Fly Deployment

Fly.io Deployment Knowledge

Overview

Comprehensive guide for deploying applications to Fly.io, covering configuration, Dockerfiles, volumes, and common pitfalls. Based on official documentation and real deployment experience.

Sources

• Fly.io Configuration Reference
• Fly Volumes Overview
• Fly.io Monorepo Deployments
• Vercel's Official Next.js Dockerfile

Critical Limitations

Volumes

A Machine can only mount ONE volume. This is the most common gotcha.

# WRONG - Will fail with "only 1 volume supported"
[[mounts]]
  source = "data"
  destination = "/data"

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

# CORRECT - Single volume with subdirectories
[[mounts]]
  source = "app_data"
  destination = "/data"

# Then use /data/repos, /data/db, etc. in your app

Other volume constraints:

• Volumes are pinned to specific physical hosts
• Cannot shrink volumes, only extend
• Not available during Docker build or release_command
• One-to-one mapping: 1 volume = 1 machine

Build Context

When using fly deploy --config path/to/fly.toml:

• The dockerfile path is relative to where fly.toml is located
• The build context is the current working directory

For monorepos, place fly.toml files in the project root with dockerfile paths like apps/api/Dockerfile.

fly.toml Configuration

Minimal Working Example

app = "my-app"
primary_region = "dfw"

[build]
  dockerfile = "Dockerfile"

[env]
  PORT = "8080"

[http_service]
  internal_port = 8080
  force_https = true
  auto_stop_machines = true
  auto_start_machines = true
  min_machines_running = 0

[[vm]]
  memory = "512mb"
  cpu_kind = "shared"
  cpus = 1

With Volume (for stateful apps)

app = "my-api"
primary_region = "dfw"

[build]
  dockerfile = "apps/api/Dockerfile"

[env]
  DB_PATH = "/data/app.db"
  PORT = "8080"

[http_service]
  internal_port = 8080
  force_https = true
  auto_stop_machines = false  # Keep running for API
  auto_start_machines = true
  min_machines_running = 1

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

[[vm]]
  memory = "1gb"
  cpu_kind = "shared"
  cpus = 1

With Build Args (for Next.js NEXT_PUBLIC_* vars)

[build]
  dockerfile = "apps/web/Dockerfile"
  [build.args]
    NEXT_PUBLIC_API_URL = "https://my-api.fly.dev"

[env]
  PORT = "3000"
  NODE_ENV = "production"
  NEXT_PUBLIC_API_URL = "https://my-api.fly.dev"

Dockerfile Patterns

Next.js Standalone (Official Pattern)

FROM node:20-alpine AS base

# Dependencies
FROM base AS deps
RUN apk add --no-cache libc6-compat
WORKDIR /app
COPY package.json package-lock.json* ./
RUN npm ci

# Builder
FROM base AS builder
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY . .

ARG NEXT_PUBLIC_API_URL
ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL

RUN npm run build

# Runner
FROM base AS runner
WORKDIR /app

ENV NODE_ENV=production
ENV NEXT_TELEMETRY_DISABLED=1

RUN addgroup --system --gid 1001 nodejs
RUN adduser --system --uid 1001 nextjs

COPY --from=builder /app/public ./public
COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static

USER nextjs

EXPOSE 3000
ENV PORT=3000
ENV HOSTNAME="0.0.0.0"

CMD ["node", "server.js"]

Requirements:

• next.config.js must have output: 'standalone'
• Install sharp if using Next.js Image optimization

FastAPI/Python

FROM python:3.11-slim

RUN apt-get update && apt-get install -y \
    git \
    curl \
    && rm -rf /var/lib/apt/lists/*

WORKDIR /app

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY . .

ENV PYTHONUNBUFFERED=1
ENV PYTHONDONTWRITEBYTECODE=1

EXPOSE 8080

HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
    CMD curl -f http://localhost:8080/health || exit 1

# IMPORTANT: Use exec form and --proxy-headers for Fly's TLS proxy
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8080", "--proxy-headers"]

Multi-stage for External Tools

Pull binaries from official images instead of version-pinning downloads:

# Pull tools from official images (always latest)
FROM ghcr.io/gitleaks/gitleaks:latest AS gitleaks
FROM aquasec/trivy:latest AS trivy

FROM python:3.11-slim

# Copy binaries
COPY --from=gitleaks /usr/bin/gitleaks /usr/local/bin/gitleaks
COPY --from=trivy /usr/local/bin/trivy /usr/local/bin/trivy

# ... rest of Dockerfile

Monorepo Deployment

Directory Structure

project-root/
├── fly.api.toml          # API config (in root)
├── fly.web.toml          # Web config (in root)
├── apps/
│   ├── api/
│   │   ├── Dockerfile    # Paths relative to root
│   │   └── ...
│   └── web/
│       ├── Dockerfile    # Paths relative to root
│       └── ...

