Fly.io

Fly.io runs your Docker containers on hardware close to your users. Machines boot in ~300 ms, scale to zero when idle, and deploy globally with a single command.

Prerequisites

# Install the Fly CLI
curl -L https://fly.io/install.sh | sh
 
# Sign up / login
fly auth login

First deploy

From your project root (with a Dockerfile):

fly launch

Fly auto-detects the Dockerfile, picks a region, and generates fly.toml. Accept the defaults or tweak interactively.

Then deploy:

fly deploy

Your app is live at https://<app-name>.fly.dev.

fly.toml (reference)

app = "my-ultimo-app"
primary_region = "iad"
 
[build]
  # Uses Dockerfile by default
 
[env]
  PORT = "3000"
  RUST_LOG = "info"
 
[http_service]
  internal_port = 3000
  force_https = true
  auto_stop_machines = "stop"
  auto_start_machines = true
  min_machines_running = 0
 
  [http_service.concurrency]
    type = "connections"
    hard_limit = 500
    soft_limit = 250
 
  [[http_service.checks]]
    interval = "10s"
    timeout = "2s"
    grace_period = "5s"
    method = "GET"
    path = "/health"
 
[[vm]]
  memory = "256mb"
  cpu_kind = "shared"
  cpus = 1

Scaling

Multiple regions

# Add machines in London and Tokyo
fly scale count 2 --region lhr
fly scale count 2 --region nrt

Vertical scaling

fly scale vm shared-cpu-2x --memory 512

Auto-scaling

The auto_stop_machines / auto_start_machines settings in fly.toml enable scale-to-zero: machines stop when idle and start on the next request (~300 ms cold start for Rust binaries).

Secrets

# Set secrets (encrypted, never visible in logs)
fly secrets set DATABASE_URL="postgres://..." JWT_SECRET="..."
 
# List secrets
fly secrets list

Custom domain

fly certs add your-domain.com

Then point a CNAME to <app-name>.fly.dev (or an A record to the Fly IP).

Volumes (persistent storage)

fly volumes create data --size 1 --region iad

In fly.toml:

[mounts]
  source = "data"
  destination = "/data"

Database

Fly Postgres

fly postgres create --name my-db
fly postgres attach my-db

This sets DATABASE_URL as a secret automatically.

External databases

Set the connection string as a secret:

fly secrets set DATABASE_URL="postgres://user:pass@host:5432/db?sslmode=require"

Monitoring

# Live logs
fly logs
 
# Machine status
fly status
 
# Metrics dashboard
fly dashboard

CI/CD with GitHub Actions

# .github/workflows/fly-deploy.yml
name: Deploy to Fly.io
 
on:
  push:
    branches: [main]
 
jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: superfly/flyctl-actions/setup-flyctl@master
      - run: flyctl deploy --remote-only
        env:
          FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN }}

Generate the token: fly tokens create deploy -x 999999h

WebSocket support

Fly.io natively supports WebSocket connections. No extra configuration needed — requests to your app on the same port are automatically upgraded when the client sends an Upgrade: websocket header.

For long-lived WebSocket connections, consider:

# Increase the idle timeout (default 60s)
[http_service]
  idle_timeout = "300s"