Design a chat application (like WhatsApp).

Features: 1:1 and group messages, online status, delivered/read receipts, history, offline delivery.
Clients keep WebSocket connections to chat servers; a session service maps user → server.
Send flow: message → server → persist (partitioned by conversation, ordered by time/ID) → route to the recipient's server via pub/sub → push; if offline, push notification and deliver on reconnect.
Message IDs per conversation for ordering and deduplication (client retries).
Groups: fan-out on write for small groups.
Presence with heartbeats and TTLs; media in object storage + CDN; mention end-to-end encryption.