Starting your Phoenix App on a Different Port in Development

I’ve recently run into the issue where I want to run two different Phoenix apps at the same time on my local machine. I wanted to run one app on port 4000 and another app on port 4001.

I thought that I was going to be able to do something like:

$ mix phoenix.server -port 4001

[info] Running MyApp.Endpoint with Cowboy
using http on port 4000

That didn’t work, so I dove into the docs to see what I could do with mix phoenix.server. Unfortunately, there are no command-line arguments for passing in the port.

Doing more Internet research, I saw a suggestion by Chris McCord about how to override the default port of 4000 on a Phoenix App here.

There are two solutions to override the default port of 4000. The solution you pick depends on what you need.

Solution 1: Replace the hardcoded port in config/dev.exs with a new port number

Looking at a sample app, port is hardcoded as 4000. You can change this to 4001 and running mix phoenix.server will start your app on port 4001 everytime.

$ mix phoenix.server

[info] Running MyApp.Endpoint with Cowboy
using http on port 4001

This solution works if you know you want to run a specific app on a specific port every single time and keeps your starting command neat.

Solution 2: Dynamically override the port in config/dev.exs with an environment variable

To take this one step further, you can use an environment variable to set the port. To do this, you can replace 4000 here like this:

http: [port: System.get_env("PORT") || 4000]

And then, when running your Phoenix server, you can do the following:

$ PORT=4001 mix phoenix.server
[info] Running MyApp.Endpoint with Cowboy
using http on port 4001

Phoenix will now use the environment variable port number you pass in to run your app and will default to 4000 if you do not pass an environment variable in. This makes solution 2 more flexible than solution 1.