Real-Time Systems

Persistent connections, presence, pub/sub fanout, geo-indexed dispatch: designs where seconds matter.

After this path you will be able to

Design systems requiring live updates — chat, tracking, collaborative tools — and articulate the trade-offs between WebSockets, SSE, and long-polling at each scale tier.

Interview approach for this path

1.Clarify the data direction first: is it server-to-client only (SSE is enough) or bidirectional (WebSocket)?
2.Pick the right protocol and explain why: WebSocket for chat, SSE for notifications, long-polling for legacy environments.
3.Address connection scale: millions of persistent connections require a dedicated connection tier, not your app servers.
4.Explain pub/sub fan-out: when a message arrives, how do you push it to all connected clients on different boxes?
5.Handle presence and heartbeats: how do you know a user is still connected, and how quickly do you detect disconnections?
6.Discuss geo-distribution: for global users, how do you minimize round-trip time to the WebSocket server?

Systems in this path

3 total

Concepts reinforced throughout

Long-Polling vs WebSockets vs SSE Consistent Hashing Message Queues

Up next

FAANG Interview Fast Track

The five systems that appear most in senior engineering loops — from a warm-up URL shortener to a full fan-out social feed and a payment system that cannot double-charge.

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