How a Midnight Migration Tested a Programmer’s Problem‑Solving Skills

Problem‑Solving Ability

A programmer is defined by the ability to solve problems, especially under production pressure, where calm analysis and decisive action separate great engineers from those who merely understand code.

Late‑Night Technical Story

The team faced a migration from an old Oracle‑based monolithic platform handling billions of daily transactions to a new micro‑service architecture built on MySQL HA. The migration had to occur without disrupting merchants, likened to changing a car’s wheels while it’s moving.

Migration Plan and Unexpected Policy

A policy rollout forced an accelerated migration of all remaining merchants on New Year’s Eve. Departments coordinated communications, testing, and deployment, aiming to move millions of merchants within a few hours.

Midnight Crisis

During the migration, progress stalled at 10 000 merchants per half hour, threatening a multi‑day delay and severe business impact. The team quickly identified that test data based on small merchants underestimated the load of large core agents.

Root‑Cause Analysis

Logs revealed the migration program processed agents sequentially without multithreading, and shared global variables caused thread‑safety issues when parallel requests were attempted.

Ad‑Hoc Multithreading Solution

Developers opened multiple browser pages, each triggering a migration for a different agent, effectively achieving concurrent execution. They fixed the thread‑safety bug by wrapping shared state in ThreadLocal, allowing each thread its own copy.

Scaling the Fix

To handle the load, the migration UI was deployed on ten servers. Operators launched concurrent migrations from each server, grouping agents in batches of fifteen, and completed the full migration within two hours.

Aftermath and Lessons

The incident demonstrated that simple technical tools—multithreading, proper logging, and systematic analysis—can resolve critical production emergencies. It reinforced that continuous practice, real‑world problem solving, and post‑mortem documentation are essential for becoming a capable programmer.