Hyeonjun Kil
Backend Engineer / Platform Part Leader
kboxstar@gmail.comSeoul, South Korea
github.com/kilhyeonjunwww.linkedin.com/in/kilhyeonjunBlog: blog.kilpenguin.com
PROFESSIONAL SUMMARY
Backend engineer with 5 years of experience building a shared platform for 6 live games (12 branches), including multi-tenant chat that held 20,002 CCU for 900s in a dev load test. My background spans DB optimization, concurrency, and legacy migration across education, healthcare, clinical trials, and gaming. As hands-on lead of a 3-person BE/FE/AI Platform Part, I manage shared done criteria and quarterly OKRs, and verify AI-assisted output with TDD, integration tests, and review.
EXPERIENCE
GameDuo
2025.01 - Present
Mobile game development and publishing company. Shared backend platform design for 6 live game environments (12 operating branches), data pipeline engineering, and common package system operations
DEV Team Platform Part Leader (2026.04 - Present)
- Single-shard chat bottleneck at 6,000 msg/s: ack p95 767ms, 99.39% acceptance. Personally led sharding and load validation. Separately set shared done criteria and verification templates for the 3-person BE/FE/AI part, tracking work in quarterly OKRs with capacity targets (6,000 CCU, 18,000 RPS, 90,000 RPS burst). Load-test ack p95 767ms→6.6ms, scaling 1→4 shards.
- Archive backfill saturated RDS CPU at 99.8%, throughput 9.3 items/min. Performance Insights isolated a query causing 99.5% of load; replaced N+1, DATE() predicates that prevented index use, and full-column reads with IN() batching, direct date comparison, and selected columns. Backfill throughput up 26x (9.3→247 items/min), Lambda p50 63.2s→1.36s, 15,333 completed with 0 pending.
- Company-wide AI agents needed centralized per-user policies, device identity, and usage tracking. Owned design, operations, and merge approval for device registration, policy, remote-control, and usage tooling; AI agents implemented it before operational handover. Assigned the usage-reporting switch to 92 people on the active roster; all 87 reporting online devices applied it, and 525 automated security tests passed.
- Coding standards, domain knowledge, and verification rules were scattered across individual setups. Unified rules, agents, hooks, and domain knowledge in Git with local RAG search. Every member shares the same AI working context via git pull, with pre-review criteria captured in an automated checklist.
DEV Team Server Developer (2025.01 - 2026.03)
- High DB and CPU load during metrics sync. Evaluated partitioning: write IOPS fell, but read IOPS rose and CPU load persisted. Chose lower Lambda concurrency over a structural change with little benefit for its code complexity.
- Batch processing collection limits (60 days). Replaced batch runs with EDA on-demand processing to distribute peak load. Expanded metrics range 6x (60→360 days), processing time 2h→5min.
- Code duplication and convention inconsistency across 7 projects. Introduced shared NestJS packages and auto-deploy (10 utility modules + Game Server Kit). Upgrade rollout across 7 projects 3hrs→15min via automation, with shared-code changes consolidated into one package.
- Slow marketing queries across Google Ads/Meta/TikTok data. Normalized read path + index tuning to avoid full-scan on 100-column table. Performance improved 97% (18s→0.5s).
- Regulatory risk from game probability disclosure requirements. Integrated a DynamicModule probability package across games and built a CDK audit-log pipeline. Achieved 94%+ integration test coverage (12 suites, 115 tests).
PROJECTS: Real-Time Game Chat Server (Multi-Tenant), Marketing Integrated Platform, Internal Common Library System
JNPMedi — Dev2 Team Docs Squad Backend Engineer
2023.06 - 2024.08
Clinical trial data management solution startup with KRW 16B cumulative investment (MSA-based)
- Intermittent automatic e-signature failures caused by database deadlocks. Reproduced foreign-key (FK) shared/exclusive-lock deadlocks using SQL and lock tables. Derived two remedies: transaction ordering and SELECT FOR UPDATE pre-acquisition.
- VDR project schedule delay under MVP launch pressure. Emergency sprint with 2 BE + 1 FE: Event-Driven PDF conversion + permission-based workflow. Launched the MVP in 3 weeks and ended synchronous large-PDF conversion timeouts.
- Missing delivery/bounce tracking left email failures visible only after the fact. Received SES Configuration Set events through SNS and logged results in Datadog. Built Slack alerts for delivery failures and shared the implementation through a tech blog and internal seminar.
