Олег

OLEH OLSHANSKYI
Java Backend Engineer | Spring Boot • Microservices • Payment Integrations
Łódź, Poland | [open contact info](look above in the "contact info" section) | [open contact info](look above in the "contact info" section)
GitHub | LinkedIn | Telegram

SUMMARY

Java Backend Engineer with 2+ years of experience building production REST APIs, microservices,
and full-stack web applications using Spring Boot and Svelte/SvelteKit. Experienced in payment
integrations, authentication systems, and real-time applications. Led backend development for a startup
platform that generated approximately $24K in first-month revenue after launch.

SKILLS

Backend: Spring Boot, Spring Security, Spring Data JPA, Spring WebFlux (Reactive), Hibernate,
R2DBC, REST APIs, gRPC, Microservices, RabbitMQ, WebSockets, Multithreading & Concurrency,
Design Patterns, SOLID, Clean Architecture, Clean Code
Payments: Payment Gateway Integration, Multi-provider Payment Routing, Webhook Handling,
Signature Verification, Idempotent Payment Processing, Stripe, Cryptomus (Cryptocurrency), Regional
Payment Gateways (EU & Ukraine)
Data & Infrastructure: PostgreSQL, MySQL, MongoDB, Redis, Docker, Docker Compose, GitHub
Actions (CI/CD)
Frontend: Svelte, SvelteKit, TailwindCSS, Axios, Accessibility (a11y), Internationalization (i18n)
Languages: Java (17, 21, 23, 25), JavaScript, TypeScript, Python, SQL
Tools: Git, Maven, Gradle, Postman, Swagger/OpenAPI, SLF4J, Logback
Other: FastAPI, Django, Flask, Aiogram (Python)

EXPERIENCE

Founder & Fullstack Developer — Self-Employed Jul 2024 – Present

Freelance web development · Java/Spring Boot backend + Svelte/SvelteKit frontend

• Delivered full-stack client websites for paying clients, by independently running the freelance
business end-to-end — requirements, backend REST APIs in Java/Spring Boot, frontend in Svelte/
SvelteKit, and delivery.
• Enabled secure real-money checkout for a client application, by integrating T-Pay payment
processing into a production Spring Boot backend.
• Secured user access for client applications, by implementing JWT-based and Google OAuth
authentication with Spring Security.
• Reduced backend processing load, by offloading tasks such as email delivery to Redis-based
background workers.
Backend Developer — FameRP Apr 2025 – Dec 2025

Online multiplayer platform · Java/Spring Boot backend + Svelte/SvelteKit frontend (concurrent with freelance work)

• Architected and implemented scalable REST APIs in Java and Spring Boot, emphasizing modular
design, SOLID principles, and long-term maintainability.
• Built the platform's payment infrastructure end-to-end, integrating RozetkaPay, Cryptomus, and
Betatransfer with webhook handling and signature verification for multi-provider routing.
• Developed a WebSocket-based support platform and Discord bot integrations, enabling real-time
communication between moderators and hundreds of daily active users.
• Delivered the platform's public-facing website end-to-end, building the frontend in Svelte/SvelteKit
and backend REST APIs in Spring Boot.

PROJECTS

Distributed VPS Hosting Platform · Spring WebFlux · gRPC · Python · KVM/libvirt

• Architected a distributed VPS provisioning platform where a reactive Java API communicates with
Python microservices over gRPC to automate virtual machine deployment across multiple physical
nodes.
• Designed a modular microservice architecture following SOLID principles and clean architecture,
enabling independent scaling and maintenance of API, VM orchestration, billing, and infrastructure
services.
• Implemented complete VPS lifecycle management, including Stripe payments, provisioning,
renewals, operating system templates, automatic IP allocation, and resource management.
• Built secure authentication with JWT, refresh-token rotation, multi-device session management, role-
based access control, and production monitoring using Spring Actuator and structured logging.

GooViral — Video Pack Marketplace (Client Project) · Java backend · Stripe · Cloudflare R2

• Built and deployed a working payment-to-delivery pipeline for a client selling a 15,000-clip video pack:
Java backend, Stripe payment processing, and automated email delivery of secure download links to
files hosted on Cloudflare R2.
• Delivered the project to production with automated payment confirmation, secure file delivery, and
Cloudflare R2 integration.

SICH TACTICAL — Community Event App · Telegram Mini App · Java backend

• Developed a Telegram Mini App for an airsoft community, enabling members to browse upcoming
events, register for games, and receive updates directly through Telegram.
• Developed the backend in Java to handle event data, registrations, and integration with the Telegram
Bot API.

LANGUAGES

Ukrainian (Native) · Russian (Fluent) · Polish (B2) · English (B1)
EDUCATION

Zespół Szkół Geodezyjno-Technicznych w Łodzi (Geodetic-Technical School Complex, Łódź)
IT Technician Diploma (Technik Informatyk) · State vocational certification: Software Development (INF.02) —
passed, Network & Database Administration (INF.03) — expected 2027 · 2022 – Expected Jun 2027