System Design Patterns: From Fundamentals to Real Systems
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Bidirectional Streaming

1. The Incident

Your team ships chat, using WebSockets: both sides sending live. The demo is flawless. Staging is flawless. You launch.

Production runs three gateway servers behind a load balancer. Alice connects and is assigned to server A. Bob, in the same chat room, is assigned to server B. Alice sends "hey." Server A looks at its list of connections for that room, finds only Alice, and delivers her message to everyone on that list. Which is nobody. Bob sees nothing.

The bug reports are confusing, because there are no errors

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