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This guide discusses how to manage multiple Fly applications within a Rails projects. This is useful for Rails projects that need to run other services, like running a pool of Puppeteer servers that your Rails app calls to take screenshots of web pages.

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What is a Fly application?

Your Rails application is a Fly application, which means it has the following few things:
  • A fly.toml file in the root directory
  • A Dockerfile in the root directory that describes the image
  • A “server” running on Fly’s infrastructure.
So how do you spin up multiple Fly applications for a single Rails project?

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Creating a Fly application within a Fly application

The important thing about creating multiple Fly applications within a project is keeping them organized. For our example, we’ll setup a Redis server application and keep it in the ./fly/applications folder within our project repo. Let’s get started by running the following commands:
From inside the fly/applications/redis folder, run:
This command will create a Dockerfile and fly.toml file that can be further configured for your application’s needs. Next, deploy the application:

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Accessing from the root application

Fly creates DNS hosts for each of your applications that are not surprising.

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Deploying updates

In the future, when it’s time to deploy updates to your Fly application within a Fly application, run:
That’s it.