Career Description — Hyeonjun Kil

Resolved delayed balance issue response caused by inability to track data change history during game operations

• •Inability to track data change history across environments and projects — Designed Git-like version control system with UUID-based cross-environment/project entity tracking, Ensured data consistency across 6 games
• •Manual entity change recording prone to omission — Applied Event Sourcing-based change tracking with Auditable decorator + TypeORM Subscriber pattern, Automated entity change recording pipeline
• •Multi-environment version conflicts during data merge — Developed 3-Way Merge Engine with parent/child entity conflict detection and unique constraint handling, Enabled reliable version merging across 6 games
• •Expensive full-snapshot comparison for every version diff — Designed Version Diff Engine with dual strategy: incremental comparison and snapshot comparison based on Base Audit availability, Established cost-efficient version diffing
• •Entity tracking failures due to PK dependency during migration and merge — Introduced shared identifier-based entity tracking decoupled from PK dependency, Ensured accurate entity tracking during migration, comparison, and merge

Unified marketing platform managing Google Ads/Meta/TikTok campaigns, creatives, and metrics in a single system, built on dedicated marketing DB separation

• •Fragmented ad platform management across Google Ads/Meta/TikTok — Delivered unified campaign automation API for creation, deployment, modification, and retention metrics in a single platform, Consolidated multi-platform campaign management into one system
• •NAS-S3 sync reliability and restore failures — Separated SQS server-worker flow, automated outbox events, tracked asset_nas_sync state, Secured restore reliability with state-tracked sync pipeline
• •Meta asset sync performance bottleneck — Converted ORM single-row saves to bulk processing, Image sync reduced 72% (25.7s→7.1s), DB transactions reduced 95% (10~16s→0.3~0.5s)
• •High creative processing Lambda costs on x86 architecture — Migrated Lambda to ARM64 Graviton2 architecture, Reduced creative processing Lambda costs by 20%
• •High BigQuery query costs for marketing data reads — Adopted BigQuery Storage Read API with gRPC streaming-based high-performance data reading, Reduced BigQuery costs by 82% ($6.25→$1.1 per TB)
• •Marketing query latency (18s) from 100+ column denormalized table — Normalized table into main/time-series/prediction, tuned indexes + cursor pagination, Performance improved 97% (18s→0.5s)

Resolved batch job limitations from marketing metrics 60-day → 360-day expansion and serverless transition for batch processing

• •Full serverless migration risked operational stability — Designed hybrid architecture keeping API server on EC2 while separating batch/job processing to Lambda, Attained workload-optimized resource usage without disrupting live services
• •Batch processing limited to 60-day collection range with 2-hour runtime — Established Event-Driven flow with SQS+Lambda+EventBridge, Reduced batch time 2h→5min, expanded collection 6x (60→360 days)
• •Data consistency risk during event publishing — Adopted Transactional Outbox Pattern for scheduled and delayed event publishing, Ensured data consistency across distributed event processing
• •Lambda throttling, high log costs, and build OOM issues — Applied Batch Size bulk processing, CloudWatch log optimization, and build OOM remediation, Stabilized Lambda operations and reduced operational costs
• •DB connection exhaustion during massive Lambda execution — Introduced RDS Proxy connection pooling, Resolved connection exhaustion and stabilized database access

Built S3-based sync and automated DDL management system to resolve dynamic game data inconsistency across environments

• •Dynamic game data inconsistency across environments — Built S3-based cross-environment data synchronization across development/staging/production, Unified game data state across all environments
• •Manual DDL schema management causing sync failures — Created automated DDL management engine with dynamic PK column type resolution, column type mismatch detection with MODIFY, and automatic index creation/RENAME, Automated schema sync across environments
• •Large-scale Cloud Data ingestion and S3 upload bottlenecks — Analyzed and optimized ingestion and upload pipeline, Resolved large-scale processing delays
• •Sync job instability causing operational issues — Implemented job separation, transitioned scheduling approach, tuned timeouts, and introduced non-blocking processing, Secured operational reliability for sync pipeline
• •Four CloudData bugs: Redis cache invalidation, DDL SKIP metadata, copy key deletion, excludeCloudData propagation — Diagnosed and fixed all four issues systematically, Stabilized CloudData operations
• •Unintended full Cloud Data deletion risk from missing option forwarding — Added excludeCloudData option forwarding to four POST migration paths, Blocked unintended data deletion risk

