Case Study: Migrating a Legacy Monolith

1. The Question

"You've joined a travel-booking company as a senior engineer. One Rails monolith, twelve years old: search, booking, payments, reviews, support tooling, all of it. 200 engineers, deploys are twice-weekly and terrifying, and the CTO has announced a move to microservices. Design the migration."

This is the classic senior-loop question. It is part architecture, part risk management, part organizational judgment. Almost every company that has a monolith asks some version of it, and most companies have one.

A monolith is one deployable application that holds every feature