Explain OOP design and API rollout

This interview round focused on practical backend engineering rather than pure algorithms. Discuss both of the following.

1. Object-oriented design for new business logic. You are asked to implement a new piece of business logic in an existing backend service using object-oriented design. How would you identify the core domain objects, define their responsibilities, separate orchestration from business rules, and make the implementation testable and extensible?

2. Changing a public API safely. You need to modify an existing public API that clients already use. What release process would you follow to ship the change safely? Cover backward compatibility, versioning, testing, rollout, monitoring, and communication with clients.