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 Microservices & APIs: Deploying full-stack applications and databases physically close to users.
• Modern Full-Stack Frameworks: Next.js, Remix, Phoenix LiveView, Rails, and Node.js with instant global scaling.
• LiteFS Distributed SQLite: Replicating SQLite databases globally with low-latency local reads and streaming primary writes.
• GPU Inference at the Edge: Running open-weight AI models on edge GPUs with rapid cold starts.

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

Declarative Configuration with fly.toml

Defining machine resources, scaling rules, and health checks:

# fly.toml
app = "api-edge-service"
primary_region = "ord" # Chicago primary

[build]
  dockerfile = "Dockerfile"

[http_service]
  internal_port = 8080
  force_https = true
  auto_stop_machines = 'stop'   # Scale to zero when idle
  auto_start_machines = true
  min_machines_running = 1

  [http_service.concurrency]
    type = "requests"
    soft_limit = 200
    hard_limit = 250

[[vm]]
  size = "shared-cpu-2x"
  memory = "2048mb"

[[services.http_checks]]
  interval = "10s"
  timeout = "2s"
  grace_period = "5s"
  method = "get"
  path = "/healthz"

Global SQLite Replication with LiteFS

Configuring distributed SQLite clusters with automated failover:

# etc/litefs.yml
fuse:
  dir: "/litefs"

data:
  dir: "/var/lib/litefs"

exit-on-error: false

proxy:
  addr: ":8080"
  target: "localhost:8081"
  db: "production.db"

lease:
  type: "consul"
  advertise-url: "http://${FLY_ALLOC_ID}.vm.${FLY_APP_NAME}.internal:20202"
  candidate: ${FLY_REGION == "ord"} # Only Chicago is eligible for primary