Backend Engineering Services
Rostyslav Zakroishchykov helps product teams and founders ship dependable backend systems—from AI-powered document pipelines and RAG chat products to payment flows and high-scale APIs.
RAG & AI Document Processing
End-to-end retrieval-augmented generation pipelines: PDF and document ingestion, chunking, embeddings, vector search, context ranking, and LLM integration. Includes OCR, preprocessing, and structured metadata extraction from complex layouts and low-quality scans.
Payment & Order Processing Systems
Unified payment gateway integrations with secure signing, webhook verification, idempotent transaction handling, and reconciliation between provider callbacks and internal order state. Event-driven workflows with PostgreSQL lifecycle states and Celery background workers.
API & Backend Development
Production-ready REST APIs in Python (FastAPI, Django) and Node.js (Express, TypeScript) with centralized error handling, Zod/express-validator input validation, JWT authentication, RBAC, and rate limiting.
Database & Performance Engineering
SQL schema design, query optimization, strategic indexing, and multi-layer Redis caching with event-driven invalidation to reduce database load and improve API latency under high traffic.
Architecture Refactoring & Technical Leadership
Refactoring monolithic codebases using SOLID, Clean Architecture, and dependency injection. Leading backend delivery, mentoring engineers, and designing resilient systems for traffic spikes and third-party API failures.
Working together
Available for contract and long-term backend work. Remote-friendly with experience collaborating in English with US-based teams across AI, fintech, and infrastructure products.
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