Design a Real-Time Chat Service for One-to-One and Group Messaging

Design a real-time messaging service for a social network's chat product. Users exchange text messages in one-to-one conversations and in group conversations. Recipients who are online receive messages immediately, recipients who are offline receive them when they reconnect, and a user's conversation history is available on every device they use. ```hint Find the recipient's connection Think about how the server locates the live connection of a recipient who may be attached to any one of many gateway machines, possibly from several devices. ``` ```hint Decide who sets the order Decide what "in order" must mean for a conversation and which single component is allowed to assign that order. ``` ### Clarifying Questions - How many daily active users are there, how many messages are sent per day, and what is the largest group size? - Which features are in scope: delivery and read receipts, typing indicators, presence, media attachments, end-to-end encryption? - Must every one of a user's devices receive every message and share the full history? - How long is message history retained? - Should offline users receive push notifications? ### What a Strong Answer Covers - Persistent client connections and a routing layer from user and device to gateway - Per-conversation ordering and idempotent sends that survive retries - A storage model for conversations and messages optimized for appends and recent-history reads - Offline delivery and multi-device sync on reconnect - Group fan-out and how very large groups change the design - Receipts and presence without overwhelming the system - Failure handling for dropped connections, gateway crashes, and duplicate sends ### Follow-up Questions - How do you guarantee that no message is lost or duplicated when a client's connection drops in the middle of a send? - How would end-to-end encryption change the server design, especially for multiple devices and groups? - A group grows very large. What changes in fan-out, receipts, and notifications? - How would you implement "seen by" for group messages efficiently?