Іван

Ivan Berestianyi
Python Backend Developer
[open contact info](look above in the "contact info" section) | [open contact info](look above in the "contact info" section) | Kyiv, Ukraine | LinkedIn | GitHub | Telegram

Python Backend Developer with 2 years of commercial experience building production systems for
high-scale B2B and commerce platforms, with a focus on payment systems, distributed microservices, and
backend architecture.
Built a procure-to-pay platform's first payment system (Stripe, Bill.com) handling 500+ transactions per
hour, and delivered end-to-end online payments that unlocked a company's first online sales channel.
Strong hands-on experience with Python, FastAPI, PostgreSQL, gRPC, Celery, and Redis, applying
production-grade reliability patterns — the outbox pattern, idempotency, and DB-level locking — to keep
financial transactions consistent under load.
Experienced across the full delivery cycle: REST API and data-model design,
authentication/authorization, testing and CI/CD, monitoring and observability with Grafana, and production
support. Track record of turning business needs into measurable outcomes.

Skills
● Backend: Python 3.12, FastAPI, Flask, Django, Django REST Framework, Celery, SQLAlchemy, aiohttp, pytest
● Frontend: React, JavaScript, TypeScript (basic)
● AI: OpenAI API, AI Agent Workflows, MCP-based Automation, Claude Code, GitHub Copilot
● Data: PostgreSQL, MySQL, Redis
● APIs & Architecture: REST API, gRPC, OAuth2/JWT, Microservices, Distributed Systems, Async Processing,
Outbox Pattern, Idempotency, Clean Architecture, SOLID
● DevOps & Tooling: Docker, docker-compose, CI/CD, Nginx, Git
● Languages: English (B2) · Ukrainian (Native)

Work experience
Junior/Middle Python Developer | Precoro June 2025 - August 2026
● Contributed to the expansion of a procure-to-pay platform serving 1000+ companies with $150B+ in
spend managed.
● Built the product's first payment system (Stripe and Bill.com as providers), enabling clients to open a
financial account for paying invoices and issuing virtual cards; the feature contributed to ~20% client
base growth and reliably handles 500+ transactions per hour.
● Migrated microservice-to-microservice communication to gRPC, added linting and testing to each
microservice's CI/CD pipeline, and introduced AI review at the PR approval stage, cutting repeat PRs by
~30% and speeding up code review.
● Designed and implemented the FastAPI microservice backend, including REST API, data validation and
serialization with Pydantic, JWT-based authentication and authorization with role-based access control,
migrations, and background task processing with Celery.
● Worked extensively with PostgreSQL in production, including query optimization, execution-plan analysis,
indexing, complex SQL, transactions, locking, and large-scale data migrations.
● Set up Grafana-based monitoring and observability across all microservices: dashboards with key metrics
(latency, throughput, error rate, resource utilization) and Slack alerts, accelerating incident detection and
reducing diagnosis time.

● Applied production-grade reliability patterns for payments — DB-level locking to handle race conditions,
the outbox pattern for reliable event delivery, and idempotency keys to prevent duplicate charges on
retries.
● Tech stack: Python, FastAPI, Flask, PostgreSQL, gRPC, Celery, Celery Beat, Redis, React, microservices,
Docker, CI/CD, Pydantic, JWT, Grafana, Git, REST API, Nginx, MCP-based Automation, Claude Code.

Junior Python/Node.js Developer | SportLife December 2024 - June 2025
● Designed and built the Python backend for online payments (memberships & gift cards), including
payment API integration, structured logging, email/SMS notifications, and automatic payment–order
reconciliation; exposed this logic to the Wix storefront through a Node.js/Velo frontend layer.
● Enabled the company's first-ever online sales: the business now sells through the website end-to-end,
with no manual payment confirmation.
● Built an automated email/SMS notification and campaign system, delivering triggered messages across
the payment and membership lifecycle.
● Implemented automatic structuring and sync of business data into Google Sheets (read/write via API) for
operations and reporting.
● Set up structured logging and error handling across integrations, cutting diagnosis time for failed
payments.
● Tech stack: Python, Node.js (Velo), Wix, REST API, Google Sheets API, Celery, PostgreSQL, email/SMS
integrations, structured logging.

Trainee Python Developer | Right&Above May 2024 - December 2024
● Upgraded the internal authentication service from a legacy Python API (sessions and custom tokens) to
OAuth2/JWT, implementing token issuance (/token with password and refresh grants) and short-lived
access/refresh tokens stored in Redis with rotation.
● Built an internal daily-reports service (create, edit, and approve reports with deadlines and team roll-ups):
designed the REST API and PostgreSQL data model, and set up background jobs (reminders, aggregation)
on Celery and Redis, scheduled via Celery Beat.
● Covered key logic with unit and integration tests (pytest), improving coverage and catching regressions
before release; used Docker and docker-compose for local development and worked within the project's
CI pipeline.
● Tech stack: Python, Django, PostgreSQL, SQLAlchemy 2, Redis, Celery, Celery Beat, OAuth2/JWT, pytest,
Docker, docker-compose, REST API, Git.

Python Developer (Automation) in medical center (Part-time) June 2023 - May 2025
● Built an internal service automating routine medical documentation on the “Helsi” portal —
authentication, form templates, headless Selenium form filling, file uploads, and submission status
tracking.
● Eliminated hours of manual paperwork for clinicians, reducing documentation time by 3–4 hours per day
across the team after rollout.
● Tech stack: Python, Selenium, aiogram, asyncio, aiohttp, REST API, PostgreSQL, SQLAlchemy, Docker.

Education

Bogomolets National Medical University Kyiv, Ukraine
Master's Degree in Medicine September 2019 - May 2025