increase customer balance

• Customer ledger shows:

  • Date
  • Invoice
  • Amount

• Payments reduce balance

Business Value

• Replaces manual Udhar notebooks
• Prevents revenue leakage
• Improves cash flow visibility

Analytics & Reporting

Dashboard APIs

• Today’s sales
• Monthly revenue
• Pending Udhar
• Low stock alerts

Reports

• Sales history
• Date-range filtering
• Invoice re-download

Built using MongoDB aggregation pipelines for performance.

Frontend Architecture

• Vite + React + TypeScript
• POS-style billing UI
• Axios with JWT interceptors
• Protected routes
• Modular API layer

UI → API Layer → Backend

Why this separation?

• Maintainability
• Testability
• Easy future mobile app support

Security Considerations

• Password hashing (bcrypt)
• JWT validation middleware
• Business-level data scoping
• Role-based route protection
• No direct client-side trust

Deployment Architecture

Component	Platform
Backend	Render
Frontend	Vercel
Database	MongoDB Atlas

Supports:

• Horizontal scaling
• Environment-based configs
• Production logging

What This Project Demonstrates

This project showcases:

• SaaS system design
• Multi-tenant architecture
• Secure authentication & authorization
• Transactional consistency
• Real-world business workflows
• Full-stack TypeScript proficiency

Future Roadmap

• Subscription plans
• Offline-first mode
• Reorder suggestions
• GST reports