API design
How the system talks
- Idempotent sale-creation endpoint so a retried request never double-charges a cart
- Every sale, refund, and discount override written to an audit log for shift reconciliation
projects / POS-System
Point of Sale (POS) System
Fast, offline-tolerant checkout for retail counters.
- React.js
- Next.js
- Node.js
- Express.js
- MySQL
- Faster
- Checkout time per transaction
- Live
- Inventory sync on every sale
- Auditable
- End-of-shift cash reconciliation
The challenge
A retail client needed a checkout system that stayed fast under real store conditions — quick item lookup, flexible discounts, and no data loss during brief connectivity drops.
The solution
Built a POS web app covering cart management, barcode-based product lookup, discounts/taxes, receipt printing, and shift/cash-drawer reconciliation, backed by a Node.js API and MySQL.
- Checkout
- Inventory Sync
- Receipts
- Shift Reports
System design
- FrontendReact/Next.js counter UI optimized for keyboard + barcode scanner input
- API GatewayExpress.js REST API handling sales, refunds, and shift sessions
- ServicesInventory service decrements stock on each completed sale
- DatabaseMySQL storing transactions, line items, and shift audit logs
Engineering notes
Performance
What keeps it fast
- Local cart state kept in-memory so item scanning and totals update instantly
- Product lookups indexed by barcode/SKU for near-instant search at the counter
Challenges
What was hard to get right
- Keeping the checkout flow usable for non-technical cashiers under time pressure
- Making sure stock counts stay accurate when sales and refunds happen back-to-back
Lessons learned
What I'd carry into the next project
- UI speed at the counter matters as much as backend correctness — every extra click costs real time
- Clear audit trails build staff trust in the system faster than any training session