Firecracker microVM

A virtual machine is a computer program that outwardly behaves like a complete, independent computer. Dozens or hundreds of such virtual machines can run simultaneously on a single physical server without knowing about each other. Firecracker microVM is a particularly lean variant of this, developed by Amazon and released as an open-source project in 2018. It starts in under 125 milliseconds — faster than the blink of an eye — while using so little memory that thousands of them can run simultaneously on a single server. The trick: Firecracker leaves out everything that makes a classic virtual machine sluggish and heavy.

Why Firecracker is indispensable for cloud services

Many modern cloud services work according to a principle called serverless computing: the user uploads a small program, and the cloud executes it exactly when it’s needed — for milliseconds or seconds, then it’s done. AWS Lambda is the best-known service of this kind. The problem: every execution of a foreign program on a shared server is a security risk. What if the code is malicious or crashes?

Classic virtual machines solve the problem with isolation — but they take seconds to start and consume gigabytes of memory. Containers, a lighter alternative, start quickly but offer weaker separation between running programs. Firecracker strikes a middle ground: the strong isolation of a full-fledged virtual machine, combined with the startup speed and low resource consumption of a container. For providers like Amazon, this is the difference between an economically viable and an unprofitable service.

How Firecracker becomes so lean

Conventional virtual machines simulate complete hardware: graphics cards, USB ports, sound cards — most of which a server-based program never needs. Firecracker simulates only a handful of essential components: processor, memory, network card, and storage. Nothing more. This drastically reduces the attack surface — that is, the points where an attacker could break into the system.

Firecracker runs directly on the host server’s Linux kernel and uses a built-in feature called KVM (Kernel-based Virtual Machine), which causes the server’s processor to execute a virtual machine’s code as if it were real hardware code. This makes Firecracker fast. The program itself is written in the programming language Rust, known for its memory safety — another safeguard against bugs that could open security vulnerabilities.

Every Firecracker instance also runs with what’s called a Jailer — a guardian process that forbids it from accessing parts of the operating system that are none of its business. This means a microVM cannot cause harm even if the code inside it is malicious.

Firecracker in everyday life: invisible, but omnipresent

Anyone using AWS Lambda — for example, to automatically resize images when someone uploads a photo — is using Firecracker without knowing it. AWS Fargate, a service for running containers in the cloud, also relies on it. Firecracker is completely hidden within these products; you only see the result.

Firecracker comes up in the tech press whenever the infrastructure behind AI services is discussed. Models like Amazon’s own AI assistants or third-party providers running their language models via AWS need exactly the properties Firecracker delivers: thousands of parallel, isolated execution environments that are instantly available and disappear after use. Other cloud providers have since developed similar technologies as well — but Firecracker is considered the reference point for the entire field.

Because Firecracker is freely available as an open-source project, smaller companies also build their own infrastructure on top of it. The provider Fly.io, for instance, runs its entire global app-execution service on Firecracker. This is unusual: a technology built for one of the world’s largest corporations is simultaneously freely usable by anyone.