Design a Chat Application with Durable Conversation History

Design a chat application with persistent conversations and real-time messaging. Explain the client state, APIs, delivery path, and reconnect behavior. ### Constraints & Assumptions The source explicitly names chat-system design but does not specify group size, encryption, attachments, or delivery guarantees. State a bounded initial scope, such as text messages in small conversations, and identify how it would grow. ### Clarifying Questions What ordering is required within a conversation? Are edits and read receipts needed? How should offline clients catch up? What does sent versus delivered mean? ### What a Strong Answer Covers Durable messages, stable identities, authenticated membership, paginated history, real-time delivery, optimistic client state, and recovery from missed or duplicated events. ### Follow-up Questions What if the send response is lost after persistence? How do multiple devices converge? What changes for a very large group or attachments?