Backend Survival Path
Build a CS base, then follow Linux and deployment practice.
18 posts2 steps
Notes on real problems from backend development and operations.
Java · Spring · Database · Infra · AI tools
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Recent Posts
Spring Transactions and Concurrency: When Two Users Edit at Once, Whose Data Survives?
A practical guide to Spring transactions and concurrency control. It covers common @Transactional mistakes, the Lost Update problem, the difference between optimistic locking (@Version) and pessimistic locking (FOR UPDATE), and how to prevent deadlocks.
A Login Recorded Under the Wrong User: The Ghost Left in ThreadLocal
While we were adding an audit log, one login showed up under the wrong name: B logged in, but the record said A. This postmortem traces the ghost to a SecurityContext left behind in ThreadLocal by a filter that never called clearContext() on reused Tomcat threads.
WITH RECURSIVE Caught the Culprit: An Incident Closed Without Changing a Line of Code
Deleting certain spaces kept failing, yet the code was innocent. A WITH RECURSIVE query swept a tree of unknown depth, found a six-year-old legacy row that broke the delete logic’s assumption, and closed the case with zero deploys.
There’s No Stock Column: Inventory Computed in SQL on Every Read
In this inventory system, the most important number, remaining stock, isn’t stored in any table. Here’s a dissection of the SQL that computes it on every read, and the store-versus-compute trade-off behind that design.
The 15-Second API: The JPA N+1 Problem Sends Its Bill
A list API took 15 seconds, and the debug log showed thousands of SQL statements. The JPA N+1 problem was only part of the bill: a bag restriction, Cartesian products, and an unpaged full load came with it.
When Did I Last Write SQL by Hand? The Day grep Overturned an AI Refactoring Plan
An AI proposed unifying our data access layer on JPA before we could add an audit log. A few lines of grep showed the premise was wrong: every write already went through JPA. The same investigation showed I could no longer read our SQL, so I started relearning it.