Development and operation of NestJS utility (10 modules) + Game Server Kit (2 packages) for multi-project code consistency

• •Common code duplication across services increasing maintenance cost — Designed 10-module library system: core, repository, cache, lock, slack, crypto, smb, hash, type, iac, Unified shared code across 7 projects with standardized dependency management
• •Repository module lacking bulk operations and audit logging capabilities — Enhanced Repository module with Bulk/Audit Log/TypeORM narrowing using overloading + TypeScript generics; refactored 2000+ lines by SRP, Improved module extensibility and maintainability
• •Concurrency conflicts in multi-instance environments — Engineered distributed lock module with ElastiCache (Redis)-based distributed lock decorator, Enabled safe concurrency control across multi-instance deployments
• •Manual library upgrades taking 3 hours across 7 projects — Added workflow_dispatch+matrix and changed-package CI tests on GitHub Packages, Reduced 7-project upgrade runtime 3h→15min
• •Game server shared code tightly coupled in monolithic package — Extracted game-server Kit: moved Sheet+5 submodules to @gameduo/glider-sheet, resolved conflicts via 5-branch divergence analysis, Realized independent package versioning and deployment
• •Slow CI pipeline (15m47s) due to test infrastructure inefficiency — Switched ts-jest isolatedModules and explicit Entity types, Cut CI time 61% (15m47s→6m06s); passed 81 suites/978 tests

Built probability calculation package and CDK-based audit log analytics infrastructure to address regulatory risk from lack of game probability verification

• •No reusable probability calculation module across game projects — Packaged NestJS DynamicModule with 5 functions + Kinesis logging as @gameduo/glider-probability, Integrated into 12 games with standardized probability calculation
• •No audit log analytics infrastructure for probability verification — Codified CDK analytics pipeline: Kinesis→Firehose (Dynamic Partitioning+Parquet)→S3→Glue→Athena, Unlocked end-to-end probability audit log querying
• •Mock-based tests lacking regression confidence for infrastructure code — Replaced mocks with LocalStack + Testcontainers integration tests, Secured 94%+ coverage (12 suites/115 tests)

Contribution: Restructured changmin's initial implementation into a package. Independently handled P0 bug fixes, test stabilization (94%+ coverage), CDK infrastructure, and deployment across 12 projects.

Resolved inefficiency in collecting participant consent and e-signature regulatory compliance using Cyan (in-house Node.js framework)

• •Inefficient individual consent dispatch for clinical trial participants — Created batch dispatch system for participant group management and batch consent dispatch, Streamlined consent collection per clinical trial
• •Opaque event flow from batch + S3 event-based processing — Migrated to EDA + Outbox pattern, Enabled event flow visualization and local test environment setup

Contribution: Designed EDA transition, implemented plugin architecture, built batch participant registration with Signer table migration. 2-person BE team.

Resolved critical notification failures due to database Deadlocks in clinical trial e-signature notifications

• •Production Deadlock incidents in clinical trial e-signature notifications — Analyzed shared-to-exclusive lock timing during foreign key constraint enforcement, Identified root cause and prevented Deadlock recurrence
• •Synchronous notification processing causing cascading failures — Designed and implemented AWS SNS → SQS → Lambda Event-Driven pipeline, Decoupled notification delivery from main transaction flow

Contribution: Independently performed deadlock reproduction, analysis, and resolution. Reproduced deadlock scenario with SQL in DBeaver and verified via INNODB lock tables.

Resolved lack of delivery status tracking and data consistency issues in email system

• •No email delivery status tracking — failures detected hours later — Configured SES Configuration Set → SNS → event processing pipeline with Datadog logging + Slack real-time alerts, Bounce/complaint detection cut to seconds with instant Slack alerting

Contribution: Designed and implemented SES Configuration Set → SNS → event processing pipeline. Led tech blog publication and internal seminar presentation.

Resolved user inconvenience and system scalability limitations from 1:1 user-organization structure

• •1:1 user-organization structure limiting multi-organization management — Expanded user-organization relationship from 1:1 to 1:N structure, Enabled multi-organization management per user
• •API migration risking service downtime — Ensured backward compatibility throughout migration process, Achieved zero-downtime API migration with service continuity

Contribution: Full-stack change: schema migration, auth flow modification, and batch update of downstream service (Docs/Billing/Mailing) account-organization reference logic.

