When you type a message, scroll through your news feed, or watch a video on Facebook, it is easy to take for granted the complex infrastructure working behind the scenes. Understanding what Facebook is written in provides a window into the massive engineering effort required to power a platform serving billions of people daily. The technology stack is a carefully curated blend of languages and tools, chosen not for trends, but for specific needs around performance, reliability, and developer velocity.
The Core Backend: Powering the Social Graph
At the heart of Facebook’s infrastructure lies its backend, responsible for managing the social graph, handling user authentication, and serving content. For many years, PHP formed the backbone of this system, largely because Facebook was founded on this language and built a significant portion of its early infrastructure with it. To overcome the performance limitations inherent in PHP, Facebook developed HipHop Virtual Machine (HHVM), an innovative runtime that compiles PHP and Hack code into highly efficient machine code. This move was instrumental in allowing the platform to scale efficiently without abandoning the rapid development cycle offered by dynamic languages.
Hack: A Statically Typed Partner to PHP
While PHP remains a foundational element, Facebook has strategically augmented its backend with Hack, a language created specifically for the company. Hack runs on the HHVM and introduces static typing to the traditionally dynamic PHP environment. This combination provides the best of both worlds: the flexibility of PHP for rapid iteration and the safety, maintainability, and performance optimizations of static types. For complex backend services and large-scale infrastructure, Hack has become the preferred language, enabling developers to catch errors early and refactor code with confidence as the codebase continues to grow.
Frontend Engineering: Building the User Interface
The user interface you interact with in your browser or mobile app represents a different set of technological choices. On the web, Facebook heavily relies on JavaScript to create dynamic, single-page applications that feel responsive and fast. The company has developed and open-sourced several powerful frontend frameworks, most notably React. React’s component-based architecture allows for building reusable UI elements, which streamlines development and ensures a consistent experience across the entire platform. On mobile, native development dominates, with iOS applications typically written in Swift and Android applications built using Kotlin.
The Role of Relay and GraphQL
Managing the data flow between the frontend and the massive backend infrastructure is a critical challenge. To address this, Facebook created Relay and the GraphQL query language. Relay is a JavaScript framework for building data-driven React applications, while GraphQL is a query language that allows the frontend to request exactly the data it needs in a single, efficient request. This architecture reduces over-fetching and under-fetching of data, leading to improved performance and a more predictable development process for complex UIs.
Infrastructure and Data: The Hidden Backbone
None of the user-facing features would be possible without the robust infrastructure supporting them. Facebook’s data centers are a marvel of engineering, running a custom version of Linux optimized for their specific workloads. To handle the immense volume of data generated every second, the platform relies on a distributed storage system known as Tao. Tao functions as a massive, highly available key-value store, specifically designed to handle the social graph and serve content like photos and videos with remarkable speed and efficiency.
Programming Languages in Infrastructure
Lower-level infrastructure components, such as the custom web servers and network libraries, are often written in languages that offer maximum performance and control over system resources. C++ plays a vital role in these areas, powering critical components that require direct hardware interaction and minimal latency. Python is also heavily utilized for automation, scripting, and operational tasks, acting as the glue that helps manage and orchestrate the vast array of systems keeping Facebook running smoothly.
