Hyeonjun Kil - Resume

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 2025.01 — Present

GameDuo [Detail]

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 2023.06 — 2024.08

JNPMedi [Detail]

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 2022.04 — 2023.05

Medgo [Detail]

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 2021.07 — 2022.03

SimpleHAN [Detail]

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)

AppKeeper mobile app launch • 2014

Presentations & Writing

Boosting Developer Productivity with Amazon Q Developer [Video] [Review]

Conference Talk — 2025.10

Games on AWS 2025 Customer Session

Email Delivery Monitoring through AWS SES Event Logs [Link]

Tech Blog — 2024.03

Mailing System Email Delivery Result Tracking Feature

Internal Seminar — 2024.02

Training Programs

Hanghae Plus Backend 6th [GitHub] 2024.09 ~ 2024.12

Hanghae99 · [Review]

TDD and Clean Architecture, Concurrency Control, Redis

Week 1 Hall of Fame, Black Badge Completion

Open Source

kiro-gateway [GitHub]

• Remove :ro from Docker SQLite volume mount #79 — 2026-02

CodexBar [GitHub]

• Support kiro-cli 1.24+ Q Developer format #288 — 2026-02

oh-my-opencode [GitHub]

• Integrate plugin skills into unified skill discovery #974 — 2026-01

atlassian-skills [GitHub]

• Use ADF format for comments in Jira Cloud API v3 #7 — 2026-01

Professional Development

System Design Study Vol. 2 [GitHub] — System Design Interview: An Insider's Guide Volume 2

2026.01 ~ Present

Node.js Design Patterns Bible Study [GitHub] — Node core concepts, concurrency, event loop, module system

2025.10 ~ 2025.12

System Design Study Vol. 1 [GitHub] — System Design Interview: An Insider's Guide

2025.08 ~ 2025.10

AWS SAA Study Group — JNPMedi internal study group, Cloud Architecture deep dive

2023.09 ~ 2023.12