Let’s design a distributed email service similar to Gmail that allows millions of users to send, receive, and organize messages asynchronously. In essence, this service acts as a massive digital post office: it accepts incoming mail, routes it to the correct recipient’s storage, and provides a user-friendly interface for reading, searching, and managing conversations. Key entities in the system include:

• User – An account holder identified by a unique email address. Each user has a profile, authentication credentials, and personalized settings (e.g., signature, theme, language).