Deploy Commands

# From project root
fly deploy --config fly.api.toml
fly deploy --config fly.web.toml

Dockerfile Paths for Monorepo

Since Dockerfiles are built with root as context:

# In apps/api/Dockerfile
COPY apps/api/requirements.txt .
COPY apps/api/*.py /app/

# In apps/web/Dockerfile
COPY apps/web/package*.json ./
COPY apps/web/ .

Common Commands

# Create apps
fly apps create my-app --org my-org

# Create volume (pick region with good capacity)
fly volumes create app_data --size 10 --app my-app --region dfw

# Check region capacity first
fly platform regions

# Set secrets
fly secrets set \
  API_KEY="xxx" \
  DB_URL="yyy" \
  --app my-app

# Deploy
fly deploy --config fly.api.toml

# View logs
fly logs --app my-app

# SSH into machine
fly ssh console --app my-app

# Check status
fly status --app my-app

# List volumes
fly volumes list --app my-app

Region Selection

Check capacity before choosing:

fly platform regions

Pick regions with positive capacity scores. Negative scores mean constrained resources.

Good US options (typically):

• dfw - Dallas
• lax - Los Angeles
• ord - Chicago

Troubleshooting

"only 1 volume supported"

You have multiple [[mounts]] sections. Consolidate to one volume with subdirectories.

Dockerfile path doubling

Example: /apps/api/apps/api/Dockerfile

Cause: fly.toml is in a subdirectory with relative dockerfile path. Fix: Place fly.toml in project root with full path: dockerfile = "apps/api/Dockerfile"

Build context wrong

Symptom: COPY apps/web/ . fails with "not found" Fix: Deploy from project root: fly deploy --config fly.web.toml

TypeScript build fails in Docker

Test locally first:

cd apps/web && npx tsc --noEmit

App won't start - connection refused

Check that app binds to 0.0.0.0, not localhost or 127.0.0.1.

HTTPS/proxy issues

For apps behind Fly's TLS proxy, add --proxy-headers to uvicorn or equivalent for your framework.

Next.js "useX must be used within Provider" during build

Symptom: Build fails with errors like:

Error: useAuth must be used within an AuthProvider
Error occurred prerendering page "/dashboard"

Cause: Next.js tries to statically pre-render pages at build time. If pages use React Context hooks (like useAuth, useTheme, etc.) but the Provider isn't wrapping the component tree during SSG, the build fails.

Fix: Ensure your context Provider wraps the entire app in layout.tsx:

// src/components/Providers.tsx
'use client'

import { ReactNode } from 'react'
import { AuthProvider } from '../hooks/useAuth'

export default function Providers({ children }: { children: ReactNode }) {
  return <AuthProvider>{children}</AuthProvider>
}

// src/app/layout.tsx
import Providers from '../components/Providers'
import AppLayout from '../components/AppLayout'

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en">
      <body>
        <Providers>
          <AppLayout>{children}</AppLayout>
        </Providers>
      </body>
    </html>
  )
}

Key points:

• The Providers component must have 'use client' directive
• Wrap providers around any component that uses context hooks
• This applies to any context: Auth, Theme, Query clients, etc.

Next.js "/app/public": not found

Symptom: Build fails with:

COPY --from=builder /app/public ./public
failed to calculate checksum: "/app/public": not found

Cause: The standard Next.js Dockerfile copies the public directory, but your project doesn't have one.

Fix: Create an empty public directory:

mkdir -p apps/web/public
touch apps/web/public/.gitkeep

Note: The public directory is where Next.js serves static assets (favicon, images, robots.txt, etc.). Even if empty, the directory must exist for the Dockerfile COPY to succeed.

Cost Optimization

Free tier includes:

• 3 shared-cpu-1x VMs with 256MB RAM
• 3GB persistent storage
• 160GB outbound bandwidth

To minimize costs:

• Use auto_stop_machines = true for non-critical apps
• Use min_machines_running = 0 when possible
• Start with 256mb memory, increase if needed
• Use single volume with subdirectories instead of multiple volumes