The Unusual Programming Language Behind WhatsApp

In 2014, Facebook shocked the tech world by acquiring WhatsApp for an astonishing $19 billion.

At the time, WhatsApp had around 450 million active users. Yet one statistic stood out more than the price tag itself: the company reportedly had only about 32 engineers.

That works out to roughly 14 million users per engineer.

Naturally, people wondered how such a small team could support one of the world’s fastest-growing communication platforms. The answer wasn’t a secret algorithm or an army of servers hidden somewhere in a data center.

One of the most important engineering decisions behind WhatsApp was its choice of a programming language that many software engineers have never used, and some have never even heard of.

That language was Erlang.

The Problem Started Long Before WhatsApp Existed

To understand why WhatsApp chose Erlang, we need to travel back to 1986.

The Swedish telecommunications company Ericsson faced a difficult challenge. Telephone networks were growing rapidly, and the software running those networks needed to support millions of simultaneous conversations.

The systems had to remain reliable, recover from failures quickly, and operate continuously without lengthy downtime. If a telephone switch crashed, thousands of conversations could be interrupted. Traditional programming approaches weren’t designed for problems at that scale.

So Joe Armstrong and his team at Ericsson created a new programming language called Erlang. It wasn’t designed to be fashionable or win programming language popularity contests. It was built for two specific goals:

• Concurrency (handling millions of things at once)
• Fault tolerance (never crashing)

Decades later, those same goals would become incredibly valuable for WhatsApp.

Why WhatsApp’s Problem Looked Like a Telecom Problem

When WhatsApp launched in 2009, its founders wanted something that felt instantaneous. Messages should arrive quickly. Users should remain connected. The application should continue working even as millions of people joined the platform.

At first glance, a messaging application seems very different from a telephone network. But from an engineering perspective, they’re surprisingly similar.

Both involve massive numbers of people maintaining independent communication sessions at the same time.

Every WhatsApp user represents an active connection. Every message represents communication flowing through that connection. As WhatsApp grew, it faced a challenge remarkably similar to the one Ericsson had solved decades earlier. The company needed software capable of handling enormous numbers of concurrent conversations reliably.

That’s exactly what Erlang had been built to do.

The Secret Weapon: The BEAM Virtual Machine

Erlang runs on a specialized virtual machine called BEAM. This is where things get interesting.

In many traditional systems, handling large numbers of simultaneous users can become expensive because each connection consumes heavy resources. Erlang approaches the problem differently. Instead of relying heavily on operating system threads, Erlang creates extremely lightweight processes managed by the BEAM virtual machine.

To put this in perspective:

• Standard OS Thread: ~1 Megabyte of memory.
• Erlang BEAM Process: ~2.6 Kilobytes of memory.

This allowed a single WhatsApp server to manage over 2 million concurrent activities without consuming excessive memory.

Imagine a traditional system using a small fleet of large, heavy delivery trucks. Each truck can carry a lot, but they’re expensive and clog up the road. Erlang takes a different approach. Instead of a few giant trucks, it deploys millions of tiny workers, each responsible for a small task.

The result is a system that can scale to huge numbers of independent activities simultaneously. For a messaging platform supporting millions of active users, that becomes a powerful advantage.

The Philosophy That Sounds Completely Wrong

One of Erlang’s most famous core philosophies is called:

“Let It Crash.”

At first, that sounds completely irresponsible. Most software engineers spend a significant amount of time trying to prevent failures. Erlang takes a different approach. Instead of assuming failures can be eliminated entirely, it assumes failures are inevitable.

The goal isn’t to stop every crash; the goal is to ensure that crashes remain isolated and recover quickly.

In Erlang, processes are completely independent. If one process encounters a problem, it doesn’t automatically bring down the rest of the system. Supervisors monitor those processes and can restart failed ones almost immediately.

Imagine a single user’s connection encounters an issue. Rather than affecting thousands or millions of other users, that specific process dies, a fresh one is instantly spun up in milliseconds, and the rest of the platform continues operating normally.

This philosophy helped make Erlang particularly attractive for systems where reliability matters more than perfection.

Of Course, Erlang Wasn’t the Whole Story

It would be inaccurate to say that Erlang alone made WhatsApp successful. Many other engineering decisions contributed to WhatsApp’s growth. The platform relied on efficient infrastructure, persistent connections, distributed systems, and thoughtful product design.

And once a user presses “Send,” an entirely different set of systems take over:

• Messages need to be encrypted.
• They need to be routed across the internet.
• They may need to be temporarily stored if a recipient is offline.
• Eventually, they must be delivered to the correct device. [1]

Erlang helped WhatsApp handle massive numbers of concurrent connections, but it was only one piece of a much larger system.

The Bigger Lesson

The story of WhatsApp and Erlang highlights an important lesson about engineering.

Great systems aren’t built by blindly chasing the newest technology. They’re built by understanding the problem deeply and choosing the right tool for it ,even if that tool was created decades ago.

A programming language created for telephone switches in the 1980s ended up helping power one of the largest communication platforms in the world. Not because it was trendy, and not because it was popular, but because it solved exactly the problem WhatsApp needed to solve.

And sometimes, that’s what great engineering looks like.

If you’re curious about what happens after you press Send on WhatsApp , from encryption to delivery to offline message handling, I created a visual walkthrough explaining the complete journey of a WhatsApp message on my channel. Check out the link below!

🎥 Watch the video: https://youtu.be/a9Ew3RkBmb4