Frontend development for clinical Trial Master File (TMF) management system

• •No document management interface for clinical Trial Master File system — Implemented document upload, classification, version management, and Audit Trail functionality for Admin/Dashboard, Delivered complete TMF management frontend

Contribution: FE developer in 1 BE + 2 FE team. Implemented Admin/Dashboard document upload, classification, and audit trail features.

Emergency deployment for delayed VDR project under MVP launch pressure

• •Synchronous PDF conversion blocking document upload flow — Designed Event-Driven PDF conversion triggered by S3 upload → Lambda lightweight conversion → SNS/SQS notification, Delivered non-blocking document processing pipeline
• •No role-based document access control for medical workflows — Engineered permission-based workflow managing document access and approval by medical role, Secured compliant document access control
• •VDR project schedule delay under MVP launch pressure — Executed 3-week emergency sprint prioritizing core features, Delivered MVP on schedule meeting launch deadline

Contribution: 1 of 2 BE developers. Designed and implemented EDA document processing pipeline and large-scale async PDF conversion worker. Delivered MVP within 3-week emergency sprint.

Subscription, plan, and license management system for all Maven services

• •No support for organization-specific subscription plans — Shipped org-specific custom plan Internal API, Facilitated tailored subscription offerings per organization
• •Non-renewable plans could be incorrectly renewed or re-subscribed — Added renewal/re-subscription blocking logic for non-renewable plans, Ensured business policy consistency for subscription lifecycle

Contribution: Implemented org-specific custom plans, event queue-based inventory auto-deletion, and renewal eligibility logic. Multiple PRs directly contributed.

Built JWT-based unified authentication system to resolve subdomain session conflicts

• •No unified authentication system for multi-subdomain architecture — Designed and implemented NestJS + JWT token authentication system, Established centralized auth infrastructure
• •Session-based auth causing subdomain transition failures — Migrated from session-based to token-based authentication, Enabled reliable user transitions across subdomains

Contribution: Independently designed and implemented over 3 weeks. Covered entire auth system for web (doctor/pharmacist/admin portals) + mobile app.

Resolved code duplication and inconsistency from independent service management by consolidating shared modules

• •No centralized permission verification across services — Implemented user permission verification middleware, Enhanced security with unified access control

Resolved lack of real-time status updates and manual notification workflows in doctor/pharmacist back-office systems by introducing Socket.io and Web Push

• •Duplicate appointment bookings causing scheduling conflicts — Implemented Doctor ID + time slot composite unique key constraint, Prevented duplicate appointments at the database level
• •Status reflection delay (tens of seconds) in consultation/dispensing/delivery — Deployed Socket.IO Room-based real-time status sync reflecting events to relevant users instantly, Reduced delay to under 1 second, eliminated patient wait complaints
• •External service failures (delivery, payment) cascading to main application — Architected internal API gateway integrating delivery (Hoodadak) and payment services with Circuit Breaker pattern, Achieved fault isolation preventing cascading failures

Contribution: Directly implemented Socket.IO real-time sync, Web Push notifications, API gateway, and reservation dedup in 2-person BE team.

Automated pharmacist's manual prescription entry process using OCR

• •Manual prescription text entry from images (3-5 min per case) — Integrated Naver Clova OCR API for automatic text extraction from prescription images, Automated prescription data entry
• •Manual medication guide creation inefficiency — Developed automated medication guide generation using MFDS (Korea FDA) drug information API, Reduced pharmacist processing time 90% (3-5min→30sec per prescription)

Contribution: Independently implemented over 2 months. Built entire pipeline: image intake → OCR extraction → MFDS API matching → medication guide generation.

Migrated from Express.js single-file (app.js) structure to NestJS modular architecture

• •Express.js single-file (app.js) 10,000-line codebase unmaintainable — Executed 3-phase incremental migration: app.js single file → Layered Architecture (Controller/Service/Repository) → NestJS framework adoption, Achieved modular architecture with clear separation of concerns
• •JavaScript lacking type safety causing runtime errors — Migrated JavaScript → TypeScript across the codebase, Established type safety and improved developer experience
• •No automated testing culture or regression safety net — Introduced TDD practices with comprehensive unit tests, Achieved 80% unit test coverage, zero incidents during major releases

Contribution: Led independently. Performed all 3 migration phases, JS→TS migration, TDD adoption, and team coding convention establishment.