KYUWON LEE

Backend Engineer | Java · Kotlin · Spring Boot · Python · Django

Seoul, South Korea · kyuwoon369@gmail.com · linkedin.com/in/eekyuwon · github.com/kyuwon53

Professional Summary

Backend Engineer with 5+ years who quickly grasps complex domains and re-architects operationally risky legacy systems into reliable, scalable structures — spanning commerce, industrial AI, and tax. Across payment/order, membership, and content domains I own data integrity, operational stability, and scalability, and prefer to prevent incidents structurally — through state models, idempotency, validation, and automation — rather than one-off fixes. As team lead of the backend for a developer learning-and-commerce platform, I delivered a zero-downtime migration of a legacy membership base, an annotation-based RBAC redesign, and an event-driven reward system, and root-caused mission-critical payment failures.

Experience

HanbitN — Senior Backend Engineer → Team Lead

Sep 2024 – Present

• Root-caused a mission-critical payment bug where fully processed payments still returned HTTP 401: Kotlin suspend controllers triggered a Servlet ASYNC dispatch that skipped the JWT filter and cleared the SecurityContext; fixed via the filter's dispatch policy, removing intermittent auth failures on the payment flow.
• Eliminated recurring "max active transactions" outages under peak traffic by moving high-traffic read endpoints out of per-request JTA (Atomikos) transactions — restored stability with no added infrastructure.
• Designed and led a phased, zero-downtime migration of 300K+ member records from a 30-year legacy platform via a unified membership API with dual-database sync, letting legacy and new services coexist during a gradual cutover.
• Redesigned authorization from URL-path rules to annotation-based method security (@PreAuthorize), unifying access control for members, operators, and role-based admins across shared APIs and the internal ops tool — a reusable shared-platform model.
• Built an event-driven reward system (@EventListener processor → routing facade → strategy handlers) with idempotent, duplicate-safe reward delivery and consistency across legacy and new databases.
• Hardened order/payment integrity with database-backed idempotency (unique constraints + row-level locking) and modernized payment-gateway (PG) integration and phone identity verification into a server-to-server, one-time-token flow.
• Optimized PostgreSQL performance (execution-plan analysis, subquery→join rewrites, indexing, N+1 removal) — e.g. cut a high-traffic product-listing query from ~1,800ms to a few milliseconds.
• Drove CI/CD and operational automation: team-wide Flyway schema versioning (eliminating environment drift and manual-SQL deployment failures), Jenkins batch-failure alerting, and Actuator-based health checks.

MakinaRocks — Backend Engineer

Oct 2022 – Aug 2024

• Sole owner of migrating anomaly-detection services from Python/Django to Java/Spring Boot, redesigning APIs and data models around business workflows rather than one-to-one porting.
• Cut dashboard latency from over 5 seconds to under 1 second by precomputing sensor metrics at ingestion instead of query time.
• Built RabbitMQ producers/consumers for high-frequency sensor ingestion and a Parquet archival pipeline, plus a health monitor with Slack alerts for early failure detection.
• Performed a manual PostgreSQL failover on AWS EC2 during a production outage (disaster response) and automated blue-green deployments with GitHub Actions, Docker, and EC2.

Uplus IT — Full Stack Engineer

Sep 2021 – Oct 2022

• Operated Hometax tax-filing services handling 10M+ filing transactions at peak (high-throughput, mission-critical).
• Optimized a 700K+ tax-invoice batch via workload partitioning and refactored 7,000+ lines of duplicated tax-calculation logic into reusable modules.

Technical Skills

Education

Chungnam National University — B.S. Mathematics

Certifications

SQLD (SQL Developer)