Design a real-time messaging application with channels or conversations, message history, and message deletion. Focus especially on how deletion behaves across storage, caches, search, connected clients, and offline clients. ### Constraints & Assumptions - Messages have stable IDs and belong to one conversation. - Clients may reconnect after being offline and may receive duplicate events. - A deletion must preserve enough information for clients to converge without exposing deleted content. - The exact retention, legal, and authorization policy is not prescribed; ask before selecting hard versus soft deletion. ### Clarifying Questions to Ask - Who may delete a message, and is there a time limit? - Should replies, reactions, and attachments remain? - Is deletion reversible or auditable? - Must search removal be immediate? - What should an offline client display when it reconnects? ### What a Strong Answer Covers - Message IDs, conversation ordering, and pagination - An authorized, idempotent deletion transition - Tombstones or versioned delete events - Fan-out to online and offline clients - Cache, search, attachment, and replica cleanup - Race handling for edit, react, reply, and delete ### Follow-up Questions - Support deletion of an entire conversation. - Meet a strict erasure requirement across backups. - Preserve ordering during a regional failover.