Backend
Sharing backend development troubleshooting and performance optimization experiences.
APIs Slowing Down After Deployment: Improving JVM Cold Start Problem by 85%
Introduction Recently, our team migrated from a Docker Compose-based production environment to Kubernetes. It was a choice to properly utilize features needed in an MSA environment, such as scalability, automation, and zero-downtime deployment. Related article: From No Dev Server to GitOps: My Journ...
November 18, 2025
Changed MySQL Column Type from bigint to varchar and It Got 8x Slower...! : Procedure Performance Degradation Troubleshooting
Introduction Hello. I’m Jeongil Jeong, a 3rd-year backend developer working at a proptech platform. Today, I’d like to share an interesting performance issue that occurred in our service and the resolution process. While solving a problem where a MySQL procedure suddenly became 8 times slower, I enc...
October 20, 2025
Money Disappeared? Transitioning Account Balance Verification Batch from Tasklet to Chunk & Partitioning
Hello. I’m Jeong Jeongil, a 2-year backend developer working at a lending and investment platform. While internally verifying data consistency in our operational company service, we experienced account balance and internal Point inconsistencies due to transaction processing time and batch work ineff...
December 24, 2024
External API Performance Improvement: Reducing Response Time from 5100ms to 57ms with Look-aside Cache
Hello. I’m a 2-year backend developer currently working at a lending and investment platform. In this post, I’d like to share my experience of improving external API performance using the Look-aside cache pattern. The problems I faced during this process were not simple. Various issues were intertwi...
December 16, 2024
Resolving DB Replication Lag
I’m sharing about a replication lag issue I encountered at work. Problem Occurrence Our company uses database replication to distribute load and improve performance. The system is configured as Active-Active redundancy in a Master-Slave structure, and through ProxySQL, SELECT statements are sent to ...
November 15, 2024
[Troubleshooting] QueryDSL StackOverflowError Investigation
I’m sharing about a Querydsl StackOverflow issue I encountered at work. Problem Occurrence As always, the problem starts with a Slack message. As soon as I received the message, I couldn’t help but react with “huh?” StackOverflow? Was there recursively implemented code in the server? What is StackOv...
March 15, 2024
[Troubleshooting] Failed Graceful Shutdown During Kubernetes Pod Scale In, Journey to Find Lost Async Logic
I’m sharing about a failed Graceful Shutdown issue I encountered at work. Problem Occurrence Usually, I encounter exceptions and problem situations through Slack quite often. Today, I encountered a problem differently than usual. This problem started with a back office user’s inquiry. “I think one p...
February 9, 2024
[Troubleshooting] Journey to Resolving HikariCP Deadlock When Using JPA and MyBatis Together (feat. OSIV)
I’m sharing an issue related to HikariCP Deadlock that I encountered at work. Problem Occurrence As always, the issue starts with a Slack message. Excerpt from Stack trace Caused by: org.apache.ibatis.exceptions.PersistenceException: ###Error querying database. Cause: org.springframework.jdbc.Cannot...
January 9, 2024
Spring Batch Job Concurrent Registration Deadlock Issue (Deadlock accessing creating a job on sqlserver when multiple jobs start at once)
I’d like to share a Spring Batch-related issue I encountered at work. Problem Occurrence The issue started with an error message in Slack. It was a stack trace indicating that a Deadlock had occurred on our company’s Batch server. Naturally, since Deadlock could severely impact the service, I immedi...
November 20, 2023
Creating a Common Excel Download Utility Class Using Java POI
It’s nothing grand, but I’d like to share an experience I had at work a few months ago. It started with inquiries from end users, namely my coworkers, who use the back-office system internally. The inquiry was as follows: Excel download doesn’t work or becomes strange when the row count exceeds a ce...
November 19, 2023