System Design Patterns: From Fundamentals to Real Systems

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Bidirectional Streaming

1. The Incident

Your team ships chat. WebSockets, obviously: the walkie-talkie from the module intro, both sides talking live. The demo is flawless. Staging is flawless. You launch.

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

The bug reports are surreal, because there are no errors

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