Fly Io

Fly.io

Fly.io transforms containers into Firecracker MicroVMs running on bare metal across the globe. It is essentially a "CDN for your backend".

When to Use

• Global/Edge: You want your Node.js server running in Tokyo, London, and New York simultaneously.
• Postgres: Run a Postgres read-replica cluster globally with ease.
• Docker: You have a Dockerfile and want it running in 5 seconds.

Quick Start

fly launch
# Scans source code, creates Dockerfile, creates fly.toml

fly deploy

# fly.toml
app = "my-app"
primary_region = "iad"

[http_service]
  internal_port = 8080
  force_https = true

Core Concepts

Machines

Fly v2 runs on "Machines" (fast-booting VMs). They can wake-on-request (scale to zero).

Regions

Choose from 30+ regions. primary_region for Writes, others for Reads.

Fly Proxy

Magic routing layer. Routes your-app.fly.dev to the nearest healthy machine. Also handles internal DNS (app-name.internal).

Best Practices (2025)

Do:

• Use Machines API: For programmatic control (e.g., spinning up on-demand workers).
• Use Tigris: S3-compatible object storage integrated with Fly (since Fly has no native S3).
• Scale to Zero: Save money on dev environments by allowing machines to sleep.

Don't:

• Don't assume persistent IP: Machines move. Use the strict internal DNS [app].internal.

References

• Fly.io Documentation