Troubleshooting
Real-world problem-solving experiences from production environments.
What Happens When You Skip DB Normalization — A Firsthand Experience
Everyone knows database normalization is important, but in practice, it seems surprisingly easy to overlook. There are plenty of moments where denormalization wins out for reasons like “one fewer JOIN means better performance” or “it’s just easier to store it as text.” I joined my current team in Ma...
February 3, 2026
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
DB Replication Lag: How to Solve It
When developing in an environment that uses database Replication, there’s a problem you’ll inevitably encounter at some point. It’s the replication lag problem. I also faced this issue at my company, and went through various considerations during the resolution process. In this article, I want to sh...
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
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: java.sql.SQLTransientConnectionException: HikariPool-1 - Connection is not available, request timed out after 30008ms...
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