The Bridge to the New Town - A Legacy System Migration Pattern

Imagine you are the person responsible for the application portfolio of a large insurance company. For their business the company runs one monolithic system for most of its product divisions1. This system is 25 years old, written in Assembler and sits on an IMS/DB database. This is the old town. It’s at the end of it’s potential growth and people want to move out of it to the other side of the river. Let’s call this old system (or town) CLS for complete legacy system. You want to replace this system with newer systems, a new town, in a step by step