Fly Machines: an API for fast-booting VMs

Summary

Fly Machines are virtual machines exposed through a fast REST API, with boot times of about 300ms, intended to support cost-effective standby capacity and scale-to-zero applications. Creation is deliberately separated from starting: it performs a centralized database preflight, reserves host capacity through NATS, records the machine, and prepares the OCI image locally. A later start request goes to the nearest regional API, which sends a direct message to the known host, avoiding database coordination and image pulls; examples cite roughly 10ms locally and 150ms across regions. Processes can exit when idle to leave machines intact for clean restarts, while fly-proxy can start them for incoming HTTP requests. The design trades simplicity for hardware pinning, so capacity or host failures may require fallback machines and redundancy; stopped machines currently incur image-storage charges, such as about $0.15 per month for a 1GB Docker image.

Takeaways

1. 1

   Machine creation is relatively slow because it performs centralized consistency and billing checks, reserves capacity through NATS, and coordinates host selection.

2. 2

   Starting a prepared machine avoids database and image-pull latency; the source gives approximate start-message times of 10ms within a region and 150ms between Los Angeles and São Paulo.

3. 3

   Hardware pinning simplifies startup but introduces host and capacity failure modes, so applications should use fallback locations or redundant machines.