From Copilot's autocomplete to pasting code into ChatGPT to coding agents that run in the terminal. A quick tour of four years of tools, and a record of the day I first brought an agent into the Kotlin backend I'd been writing alone for almost a year. Its first commit wasn't a feature but a fix for broken test infrastructure, and the same day the PR workflow and PR CI began. Also: why the hand-written setup from March changed nothing.

When a scheduled batch job ended up living inside the web application module, it was clear the structure was wrong. This is the 313-file change that moved entities, repositories and QueryDSL Q-classes out of the API modules into domain modules, taking our Gradle build from 4 modules to 6: why now, how I handled scanBasePackages, kapt and test utilities, and what's still left undone.

After 191 days, I dropped the locked_yn column that had been there since the first commit to handle the month-end close for orders, sales, deposits and stock reports. No production code had ever set it to true, while a date rule next to it changed three times and did the actual locking. Here's the choice between a flag that stores state and a rule computed from dates, and what each costs.

Every sale needed a transaction statement PDF that looks exactly like the paper form. Here's how I built it with Apache PDFBox 3: embedding a Korean font, drawing a table with merged cells by hand, paginating up front at 18 rows on the first page and 25 after, and compositing a company stamp image. Plus the test that broke when I added that one stamp.

Sixteen days into a new ERP backend, the status enum had changed three times. Here's why I picked a status column over hard deletes, deleted_at and Hibernate's @SQLRestriction, why I renamed the "delete" API so its name admits it doesn't delete anything, and the uniqueness and query-filter problems I took on in exchange.

Notes from Daangn Tech Meetup 2024: the early engineering strategy that let Daangn Alba move fast with event-based analytics, how the chat system turns into a message queue as servers are added, an Istio network outage hunt that ended at CPU limits, and what came up in the networking sessions with the Daangn Pay Money Service and Account Service teams.

Why I picked the Airflow we already had over Spring Batch, Spark or AWS Batch, and how I installed Apache Airflow 2.10 into a Conda environment on macOS with a constraint file, added the MySQL and AWS providers, and got the webserver and scheduler running, with the reason behind each step.

Notes from the server sessions at SLASH 24, Toss's engineering conference: safely migrating a legacy ledger with replay verification (Read/Write Verifiers), building foreign-currency core banking on MySQL and MSA instead of Oracle, handling currency exchange with the Saga pattern and compensating transactions, plus the keywords I took away from the kernel observability and MyData circuit breaker sessions.

To consolidate JWT verification scattered across several Spring Boot servers, I compared sending every request through the Auth server with verifying tokens at the API Gateway. Why I chose the Gateway, and why I wrote a custom plugin instead of using Kong's bundled JWT plugin, which returns 401 when there's no token.

Baking a JWT custom plugin written in Lua into the Kong image with a Dockerfile, attaching it to a Route in Konga, and setting up the Consumer's JWT credential in Kong Manager, along with which phase of the request the plugin runs in.