Fly.io is a public cloud for developers, run by Fly.io, Inc. since 2017. Its basic unit is the Fly Machine, a hardware-virtualized container: a software container (an app packaged with everything it needs) that runs inside its own small virtual machine, isolated by KVM, the virtualization layer built into Linux.
A developer deploys an app from GitHub or with the fly launch command of the Fly.io CLI. When the app has no Dockerfile, the recipe for building a container image, Fly.io generates one for common frameworks such as Rails, Django, Node, Laravel, Go and Rust. Machines boot in under one second and can run only when needed. Apps run in more than 18 regions on Fly.io's own hardware, with private networking and autoscaling. Data can live in Managed Postgres, in Redis from Upstash, in Tigris object storage, or on volumes attached to Machines. Sprites are separate isolated sandboxes for running untrusted or AI-generated code.
It is a hosted service priced per use: compute is billed by the second while a Machine runs, and storage by the capacity provisioned.