PROJECTS: Maven Docs System Reliability Improvement, Maven VDR MVP Development
Medgo — Dev Team Backend Engineer
2022.04 - 2023.06
Telemedicine and medication delivery platform startup with 300K cumulative downloads
- Auth, appointment, and notification changes risked regressions in a single Express app.js. 3-stage progressive migration (single-file→layered→NestJS) + JS→TS. Reached 80% test coverage and established module/layer boundaries and team coding conventions.
- Manual prescription entry bottleneck (3-5min per case). Naver Clova OCR + MFDS (Korea FDA) API integration for full workflow automation. Pharmacist processing time 3–5min→30sec per prescription.
- Status reflection delay in consultation/dispensing/delivery (tens of seconds). Socket.IO real-time sync + Web Push notifications. Reduced to under 1 second, eliminated patient wait complaints.
PROJECTS: Doctor/Pharmacist Web Service Enhancement, Backend Architecture Migration
SimpleHAN — Dev Team Backend Engineer
2021.07 - 2022.03
Custom software consulting firm
- Manual learning management (attendance, completion, progress tracking). Built LMS full-stack (QR attendance, PDF certificates, Excel bulk registration). Digitized operations for 300+ students per semester.
- Adding matching and counseling to legacy PHP risked regressions. Integrated an independent Spring Boot app via an iframe/postMessage boundary with weighted-average matching. Launched the JOB Agent service while isolating the legacy-system impact surface.
PROJECTS: Learning Management System (LMS) Development
SIDE PROJECTS
Personal Projects (Side) — Platform / Tooling Engineering
2026.04 - Present
Self-directed platform/tooling engineering outside work — development workflow automation framework and side projects
PROJECTS: Public AI CLI Runtime Launcher
CORE COMPETENCIES
DB Optimization, Concurrency & Distributed Systems
- Query-plan and lock-contention analysis; Redis distributed locks, Streams WAL, and sharding
Serverless Architecture & Cost Optimization
- AWS serverless and event pipelines; BigQuery Storage Read API migration
Legacy Modernization & Reliability
- Incremental Express-to-NestJS migration, TypeScript, shared packages, and test-backed changes
Engineering Productivity & Quality Verification
- Shared team context, review checklists, and Git hooks for AI-assisted output
TECHNICAL SKILLS
Backend: Node.js, TypeScript, JavaScript, NestJS, Express.js, TypeORM, Java, Spring Boot, Jest, TDD, REST API, gRPC, Testcontainers, Integration Test, OAuth2 — Incremental NestJS migration, shared packages, TDD, REST API design, and consumption of gRPC-based APIs
Database: MySQL, BigQuery, ElastiCache (Redis), Redis, Redis Streams, Valkey, DynamoDB, ClickHouse — Query optimization, deadlock root cause analysis, large-scale table normalization, Redis distributed locking
Cloud & Infra: AWS (Lambda, ECS/Fargate, S3, SQS/SNS, CDK, EventBridge, Kinesis, Athena), GCP (BigQuery, Pub/Sub), Docker, GitHub Actions, CI/CD, Terraform, ARM64 (Graviton) — Batch-to-serverless migration, EventBridge/Kinesis event pipeline design, BigQuery read-path migration, CDK-based infrastructure as code, CI/CD pipeline
Quality, Observability & AI Tooling: k6, Grafana, Datadog, LocalStack, Claude Code, MCP — Validates performance with k6 load tests and observability metrics, and verifies AI-assisted output through TDD, integration tests, and review
EDUCATION
Inha Technical College
Bachelor's (Advanced Major Program, Concurrent Employment), Computer Information Engineering
2022.02 - 2023.02
Inha Technical College
Associate (Concurrent Employment), Computer Information
2017.02 - 2022.02
CERTIFICATIONS
SQLD (Structured Query Language Developer) — Korea Data Agency, 2025.12
Engineer Information Processing — Human Resources Development Service of Korea, 2023.06
PRESENTATIONS & WRITING
Boosting Developer Productivity with Amazon Q Developer
Conference Talk — 2025.10
Games on AWS 2025 Customer Session
Email Delivery Monitoring through AWS SES Event Logs
Internal Seminar → Tech Blog — 2024.03
Presented internally then published as tech blog
TRAINING PROGRAMS
Hanghae Plus Backend 6th
2024.09 ~ 2024.11
Hanghae99
TDD and Clean Architecture, Concurrency Control, Redis
Week 1 Hall of Fame, Black Badge Completion
OPEN SOURCE
PROFESSIONAL DEVELOPMENT
System Design Interview Study
2025.08 ~ 2026.04
System Design Interview: An Insider's Guide Vol. 1 & 2
Node.js Design Patterns Bible Study
2025.10 ~ 2025.12
Node core concepts, concurrency, event loop, module system