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Yuliia Koziuk
Software Engineer | Backend / Full-Stack Developer (Node.js, NestJS, TypeScript, React)
Vinnytsia, Ukraine · [open contact info](look above in the "contact info" section)

SUMMARY
Software Engineer with 2 years of commercial experience designing, building and shipping backend and full-
stack web applications with TypeScript, Node.js, NestJS, PostgreSQL and React. Took ownership of
backend architecture from scratch on several production projects, delivering secure authentication and
RBAC, scalable REST APIs, AI/LLM features (RAG, prompt engineering), S3 file storage, background job
processing with Redis and BullMQ, and fully Dockerized environments. Comfortable working across the
whole stack, from data modeling and API design to React frontends and E2E testing. Master's student in
Cybersecurity who brings a security-first mindset to every system.

SKILLS
Languages: JavaScript, TypeScript, SQL
Backend: Node.js, NestJS, Express.js, Fastify, REST API, JWT, OAuth (Google / Apple Sign-In), RBAC,
microservices (TCP, RabbitMQ), BullMQ
Databases & ORMs: PostgreSQL, Redis, Sequelize, Prisma
Frontend: React, Vite, TanStack React Query, React Router
Testing: Jest, Vitest, React Testing Library, Playwright (E2E)
DevOps & Cloud: Docker, CI/CD, AWS S3 / S3-compatible storage, Firebase Cloud Messaging, Linux
AI: LLM integration, RAG, Prompt Engineering, Claude Code, ChatGPT
Tools: Git, ESLint, Linux, Windows

WORK EXPERIENCE

Junior Software Engineer — Five System
Feb 2025 – Present
Full-Stack Developer — Business Platform (NDA project) | Aug 2026 – Present
Tech stack: TypeScript, React, Vite, TanStack React Query, NestJS, Sequelize, PostgreSQL, Redis, BullMQ,
Docker, S3, Playwright
Built the full-stack architecture from scratch: NestJS + Sequelize/PostgreSQL backend, React/Vite
frontend, Dockerized local and deployment environments (PostgreSQL, Redis, S3).
Implemented authentication and role-based access control (RBAC): phone-based sign-up, JWT sessions,
closed-by-default API behind a global auth guard, admin and multiple user roles.
Designed the relational data model and REST API for core domain entities, using atomic sequencing to
prevent race conditions under concurrent writes.
Built the file storage subsystem: presigned S3 uploads, server-side metadata extraction, on-demand ZIP
export and per-resource permissions.
Implemented keyset pagination for large ordered datasets and typeahead search for cross-entity linking.
Implemented React Query caching with cache invalidation across dependent mutations and end-to-end
validation (class-validator DTOs mirrored on the frontend).
Covered critical user flows with Playwright end-to-end (E2E) tests.
Backend Developer — App with AI Features (NDA project) | Jan 2026 – Sep 2026
Tech stack: TypeScript, NestJS, Sequelize, PostgreSQL, Docker, Firebase, LLM / RAG
Owned the core backend architecture end to end (NestJS, Sequelize/PostgreSQL, Docker), from initial
project setup to production-ready features.
Implemented authentication and user management: sign-up/login, JWT sessions, email OTP verification,
password reset, Apple and Google social login, admin roles, account soft-delete and restoration.

Built the AI/LLM integration layer: RAG service, prompt library and prompt engineering.
Developed push notifications (Firebase Cloud Messaging), a subscription and billing system, user
onboarding flow, micro-goals and quiz features.
Backend Developer — Backend Service (NDA project) | Jul 2025 – Jan 2026
Tech stack: TypeScript, Fastify, Sequelize, PostgreSQL, Docker
Designed and delivered new backend features and REST API endpoints with TypeScript and Fastify,
working closely with the team from requirements to release.
Diagnosed and resolved production bugs, improving application stability and reliability.
Maintained and evolved the PostgreSQL data layer with Sequelize ORM, including schema changes and
query optimization, in Dockerized environments.
Backend Developer — Template project | Jun 2025 – Aug 2025
Tech stack: TypeScript, NestJS, Sequelize, PostgreSQL
Built backend modules and REST API endpoints with NestJS and TypeScript following clean, modular
architecture.
Fixed bugs, maintained the PostgreSQL database with Sequelize ORM and took part in code reviews with
senior engineers.

Freelance Backend Developer — Self-employed
Oct 2024 – Mar 2025
Backend Developer — Orders & Booking Web App for a Small Business | Oct 2024 – Mar 2025
Tech stack: JavaScript, Node.js, Express.js, PostgreSQL, Sequelize, JWT, Git
Built a REST API with Node.js and Express.js for a small business web application covering customers,
orders/bookings and admin management.
Designed the PostgreSQL database schema and implemented CRUD operations with Sequelize ORM.
Implemented JWT-based authentication, password hashing (bcrypt) and role-based access for admin and
customer users.
Added request validation, centralized error handling and logging middleware.
Integrated email notifications (Nodemailer) for order confirmations.
Worked directly with the client to gather requirements, estimate tasks and deliver features in short
iterations.

EDUCATION
Master's Degree in Cybersecurity — Vinnytsia National Technical University | Sep 2025 – Dec 2026
(expected)
Bachelor's Degree in Cybersecurity — Vinnytsia National Technical University | Sep 2021 – Jun 2025

LANGUAGES
Ukrainian — Native
English — Intermediate (B1)