How WhatsApp Implements Real-Time “Typing…” Updates at Scale
A Classic System Design Interview Question
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Hello guys, If you’ve ever built a chat app—or sat through a system design interview—you’ve probably been asked this deceptively simple question:
“How does WhatsApp show ‘Typing…’ instantly?”
Does the app refresh every second?
Is it polling the server continuously?
Or is there some clever trick behind the scenes?
The short answer: no constant refreshing, no polling loop.
The real answer is much more interesting—and interviewers love it.
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The Naive Answer (That Most Candidates Give)
A lot of people instinctively say something like this:
