Chat System Interview Questions

I would ask 1:1 vs groups, receipts, media, multi-device, and encryption up front. Mobile clients keep a long-lived connection to a gateway. The gateway maps userId to connection. When I send a message, it hits a chat service, gets persisted keyed by conversation and timestamp, then the gateway pushes to the other person. If they are offline, we store and forward. Small groups can fan-out on write. Huge groups should fan-out on read or I will melt the writer. Media does not live in the chat DB — object storage plus CDN, chat only stores the URL. Presence is Redis with TTLs. If they want WhatsApp-style E2E encryption, the server stores ciphertext and cannot read messages. I would also mention idempotent message ids so retries do not double-send, and unread counters as a separate small service.

I persist the message first, then try push. If the device is offline I keep it in the inbox store keyed by user and sync on connect. I would not only use in-memory pub/sub. Fan-out on write for small group chats, fan-out on read for huge groups or channels. Acknowledgements give at-least-once delivery; clients dedupe by message id. I mention push notifications as a separate channel, not as the source of truth.

1:1 I can fan-out on write to two inboxes. A 10,000 member group I should not write 10,000 copies on every message if activity is high. I store the message once in a group log and let clients pull new offsets, or I fan-out only to online members and let others catch up. Presence and typing indicators must be ephemeral and sampled. I would not reuse the 1:1 table design blindly for huge rooms.

I would not trust client clocks. The server assigns a monotonic sequence per conversation, or I use a hybrid logical clock. Clients can show optimistic UI, then reconcile. If two messages race, the conversation id plus sequence breaks ties. I mention that 'last write wins' on an edited message needs an edit version. WhatsApp-style ticks are client UX on top of server order, not a substitute for it.

Images and video go to object storage. The chat message stores a URL and metadata, not the blob. Upload is a separate API, often with a pre-signed URL. The websocket path stays small. I would CDN the media. I would not send 20 MB through the same channel as 'typing...'. Virus scan and size limits belong on the upload path.