Hyeonjun Kil
Backend Engineer
kboxstar@gmail.com Seoul, South Korea
Professional Summary
Backend engineer with 4 years and 5 months of experience across 4 domains (Education, Healthcare, Clinical Trials, Gaming), leading 20+ projects with quantifiable results: 82% BigQuery cost reduction, CDK-based analytics infrastructure as code, and internal NPM package system. Specializes in serverless/EDA-based system design and cloud cost optimization. Presented at Games on AWS 2025 on leveraging AI to scale data pipeline capacity 270x through batch-to-Lambda architecture improvement. Ensures system reliability through architecture-level root cause analysis, including resolving recurring Deadlock patterns.
Core Competencies
Cost Optimization & Performance
- Cloud cost structure analysis and pricing model optimization — 82% cost reduction ($6.25→$1.1 per TB)
- Bulk data processing bottleneck diagnosis — 72% sync time reduction via bulk operation migration
- Data model redesign and query optimization — 97% query performance improvement (18s→0.5s)
System Design & Architecture
- Event-Driven Architecture design — batch→Lambda serverless migration reducing processing 2hrs→5min, 270x pipeline expansion
- Multi-project shared package system design — code consistency and reusability across 12 projects
- Legacy progressive modernization strategy — staged architecture migration + TDD adoption achieving zero-incident deployments
Reliability & Quality
- Root cause analysis — reproduced and identified lock deadlock patterns in recurring failures, resolving at the architecture level
- Data consistency design — outbox pattern (event integrity), distributed lock (concurrency control)
- Regulatory compliance framework — integrated verification package across 12 projects + CDK-based audit log analytics pipeline achieving 94%+ coverage
Technical Sharing & Development Culture
- AI development adoption leadership — guided team on AI agent workflows (Cursor, Claude Code) and context engineering practices, driving company-wide AI adoption (Games on AWS 2025 speaker)
- TDD foundation for AI code reliability — established test-driven development framework to ensure AI-generated code quality, embedding TDD culture across the team
- Developer meeting-driven knowledge sharing — authored shared library contribution guides and merge rules, established code conventions, standardized architecture patterns across teams
Experience
DEV Team Server Developer
GameDuo
2025-01 — Present
Mobile game development and publishing company
- Resolved escalating marketing data query costs by switching BigQuery read method, achieving 82% cost reduction ($6.25→$1.1 per TB)
- Overcame batch processing limitations by migrating to serverless architecture, expanding marketing metrics collection range 6x (60→360 days)
- Eliminated code duplication across multi-game projects by building NestJS utility (10 modules) + Game Server Kit (2 packages), achieving code consistency across 12 projects
- Designed and implemented Git-like Audit Log + 3-Way Merge engine to automate balance data change tracking and merging across 6 games, resolving delayed incident response
- Unified 3 ad platform (Google Ads/Meta/TikTok) data and improved query performance 97% (18s→0.5s) through table normalization
- Addressed game probability disclosure regulation risk by integrating DynamicModule-based package across 12 projects, building CDK-based audit log pipeline (94%+ coverage)
Dev2 Team Docs Squad Backend Engineer
JNPMedi
2023-06 — 2024-08
Clinical trial data management solution startup with KRW 16B cumulative investment (MSA-based)
- Reduced 5+ monthly e-signature processing failures to zero by reproducing and identifying FK shared lock deadlock pattern, fundamentally resolving clinical trial Deadlock
- Expanded user-organization 1:1 to 1:N mapping with full API backward compatibility ensuring zero-disruption migration, enabling multi-org onboarding for enterprise hospitals and pharma
- Deployed as part of 3-person team (2 BE + 1 FE) for 3-week emergency sprint on delayed VDR project, implementing Event-Driven PDF conversion and permission-based workflow to meet MVP launch deadline
- Resolved delayed detection of email delivery failures by building event-driven monitoring pipeline for real-time bounce/complaint tracking with instant alerting (published tech blog)
Dev Team Backend Engineer
Medgo
2022-04 — 2023-05
Telemedicine and medication delivery platform startup with 300K cumulative downloads
- Modernized 3,000-line legacy codebase via 3-stage progressive migration (single-file → layered → NestJS) with JS→TS + 80% TDD coverage, maintaining zero post-deployment incidents
- Resolved session conflicts across 3 subdomains on 300K-download platform by building JWT-based token authentication in 3 weeks, enabling seamless user switching
- Automated full prescription-to-medication-guide workflow via Naver Clova OCR + MFDS (Korea FDA) API integration, cutting pharmacist processing time 90% (3-5min→30sec)
- Introduced Socket.IO-based real-time status sync + Web Push notifications, reducing status reflection delay from tens of seconds to instant and eliminating patient wait complaints
Dev Team Backend Engineer
SimpleHAN
2021-07 — 2022-03
Custom software consulting firm
- Developed LMS core features (QR attendance, PDF certificates, Excel bulk registration) full-stack over 10 months, digitizing learning management for 300+ students per semester
- Overcame legacy PHP system limitations via cross-frame integration, implementing weighted matching algorithm and counselor back-office to launch JOB Agent service
- Built cross-domain auth server to unify login across 5 country-specific websites into a single authentication system
Technical Skills
Backend: TypeScript·NestJS·Express.js·TypeORM·Java·Spring Boot·Jest — Legacy-to-NestJS progressive migration, shared package system across 12 projects, 80% TDD coverage
Database: MySQL·BigQuery·ElastiCache (Redis) — Query optimization (18s→0.5s), deadlock root cause analysis, large-scale table normalization, Redis distributed locking
Cloud & Infra: AWS (Lambda, S3, SQS/SNS, CDK, EventBridge, Kinesis, Athena)·GCP (BigQuery, Pub/Sub)·Docker·GitHub Actions — Batch-to-serverless migration (270x expansion), 82% BigQuery cost reduction, CDK-based analytics infrastructure as code
Education
Inha Technical College
Bachelor's (Advanced Major Program, Concurrent Employment) • Computer Information Engineering
2022-02 — 2023-02 GPA: 4.10/4.5
Inha Technical College
Associate (Concurrent Employment) • Computer Information
2017-02 — 2022-02 GPA: 4.16/4.5
Certifications
SQLD (Structured Query Language Developer)
Korea Data Agency • 2025-12
Engineer Information Processing
Human Resources Development Service of Korea • 2023-06
Awards
2014 Youth Future Imagination Technology Competition Gold Award (KIPO Commissioner's Award)
2014 — AppKeeper mobile app launch