Backend Engineer focused on transaction processing systems, payment architecture, and distributed backend applications.

Interested in building reliable financial systems that guarantee atomicity, consistency, and data integrity under high-concurrency environments.

Currently exploring:

• Java
• Spring Boot
• Kafka
• Payment Systems
• JVM Performance
• Distributed Systems

Repository: https://github.com/zzzyoonnn/FinFlow-backend

FinFlow is a backend project that simulates core banking operations with a focus on transactional integrity, consistency, and auditability. It explores how financial systems safely process deposits, withdrawals, and transfers while maintaining reliable account state.

Designed and implemented essential banking operations including:

• Deposit
• Withdrawal
• Transfer
• Transaction history management

• Designed withdrawal and transfer workflows within a single transactional boundary, guaranteeing atomic state changes across account and transaction entities.
• Applied domain-driven design principles by encapsulating business rules and invariants within the domain model, preventing invalid account states.
• Centralized request logging and execution monitoring using Spring AOP, reducing duplication across service components.
• Implemented transaction ledger recording with balance snapshots, enabling full traceability of account state changes and financial audits.
• Structured transaction processing flows into validation, state mutation, persistence, and response stages to improve maintainability and clarity.
• Eliminated N+1 query problems using Fetch Join, reducing database round-trips and improving transaction history retrieval performance by 30%.
• Deployed the application on AWS EC2 with MySQL hosted on Amazon RDS, establishing a production-like environment for end-to-end testing.

Through this implementation, I became particularly interested in how financial systems prevent data inconsistency, race conditions, and double execution under concurrent requests.

👉 Core Banking Service Implementation: AccountService.java

• 계좌가 많아질 경우, 어떻게 설계하면 좋을까?🤔
• N+1은 왜 발생할까?🤔(Fetch Join 적용 전후 성능 비교)
• N+1은 왜 JPA에서 발생할까?🤔(JPQL)
• N+1은 메모리에 어떤 영향을 미칠까?🤔(JVM)
• 영속성 컨텍스트는 N+1과 어떤 관계가 있을까?🤔(Hibernate)
• Fetch Join은 영속성 컨텍스트에서 어떻게 동작할까?🤔
• Hibernate 내부에서 영속성 컨텍스트는 어떻게 동작할까?🤔
• N+1은 GC를 왜 자주 발생시킬까?🤔

Repository: https://github.com/zzzyoonnn/Servlet2Spring

This project reflects my growth in understanding backend architecture evolution and framework-driven design.

Started with a JSP-based architecture and progressively evolved it into Spring MVC and Spring Boot.

• Replaced manual request handling and dependency management in Servlet-based architecture with Spring MVC and dependency injection, improving maintainability and testability.
• Migrated authentication from session-based login to Spring Security with JWT, improving scalability and decoupling authentication from server-side state.
• Integrated Spring Security to centralize authentication and authorization logic, reducing security-related code duplication.
• Deployed the migrated application on AWS using EC2, RDS, and S3, enabling cloud-native application management and deployment.

This migration process taught me the importance of modernizing systems without compromising stability, especially in environments where reliability is critical.

Backend

• Java 21
• Spring Boot
• Spring Data JPA
• Spring Security (JWT)

Database

• MySQL
• H2 (for testing)

Infrastructure

• AWS (EC2, RDS, S3)

Build Tool

• Maven

• Core Banking Systems
• Transaction Processing
• Financial Data Integrity
• High-reliability Backend Systems

I aim to build backend systems where correctness and reliability are prioritized over feature speed — especially in domains where data integrity is critical.