# Nguyễn Duy Khánh — Full Stack Developer > Full-stack developer in Hà Nội, Việt Nam, rooted in Golang backend for high-scale fintech. QR payment systems at 10 million codes a day, real-time ride-hailing, and AI-powered SaaS — built end to end. Available for new projects. - Website: https://khanhnd4.dev - Email: khanhnd4.work@gmail.com - Phone: 0362 765 148 - Location: Hà Nội, Việt Nam · ICT (UTC+7) - Availability: Available for new projects ## Services - **High-scale backend & payments** — Golang services built for volume: QR payment systems generating 10 million codes a day, e-invoice platforms dispatching 20,000 invoices daily, integrations with VIB, BIDV, VietinBank and Vietcombank, CDC pipelines, and reconciliation that balances to the đồng. - **Real-time systems & mobile** — Driver matching, live location tracking, and trip lifecycle for ride-hailing — Golang, Redis and FCM broadcasts, with background job workers and fare reconciliation. Shared Flutter apps for riders and drivers, plus the web admin console to run it all. - **AI products, end to end** — IELTS Smart scores Speaking and Writing with LLM evaluation and a pronunciation analysis pipeline — plus tiered subscriptions with per-feature quotas, a credit-based B2B scoring API, and admin dashboards. Go and PostgreSQL underneath, Next.js on top. ## Selected work From payment rails to ride-hailing — real systems in production, with real users. - **IELTS Smart** (https://ieltssmart.vn): AI-powered SaaS for IELTS Speaking & Writing practice and mock tests, for both learners and centers (B2C & B2B). Automated band scoring via LLM evaluation, plus a pronunciation and speech-analysis pipeline for Speaking. Role: End to end — the scoring pipeline, tiered subscription & billing with per-feature quotas, a credit-based B2B Partner Scoring API, center/admin dashboards, i18n, JWT auth, and audio upload via Cloudflare R2. Stack: Go, PostgreSQL, Next.js, LLM, Speech analysis, Cloudflare R2, JWT, i18n. - **KPOS** (#contact): A sales management system for retail stores: product catalog with variants and price books, stock and stocktakes, purchasing and orders, plus customers, staff, attendance and payroll — multi-branch from the start. Role: Building it end to end — the catalog and inventory model, CSV import/export, branch-scoped access, and the admin console on top. Stack: Golang, Next.js, TypeScript, PostgreSQL, Multi-branch. - **ChillGo** (https://chillgo.vn): A ride-hailing platform: real-time driver matching, live location tracking, trip lifecycle, and fare & revenue reconciliation. Role: End to end — matching and tracking in Golang with Redis, FCM broadcasts, background job workers, deep links, shared Flutter apps for riders and drivers, and a web admin console, on Clean Architecture. Stack: Golang, Redis, Flutter, FCM, Clean Architecture. - **Frame by Frame Player** (https://apps.apple.com/vn/app/frame-by-frame-player/id6737622267): A fully offline iOS app for motion analysis, built for coaches, athletes, physios and animators. Ordinary players scrub in whole seconds, so a golf swing or a lift can never be inspected — this one makes the frame the unit of work: step one frame at a time, overlay the neighbouring frames as an onion-skin motion trail, and annotate the exact frame. Role: Built the app — an on-device FFmpeg extraction pipeline with pre-flight memory estimation and cancellable sessions, onion-skin ghosting up to five frames each way colour-coded by direction, an annotation layer rendered in a background isolate so exports burn in cleanly, RevenueCat subscriptions, eight locales, and the App Store submission itself. No accounts, no backend, no analytics — nothing leaves the device. Stack: Flutter, Dart, FFmpeg, RevenueCat, StoreKit, Next.js, i18n. - **Payment System** (#contact): High-capacity QR code generation producing 10,000,000 codes daily, with real-time transaction notifications — integrated with VIB, BIDV, VietinBank and Vietcombank. Role: Backend — the generation pipeline, bank integrations, and the real-time notification flow, in high-traffic production. Stack: Golang, Kafka, PostgreSQL, Bank APIs. - **E-Invoice System** (#contact): E-invoice calculation and dispatch for partners including VIETTEL, VNPT, MISA and FPT — 20,000 invoices a day for 1,000+ users. Role: Backend — CDC for asynchronous processing, calculation, and dispatch that has to be right the first time. Stack: Golang, CDC, Kafka, PostgreSQL. - **Nail System** (https://ht-nails.com): A multi-branch nail salon management platform: booking management, employee scheduling, inventory control, and role-based access. Role: Full stack — built with NestJS, TypeORM and MySQL, with RBAC and JWT authentication. Stack: NestJS, TypeORM, MySQL, JWT, RBAC. ## Experience ### Chill Go Viet Nam Technology JSC · Hà Nội — Full-stack Developer (Aug 2025 – Present · 1 yr 1 mo) - Built real-time driver matching, location tracking, and trip lifecycle for a ride-hailing platform (ChillGo) using Golang, Redis and FCM broadcasts. - Implemented Clean Architecture, standardized API responses, background job workers, fare calculation, and driver revenue reconciliation. - Developed shared Flutter apps for riders and drivers, plus a web admin console for managing users, drivers, trips, and catalogs. ### KiotViet Corp. · Hà Nội — Backend Developer (Aug 2022 – Aug 2025 · 3 yrs 1 mo) - Developed and maintained backend services and APIs in Golang, efficiently handling high-traffic environments with over 100,000 active users. - Integrated third-party services seamlessly, enhancing functionality and user experience across multiple platforms. - Implemented comprehensive unit testing, increasing test coverage by 45% and significantly reducing bug rates in production. ### Greenify Viet Nam Co. · Hà Nội — Backend Developer (May 2021 – Aug 2022 · 1 yr 4 mos) - Engineered scalable codebases with NodeJS and ReactJS, focusing on performance and maintainability. - Designed databases to meet project requirements and ensure functional effectiveness. - Seamlessly integrated third-party services, extending application features and improving user experience. ## Tech stack - **Backend**: Golang, Node.js, NestJS, Express, gRPC - **Data & Messaging**: PostgreSQL, MongoDB, Redis, Kafka, MySQL - **Frontend & Mobile**: JavaScript, TypeScript, React, Next.js, Flutter - **Tooling**: Git, GitHub, GitLab, Python ## About I'm a full-stack developer rooted in backend work — the kind where correctness matters: a payment that must settle, a driver who must be matched in seconds, an essay that deserves a fair score. My happy place is Golang, PostgreSQL, Redis and Kafka, but I'll go wherever the bug is. I shipped my first production code in 2021 and haven't stopped since. I've built QR payment systems producing 10 million codes a day, an e-invoice platform dispatching 20,000 invoices daily for VIETTEL, VNPT, MISA and FPT, a lending platform connecting merchants with banks, and messaging gateways with CDC for reliable delivery. These days I build products end to end: a ride-hailing platform with real-time matching in Golang and Flutter, and an AI-powered IELTS scoring SaaS in Go, PostgreSQL and Next.js — from schema to screen, ideation to ship. Most of what I'm learning and breaking ends up on [GitHub](https://github.com/khanhnd-work). A few quick bits: B.IT — Posts & Telecoms Institute of Technology; Based in Hà Nội, Việt Nam (ICT); Backend-leaning full-stack, Golang first; +45% test coverage on a 100k-user product; Distributed systems reader; Ships mobile too — Flutter, both apps. ## FAQ ### How do we work together? A short call to pin down the real problem, then a written plan with milestones before I write any code. After that you get working software every week — not a status update. ### What kind of work do you take on? Backend-heavy products are my sweet spot — payments, real-time systems, integrations — but I ship the whole thing when needed: web in Next.js, mobile in Flutter, and the Golang services underneath. ### Can you build AI features? Yes — IELTS Smart scores Speaking and Writing with LLM evaluation and a pronunciation analysis pipeline, with quotas and billing wired in from day one. I build AI features that survive real users and real bills. ### What do you build with? Golang first, with Node.js and NestJS where they fit. PostgreSQL, MongoDB, Redis and Kafka for data; Next.js and TypeScript on the front; Flutter for mobile. If your stack is something else, tell me — I'd rather say it's not a fit than learn on your budget. ### Where are you based, and will our hours overlap? Hà Nội, Việt Nam — ICT (UTC+7). That's a full working day with Asia-Pacific, afternoons with Europe, and evenings with the US West Coast. Chat and email day to day, calls when a call is faster. ### Are you free, and how do we start? Yes, I'm open to new projects right now. Email khanhnd4.work@gmail.com with what you're building and when you need it — if it's a fit, you get a plan and a timeline back; if it isn't, I'll say so. ## Links - [CV (PDF)](https://khanhnd4.dev/files/nguyen-duy-khanh-fullstack-developer.pdf) - [GitHub](https://github.com/khanhnd-work) - [Site source code](https://github.com/khanhnd-work/khanhnd4.dev) - [Sitemap](https://khanhnd4.dev/sitemap.xml)