HardDesign: Chat System (WhatsApp/Slack)

Alice is connected to Chat Server 1. Bob is connected to Chat Server 2. Alice sends Bob a message. How is it delivered?

Tests your understanding of this concept.

Answer Options

AAlice's server sends directly to Bob's server via HTTP
BMessage is stored in DB, Bob polls for new messages
CMessage is published to shared Redis Pub/Sub channel, Bob's server is subscribed
DA centralized router server handles all cross-server delivery

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This question is from the Design: Chat System (WhatsApp/Slack) topic (System Design Cases).

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