Run a Next.js App

Getting an application running on Fly.io is essentially working out how to package it as a deployable image. Once packaged it can be deployed to the Fly.io platform. In this guide we’ll learn how to deploy a Next.js application on Fly.io. You can deploy your Next.js app on Fly with minimal effort, our CLI will do the heavy lifting. We’ll be using a standard app generated with create-next-app CLI tool provided by Next.js.

Generate the Next.js app

We’ll assume you have NodeJS installed already and can run npx. Refer to the documentation if you would like to use yarn, pnpm or bun. Now let’s create a new project by running the create-next-app. This will scaffold a new project asking you if you’d like to set up some basic tooling such as TypeScript.

npx create-next-app@latest hello-nextjs

✔ Would you like to use TypeScript? … No / Yes

...

Initializing project with template: app-tw

Installing dependencies:
- react

...

Installing devDependencies:
- typescript

...

Success! Created hello-nextjs at /.../hello-nextjs

Great! You should be able to run our app locally with cd hello-nextjs && npm run dev and access it at http://localhost:3000.

Deploy to Fly.io

First, install flyctl, the Fly.io CLI, and sign up to Fly.io if you haven’t already. Now let’s launch your Next.js app from the root of your application.

cd hello-nextjs
fly launch

Scanning source code
Detected a Next.js app
Creating app in /Users/mentels/work/hello-nextjs
We're about to launch your Next.js app on Fly.io. Here's what you're getting:

Organization: mentels                (fly launch defaults to the personal org)
Name:         hello-nextjs           (derived from your directory name)
Region:       Warsaw, Poland         (this is the fastest region for you)
App Machines: shared-cpu-1x, 1GB RAM (most apps need about 1GB of RAM)
Postgres:     <none>                 (not requested)
Redis:        <none>                 (not requested)
Sentry:       false                  (not requested)

? Do you want to tweak these settings before proceeding? No
Created app 'hello-nextjs' in organization 'personal'
Admin URL: https://fly.io/apps/hello-nextjs
Hostname: hello-nextjs.fly.dev

...

Visit your newly deployed app at https://hello-nextjs.fly.dev/

That’s it! Run fly apps open to see your deployed app in action. Try a few other commands:

fly logs - Tail your application logs
fly status - View your app’s current deployment status
fly ssh console - Open a terminal on your VM
fly deploy - Deploy the application after making changes

In the process we’ve generated a fly.toml and Dockerfile in the root of the project. Change them if you need to customize your deployment.

The following sections cover further steps you can apply for your Next.js app deployment.

Standalone builds (recommended)

To reduce the size of the final image deployed to Fly.io, we recommend using the standalone output in your next.config.js (or next.config.mjs) file:

/** @type {import('next').NextConfig} */
const nextConfig = {
  output: "standalone"
};

export default nextConfig;

When running fly launch or using the Dockerfile generator, if this "standalone" value is set in your Next config, your Dockerfile will be updated such that only the files necessary for a production environment will be used in the final image. Using the standalone setting and updating your Dockerfile can reduce your total image size by ~400mb, so we highly recommend this approach! You can also manually update your Dockerfile to accommodate a standalone Next.js app. After running the build script (npm run build), there should typically be a final COPY instruction towards the end of one’s Dockerfile. Replace that with the following:

COPY --from=build /app/.next/standalone /app
COPY --from=build /app/.next/static /app/.next/static
COPY --from=build /app/public /app/public

Lastly, update your CMD to the following:

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

Generating your Next.js Dockerfile

While you’re welcome to write your own Dockerfile, the easiest way to get started with this is to use the Dockerfile generator. Once installed, it can be run using npx @flydotio/dockerfile for Node applications or bunx @flydotio/dockerfile for Bun applications. You’ll see it referenced throughout this article for various use cases.

Connecting to databases

Unlike some other frameworks, Next.js does not bundle a database adapter. Instead, you are free to choose the ORM or node module you wish. Fly.io will provide a DATABASE_URL that you can use at runtime to connect to your database. How you will use this will depend on the database module you choose. Prisma is a popular choice, and connecting to your database using Prisma is done through your prisma/schema.prisma file. An example:

datasource db {
  provider = "postgresql"
  url      = env("DATABASE_URL")
}

If you are unsure how to connect using your favorite adapter, check out our Vanilla with Candy Sprinkles blog entry and select the configuration that most closely matches your application. If you still have questions, post on our community forum.

Build time secrets

This approach allows you to inject secrets (such as database URLs) that are only available at build time.

This approach will not work for SQLite3, as the build Machine still won’t have access to your volume. However, it can be used with PostgreSQL, MySQL and other such databases.

