Build, Deploy and Run a Deno Application

Getting an application running on Fly is essentially working out how to package it as a deployable image. Once packaged it can be deployed to the Fly infrastructure to run on the global application platform.

For this Getting Started article, we'll look at deploying a Deno application on Fly.

The Hellodeno Application

Our example will be a basic "hello world" example using Deno and Dinatra.

You can get the code for the example from the hellodeno Github repository. Just git clone to get a local copy. Here's all the code:

import {
} from "";

const greeting = "<h1>Hello From Deno on Fly!</h1>";

  get("/", () => greeting),
  get("/:id", ({ params }) => greeting + `</br>and hello to ${}`),

We'll call this file main.ts. We also want a deps.ts file for dependencies, here it is:

export {
  } from "";

There's nothing else to run. Deno will manage getting packages for itself.

Running The Application

Run deno run --allow-net main.ts to start the application:

deno run --allow-net main.ts
listening on

And connect to localhost:8080 to confirm that you have a working Deno application by receiving a greeting. Now to package it up for Fly. There are a number of ways to do this. You can use flyctl's own simple builder,

Install Flyctl and Login

We are ready to start working with Fly and that means we need flyctl, our CLI app for managing apps on Fly. If you've already installed it, carry on. If not, hop over to our installation guide. Once thats installed you'll want to login to Fly.

Configuring the App for Fly

Each Fly application needs a fly.toml file to tell the system how we'd like to deploy it. That file can be automatically generated with the command flyctl init command. To build this simmple deno app, we'll use the builtin deno builder.

flyctl init

? App Name (leave blank to use an auto-generated name) hellodeno

? Select organization: Demo (demo)

? Select builder: deno
    Deno builtin
? Select Internal Port: 8080

New app created
  Name     = hellodeno
  Owner    = dj
  Version  = 0
  Status   =
  Hostname = <empty>

Wrote config file fly.toml

You'll be asked for an application name first. We recommend that you go with the autogenerated names for apps to avoid namespace collisions. We're using hellodeno here so you can easily spot it in configuration files.

Next you'll be prompted for an organization. Organizations are a way of sharing applications between Fly users. When you are asked to select an organization, there should be one with your account name; this is your personal organization. Select that.

Flyctl also asks you to select a builder. Builders are responsible for constructing the Docker image of your application which is then deployed to Fly's Firecracker VMs. The simplest to use are the builtin builders, which we recommend you use here. Select Deno (Deno Builtin). If you want to know more about the various builders, see Builders and Fly.

The last thing you will be asked for an internal port value. This is the port your application communicates over. If set incorrectly, Fly will be unable to connect to the application and it will fail health checks and be terminated. The default shown, 8080, is correct for the builtin builders which, by design, get applications to talk on port 8080. So, hit return and carry on.

Inside fly.toml

The fly.toml file now contains a default configuration for deploying your app. In the process of creating that file, flyctl has also created a Fly-side application slot of the same name, "hellodeno". If we look at the fly.toml configuration file we can see the name in there:

app = "hellodeno"

  builtin = "deno"

  internal_port = 8080
  protocol = "tcp"

    hard_limit = 25
    soft_limit = 20

    handlers = ["http"]
    port = "80"

    handlers = ["tls", "http"]
    port = "443"

    interval = 10000
    timeout = 2000

The flyctl command will always refer to this file in the current directory if it exists, specifically for the app name/value at the start. That name will be used to identify the application to the Fly platform.

Next is then the build section which records which builder we would like to use to build and deploy this app. Here we are using the builtin builder for deno.

The rest of the file contains settings to be applied to the application when it deploys.

We'll have more details about these properties as we progress, but for now, it's enough to say that they mostly configure which ports the application will be visible on. The assumption the builder makes is that your application is talking on port 8080.

Deploying to Fly

We are now ready to deploy our containerized app to the Fly platform. At the command line, just run:

flyctl deploy

This will lookup our fly.toml file and get the app name hellodeno from there. Flyctl will then call on the builtin builder to build the application image, complete with the deno runtime.

Then flyctl will start the process of deploying our application to the Fly platform. Flyctl will return you to the command line when it's done.

Viewing the Deployed App

If you want to find out more about the deployment. The command flyctl info will give you all the essential details.

flyctl info
  Name     = hellodeno
  Owner    = dj
  Version  = 0
  Status   = running
  Hostname =

73f825ad 0       fra    run     running 1 total, 1 passing 0        6m34s ago

Connecting to the App

The quickest way to view your application is to run flyctl open which will open your browser on the URL for your application. Run flyctl open /name to get an extra greeting from the app.

If you want to manually enter a URL to check, remember to replace with the hostname you got from flyctl info and connect to where you should find a greeting - it will normally be upgraded to a secure connection. Use to start with a secure connection. Add /name and you'll get an extra greeting from the hellodeno application.

Bonus Points

If you want to know what IP addresses the app is using, try flyctl ips list:

flyctl ips list
TYPE ADDRESS                              CREATED AT
v4                        23h17m ago
v6   2a09:8280:1:d1e3:ff42:8342:9bdd:a0cf 23h17m ago

Arrived at Destination

You have successfully built, deployed, and connected to your first Deno application on Fly.