Design One-to-One Messaging Across Multiple Devices

Design a messenger that supports one-to-one conversations. Messages must be stored on the server so users can retrieve their conversation history. ### Part 1 — Message storage and delivery Describe the send path, persistent message model, and delivery to a recipient who is currently online. Explain how an offline recipient is notified through APNs or Google Firebase Cloud Messaging. #### What This Part Should Cover - A durable acceptance point, stable message identity, and retry behavior. - The relationship between online connections, presence information, and mobile push notifications. - The distinction between storing a message, notifying a device, and knowing that a user has read it. ### Part 2 — Multiple devices and history Users can be logged in on several devices at once. Explain how messages synchronize across those devices and how a user pages through older server-stored messages. #### What This Part Should Cover - Device-specific connections and synchronization progress, including reconnects and missed messages. - A stable conversation ordering and cursor-based history pagination during concurrent new sends. - Authorization checks that restrict every delivery and history request to conversation participants. ### What a Strong Answer Covers - A consistent account of message identity and ordering across storage, retries, live delivery, push, and history. - Recovery when connection or notification state is stale, rather than treating presence as proof of delivery. - Explicit assumptions about scale and optional features; group chat and invented latency targets are not required. ### Follow-up Questions - What happens if a message is stored successfully but the sender times out before receiving the response? - How does a device recover a gap if it receives a newer message before an older one?