First, you need to obtain the secrets you will need to deploy. Often this is only the value of DATABASE_URL. If you don’t know the value of this secret, fly console and printenv DATABASE_URL can obtain this value for you. Next, you need to modify your Dockerfile to mount a secret. The easiest way to do this is using the Dockerfile generator, like so:

npx @flydotio/dockerfile --mount-secret=DATABASE_URL

It is possible to mount multiple secrets, and you can read more about mounting secrets for further details. Finally, you need to pass the secret on each deploy:

fly deploy --build-secret DATABASE_URL=value

Replace value above with the actual secret. It might be worth putting this command into a shell script or batch file.

Deferring static site generation

An alternate approach, one that works with SQLite3 too, is to defer the running of the build step to just after deployment before your web server is started. The upside is that your build has full access to all of your deployment secrets and environment variables. This involves replacing the entry point in your Dockerfile (typically your CMD) with a script and having that script run npm build (or equivalent) prior to starting your server. You can let the Dockerfile generator take care of these changes for you:

npx @flydotio/dockerfile --build=defer

Downsides of this approach:

Your deployment Machines will need enough memory to run a build. See: fly scale memory and swap_size_mb for two options.
You may need to adjust the grace_period for any HTTP service checks.
If you are only running one Machine there will be a period of time where your server is inaccessible while the site is being statically generated.
If you run multiple Machines, the static site generation will be run on each increasing the total time before any changes are fully deployed.

Exposing environment variables to the browser

If you’re a Next.js user you might know that it supports exposing environment variables to the browser using variables with names starting with NEXT_PUBLIC_. These variables are fixed at build time, and unlike runtime environment variables (which are only available on the server), they can’t be changed unless the application is built again. In order to make these NEXT_PUBLIC_ variables available, you can add them as ARG variables in your Dockerfile, as seen in the example below. This should come after any actual build steps (if you’ve used the Dockerfile generator or the Dockerfile created from fly launch, this would be after the line FROM base as build).

# Build arguments
ARG NEXT_PUBLIC_EXAMPLE="value"
ARG NEXT_PUBLIC_OTHER="Other value"

Alternatively, if you’d prefer to use the Dockerfile generator, you can do so by running the following:

npx @flydotio/dockerfile "--arg-build=NEXT_PUBLIC_EXAMPLE:value" \
  "--arg-build=NEXT_PUBLIC_OTHER=Other value"

Disabling telemetry in production

During server startup, the following messages may appear in your log:

Attention: Next.js now collects completely anonymous telemetry regarding usage.
This information is used to shape Next.js' roadmap and prioritize features.
You can learn more, including how to opt-out if you'd not like to participate in this anonymous program, by visiting the following URL:
https://nextjs.org/telemetry

The following command can be used to modify your Dockerfile to disable telemetry:

npx @flydotio/dockerfile --env-base=NEXT_TELEMETRY_DISABLED:1

Common pitfalls

Out of memory: Killed process

If you should happen to see lines like the following in your logs:

Out of memory: Killed process

Two links that may be of help:

Fetch failure when optimizing images

If your application uses the <Image> element and your images aren’t showing and you are seeing the following in your logs:

TypeError: fetch failed

Scan backward in your logs for the following:

Warning: For production Image Optimization with Next.js, the optional 'sharp' package is strongly recommended. Run 'npm i sharp', and Next.js will use it automatically for Image Optimization.

Read more: https://nextjs.org/docs/messages/sharp-missing-in-production

Follow the instructions in the message, and then redeploy:

npm i sharp
fly deploy

Cannot find module

Your app doesn’t start and you see an error like the following in your logs:

`Cannot find module 'typescript'`

Verify that the missing module appears in package.json’s dependencies, not just devDependencies. If you’re using a Dockerfile generated by dockerfile-node, your image will be optimized for production and omit development dependencies by default. If the module is missing, add it to dependencies and re-deploy.

​Generate the Next.js app

​Deploy to Fly.io

​Standalone builds (recommended)

​Generating your Next.js Dockerfile

​Connecting to databases

​Build time secrets

​Deferring static site generation

​Exposing environment variables to the browser

​Disabling telemetry in production

​Common pitfalls

​Out of memory: Killed process

​Fetch failure when optimizing images

​Cannot find module

Generate the Next.js app

Deploy to Fly.io

Standalone builds (recommended)

Generating your Next.js Dockerfile

Connecting to databases

Build time secrets

Deferring static site generation

Exposing environment variables to the browser

Disabling telemetry in production

Common pitfalls

Out of memory: Killed process

Fetch failure when optimizing images

Cannot find module