Imagine you're building the backend for a popular e-commerce platform. A key feature is displaying recently viewed listings to users, allowing them to easily pick up where they left off. This seemingly simple feature has significant implications for user engagement and sales. A poorly implemented "Recently Viewed" service can lead to frustrated users, lost sales, and performance bottlenecks.

This problem focuses on designing a robust and scalable in-memory service to manage recently viewed listings for users. We need to consider how to efficiently store and retrieve this data while handling concurrent user requests. A straightforward approach might use simple lists, but that quickly becomes inefficient with a large user base.

Your task is to design a solution that addresses these challenges using sound object-oriented principles and design patterns. We need a design that's not only efficient but also easy to maintain and extend as the platform evolves.

Requirements