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Kostiantyn Voskoboinik
Full Stack Engineer · Ruby on Rails / React / TypeScript
[open contact info](look above in the "contact info" section) · [open contact info](look above in the "contact info" section) · Remote (EMEA / CET, US overlap)
[open contact info](look above in the "contact info" section) · github.com/vkot91

SUMMARY
Full Stack Engineer with 5.5+ years of experience, including 3.5 years at a US real-money platform with 5M+ registered users. I own
domains rather than tickets — from data modeling and production SQL to Rails applications, API contracts, and the React interfaces
consuming them. Strongest in Ruby on Rails, PostgreSQL, React, and TypeScript, with a frontend background that helps me move
comfortably between backend systems and complex UI. Experienced in owning production-critical domains, working through
ambiguous problems, and shipping end-to-end features in high-traffic systems.

SKILLS
Backend: Ruby, Ruby on Rails, ActiveRecord, Sidekiq, PostgreSQL, MySQL, Redis, RabbitMQ, REST APIs, Hotwire (Turbo & Stimulus),
WebSockets, Auth0 / Okta / OAuth2
Data & SQL: relational modeling, complex SQL, query tuning, production investigation, reporting
Frontend: TypeScript, React, Next.js (App Router), React Query / TanStack Query, Tailwind CSS, shadcn/ui, Radix, MaterialUI, Ant Design,
virtualization, rendering optimization, Core Web Vitals (LCP, CLS, INP), semantic HTML, ARIA
Testing & quality: Minitest, Capybara, Jest, React Testing Library, Playwright (e2e), self-testing and test plans, A/B rollouts, code review
Infra & tooling: Docker, GitHub Actions, AWS (Lambda, S3, SES, SQS, SNS), Turborepo, pnpm, Grafana Loki, Datadog, CI/CD pipelines
AI / LLM: AI-assisted development (Claude Code, Codex), spec-driven development, LLM API integration, prompt design, structured
outputs
Also: Node.js, NestJS, GraphQL (Apollo), Prisma, Drizzle ORM
Ways of working: Scrum (sprints, planning, grooming, planning-poker estimation), Kanban, RFCs and technical documentation,
cross-functional delivery with backend, QA, mobile, product and design; async remote collaboration

EXPERIENCE

Underdog Fantasy | USA (Remote) | 09/2022 – 03/2026
Full Stack Developer
US real-money fantasy sports platform, 5M+ registered users. Rails monolith with ERB + Stimulus for internal operations tooling, plus a
React/TypeScript consumer product.
• Owned backend development in the user-profile and live-markets domains (Ruby on Rails, PostgreSQL): designed and evolved the
relational models behind market configurations that operations adjust and override in production many times a day — 1,000+
markets configured a week.
• Rebuilt the live-markets trading workflow used by 200+ operations staff every day. I collapsed three separate screens — market
creation, pricing, and line generation — into one, with debounced server-side price recalculation driven from Stimulus, prefilled line
swaps, and dynamic price steps scaled to the price band.
• Migrated production authentication from a legacy in-house flow to Auth0 and Okta in two weeks, across two products and under
state-by-state regulatory requirements. I built the Rails-side login, session and account integration as well as the React MFA and
auth screens, ran the rollout as an A/B test against the native flow, and phased in mandatory MFA without a spike in support tickets.
• Built a personalization feature end to end — combining user history, market parameters and business rules into a single decision
path that lets eligible users place a larger entry — raising adoption among eligible users by 15%.
• Wrote and tuned complex SQL against PostgreSQL for production investigations and reporting across profile, markets and wallet
data; applied Redis caching on performance-sensitive paths of the monolith and built Rails view components with Hotwire
behaviour for server-rendered pages.
• Led the migration from XState to React Query across core flows, removing roughly 30% of state-management code. My data-access
and API patterns became team defaults, and I enforced them as a regular code reviewer.
• Acted as the go-to engineer for both domains: wrote RFCs and design docs, negotiated API contracts with partner teams, built the
auth web screens consumed by the mobile apps, and kept 90%+ unit coverage on the code I owned.
Qubit Labs | UK (Remote) | 07/2021 – 09/2022
Frontend Developer
• Built and published a ~50-component Storybook library as versioned npm packages, adopted by 3 production applications as the
shared design system.
• Launched a content platform on Next.js with server-side rendering, built for SEO-driven organic traffic.
• Delivered in 2-week Scrum sprints with planning-poker estimation, working directly with design and backend.

Earlier: NEKLO LLC / Nix Solutions | Ukraine (Remote) | 01/2021 – 08/2021

Frontend Developer
• Built React product and filtering interfaces for high-traffic sections of a Magento e-commerce store and resolved cross-browser
issues; created responsive, mobile-first components and modular libraries reused across client projects.

PROJECTS · 2026 – Present
TicketHub — event-driven ticketing platform · github.com/vkot91/tickethub
NestJS microservices with a saga-with-compensation order flow, outbox/inbox pattern over RabbitMQ, Redis seat locks (zero oversell at
1,000 concurrent buyers, verified with k6), and idempotent Stripe webhooks — the same async and consistency patterns Rails teams
reach for with Sidekiq. Two Next.js 15 App Router apps on top, with Tailwind, Radix and an interactive seat map. PostgreSQL with Drizzle
ORM, BullMQ, AWS (Lambda, S3, SES, ECS Fargate), Grafana Loki.
Social platform monorepo · github.com/vkot91/monorepo-social-media
Turborepo + pnpm monorepo sharing Zod-validated contracts: NestJS API with JWT refresh-token rotation, Next.js App Router frontend,
and Playwright e2e running in GitHub Actions against an isolated, seeded PostgreSQL/Redis environment.

EDUCATION
MSc, Computer Science (Web Applications) — Lublin University of Technology, Poland · 2016–2021

LANGUAGES
Ukrainian (native) · Polish (C1) · English (C1)