January 2008 - California, United States.

Jan Koum, an engineer at Yahoo, applies for work at Facebook - rejected.

This was not the end - he moved on with his life.

He buys an iPhone in the subsequent year and immediately recognizes the huge potential of the new App Store.

He decides to build an instant messenger with some of his former coworkers from Yahoo. They named it WhatsApp. The vision behind WhatsApp was to replace the expensive SMS.

With 1 million people signing up each day, the growth rate of WhatsApp was mind-boggling.

WhatsApp was able to support 50 billion messages a day from 450 million daily active users with only 32 engineers.

Although explosive product growth is a good problem to have, Jan Koum and the team behind WhatsApp had to adopt the best engineering practices to overcome the challenges.

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