Rostyslav Z

Work Experience

Rostyslav Zakroishchykov has 12+ years of commercial experience across AI integration, payment systems, event-driven backend architecture, and full-stack delivery for US-based clients and product companies.

AI Integration Engineer | Python Developer

Upwork · October 2024 – Present · United States (remote)

  • Engineered and deployed a Retrieval-Augmented Generation (RAG) system serving 200+ active users by designing a pipeline for document ingestion, chunking, embedding generation, and vector search-based retrieval.
  • Integrated retrieved context into LLM prompts with ranking and filtering logic to ensure relevance and reduce noise in responses.
  • Built an AI document processing service that ingests PDFs and architectural schematics and extracts metadata using a multi-stage pipeline combining OCR, preprocessing, and LLM-based interpretation.
  • Designed a workflow that converts raw documents into clean text and visual segments, then applies classification and extraction steps to produce structured JSON output, with fallback and validation logic for inconsistent layouts and low-quality scans.

Backend Engineering Lead

Compass Mining · September 2022 – October 2024 · Denver, CO (remote)

  • Implemented a payment integration layer with external providers through a unified abstraction for multiple payment gateways, including secure request signing, webhook verification, and consistent transaction state management.
  • Ensured idempotent payment processing, reliable reconciliation between provider callbacks and internal order states, and robust error handling for network and provider-side failures.
  • Designed a state-driven order processing architecture by replacing synchronous webhook execution with an event-based workflow using persistent lifecycle statuses in PostgreSQL and asynchronous Celery workers on AWS.
  • Moved payment, inventory, notifications, and CRM integrations to background jobs with retryable, idempotent execution and failure recovery, improving resilience to traffic spikes and third-party API failures.

Principal Backend Engineer

Compass Mining · January 2021 – October 2024 · Denver, CO (remote)

  • Engineered and optimized complex SQL queries and database schema by restructuring table design and introducing strategic indexing, reducing data retrieval latency at scale.
  • Refactored the codebase using SOLID principles, breaking large monolithic classes into smaller single-responsibility components and improving readability and long-term maintainability.

Backend Engineer

Upwork · March 2019 – October 2020 · United States (remote)

  • Implemented centralized error handling in Express.js via global error middleware, removing repetitive try-catch blocks and standardizing API error responses.
  • Configured environment-based setup using dotenv with separate configurations for development, staging, and production, improving deployment safety and environment consistency.

Full Stack Developer (Node.js)

CryptoDrilling · March 2017 – January 2020 · United States (remote)

  • Refactored a Node.js service using Clean Architecture principles, separating controllers, use cases, and repositories to improve maintainability, testability, and separation of concerns.
  • Introduced Dependency Injection across the Node.js codebase, decoupling modules and enabling isolated unit testing of core business logic.
  • Implemented rate limiting and request validation using Zod and express-validator, enforcing strict input schemas and request throttling to improve API security and stability.

Full Stack Developer

Upwork · October 2013 – February 2017 · California, United States (remote)

  • Designed and implemented a multi-layer Redis caching architecture replacing in-memory caching, with structured cache tiers and event-driven invalidation tied to database mutations; reduced database load by ~80% and improved API latency under high traffic.
  • Built a secure JWT-based authentication and authorization system with role-based access control, refresh token rotation, permission-scoped API endpoints, and centralized session validation.

Education

American University Kyiv

Bachelor's Degree in Information Technology · September 2022 – June 2026

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