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HJ

Hyukjun Yoon

Backend engineer who turns business flows into product features and designs service structures that scale reliably.

[email protected]+82 10-6709-1540
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About Me

I am a backend engineer who turns business requirements into product features such as payments, reservations, lessons, and notifications, then designs service structures that scale reliably. I have migrated legacy payment and authentication systems to Java/Spring and reshaped high-risk operational domains into observable, change-friendly systems. Recently, I have been applying LLM pipelines and AI code review automation in production to improve both product quality and development productivity.

Experience

Day1 Company

Backend Engineer

Aug 2024 - Present

  • •In progress: backend and app localization for entering the Japanese market
  • •Diversified authentication by adding LINE, Google, and Apple social login, and integrated a LINE friend-add prompt for Japanese users
  • •Built the app internationalization (i18n) foundation with GCS runtime-loaded language packs and bundle fallback, using a GrowthBook kill switch for zero-downtime rollout
    • •Cleaned up hardcoded Korean into message-based strings with locale fallback
  • •Added user language and timezone (locale) settings APIs and screens, and propagated language codes to authenticated user data
  • •Rebuilt issued documents (completion certificates, level-test reports) from an image-based approach to an HTML rendering architecture for multilingual support — adding a new language now means adding a string file, not redrawing per-language background images
    • •The backend only publishes issuance events; rendering is delegated to a Cloud Run service that renders HTML templates and locale strings into PDF
  • •Implemented language-based home banner segmentation
Operating a CS Support System with Spring AI - From Function Calling to an Operator Suggestion Workflow
Spring AI in Practice - Building a Seven-Step AI Diagnostic Pipeline
Building a Multi-Channel Notification Server - From PHP Legacy to Java/Spring
Splitting Login Responsibility (2) - Connecting the authorize/callback FlowSplitting Login Responsibility (3) - Verifying Old Tokens and New Backend Tokens TogetherJWT Signing Migration - From HS256 to RS256/JWKS/KMS
Introducing the Facade Pattern to Spring MVC - Improving Layered Architecture with a @Gateway AnnotationStandardizing API Responses and Error Handling - Getting HTTP Status Codes Right
From ECS to EKS - Building an Operating Environment with kustomize and Gateway APIReplacing Datadog with an LGTM Stack - OTel Sidecar with Grafana, Loki, Tempo, and Mimir

EXEM

Backend Engineer

Jul 2020 - May 2024

  • •Reworked MyBatis queries with subquery structures to avoid query-size limits from large parameter sets
  • •Combined multiple split read queries into a single query to improve response performance
  • •Implemented core business logic for a database analysis tool
  • •Improved large data read performance to respond within 100ms to 3s
  • •Provided database status and analysis metrics visually in the user interface
Apache Druid article

Skills

Backend

JavaSpring BootQueryDSLMyBatisJPAKafka

Infrastructure

AWSKubernetesDockerArgoCDTerraformNginx

Database

PostgreSQLRedisDynamoDBApache Druid

AI / LLM

OpenAIGeminiAWS Bedrock

Monitoring

GrafanaLokiTempoMimirOpenTelemetryPrometheus

Tools

GitGitHub ActionsJMeterNotion

Certifications

AWS Certified Developer - Associate (2023)CKA (Certified Kubernetes Administrator) (2022)SQLD (2022)Engineer Information Processing (2019)