Eight native clients and an Erlang backend

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How did a tiny team of 30 engineers build the world-famous messaging app more than a decade ago, and what can dev teams learn from that feat today? Jean Lee was engineer #19 at WhatsApp, joining when the company was still small, with almost no formal processes. She helped it scale to hundreds of millions of users, went through the $19B acquisition by Facebook, and later worked at Meta.

In this episode of Pragmatic Engineer, I talk with Jean about what it was like building WhatsApp. When Facebook bought WhatsApp in 2014, only around 30 engineers supported hundreds of millions of users across eight platforms.

We discuss how the founders kept things simple, saying “no” to most feature requests for years. Jean explains why WhatsApp chose Erlang for the backend, why the team avoided cross-platform abstractions, and how charging users $1 per year paid everyone’s salaries, while keeping growth intentionally slow.

Jean also shares what the Facebook acquisition was like on the inside, how she dealt with sudden personal wealth, and what it was like transitioning from an IC to a manager at Facebook – including the reality of calibration meetings and performance reviews.

We also discuss how AI enables smaller engineering teams, and why WhatsApp’s experience suggests ownership and trust might matter more than tools.

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Timestamps

(00:00) Intro

(01:39) Early years in tech

(06:18) Becoming engineer #19 at WhatsApp

(13:53) WhatsApp’s tech stack

(18:09) WhatsApp’s unique ways of working

(25:27) Countdown displays and outages

(27:07) Why WhatsApp won

(28:53) The Facebook acquisition

(33:13) Life after acquisition

(39:27) Working at Facebook in London

(44:07) Transitioning to management

(47:27) Performance reviews as a manager

(53:29) After Facebook

(58:53) AI’s impact on engineering

(1:02:34) Jean’s advice to new grads and startups

(1:06:45) Empowering employees

(1:08:17) Book recommendations

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The Pragmatic Engineer deepdives relevant for this episode:

• How Meta built Threads

• How Big Tech runs tech projects and the curious absence of Scrum

• Performance calibrations at tech companies

• Software engineers leading projects

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