This is a guide to deploying a distributed Elixir cluster on Render using libcluster, Phoenix and Mix releases. The cluster is set up to discover nodes automatically and keep the node list current as they join or leave the cluster.

We'll start off with a bare Phoenix project, modify it to use Mix releases and libcluster and deploy it on Render. The full source code for this example is available at https://github.com/render-examples/elixir_cluster.

Create a Phoenix App

• Create a new Phoenix app in the terminal. We don't need a database in this example, so we're passing the --no-ecto flag to mix.

   # install phx.new; feel free to change 1.4.9 to a different version
   $ mix archive.install hex phx_new 1.4.9
   # create a new Phoenix app
   $ mix phx.new elixir_cluster_demo --no-ecto # also fetch and install dependencies
   $ cd elixir_cluster_demo

• Update mix.exs to add libcluster to deps.

   defp deps do
     [ ...,
       {:libcluster, "~> 3.1"}
     ]

Then run mix deps.get in your terminal to update dependencies.

Configure Mix Releases

Create runtime configuration needed for Mix releases.

1. Rename config/prod.secret.exs to config/releases.exs.
2. Change use Mix.Config in your new config/releases.exs file to import Config.
3. Uncomment the following line in config/releases.exs:

   config :elixir_cluster_demo, ElixirClusterDemoWeb.Endpoint, server: true

Finally, update config/prod.exs to delete the line import_config "prod.secret.exs" at the bottom.

Configure libcluster

Our setup will create nodes with names like elixir-cluster-demo@10.200.30.4, where the IP addresses are dynamic. Render assigns IPs to nodes when they first start, and every deploy results in a new node IP. This is where libcluster comes in: it enables automatic cluster formation through multiple configurable cluster management strategies.

Given dynamic node IPs, DNS is the best way to reliably form a cluster and keep it up to date. Consequently, we will use libcluster's DNS strategy for cluster formation.

Let's add libcluster to our production config. Add the lines below to rel/prod.exs.

dns_name = System.get_env("RENDER_DISCOVERY_SERVICE")
app_name = System.get_env("RENDER_SERVICE_NAME")

config :libcluster, topologies: [
  render: [
    strategy: Cluster.Strategy.Kubernetes.DNS,
    config: [
      service: dns_name,
      application_name: app_name
    ]
  ]
]

Render automatically populates RENDER_DISCOVERY_SERVICE and RENDER_SERVICE_NAME based on the name of your service.

Finally, add libcluster to the application supervisor by adding the lines highlighted below to application.ex:

  def start(_type, _args) do
    # List all child processes to be supervised
    topologies = Application.get_env(:libcluster, :topologies) || []

    children = [
      # start libcluster
      {Cluster.Supervisor, [topologies, [name: ElixirClusterDemo.ClusterSupervisor]]},
      # Start the endpoint when the application starts
      ElixirClusterDemoWeb.Endpoint
      # Starts a worker by calling: ElixirClusterDemo.Worker.start_link(arg)
      # {ElixirClusterDemo.Worker, arg},
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: ElixirClusterDemo.Supervisor]
    Supervisor.start_link(children, opts)
  end

Display Connected Nodes

Once everything is wired up, you can access the current node using node() and other nodes in the cluster using Node.list().

Our sample app displays all connected nodes on the homepage. Edit the index view and template in your own app as shown in this commit.

Update Your App for Render

Update config/prod.exs to change the code below.

   config :elixir_cluster_demo, ElixirClusterDemoWeb.Endpoint,
     url: [host: "example.com", port: 80],

to this:

   config :elixir_cluster_demo, ElixirClusterDemoWeb.Endpoint,
     url: [host: System.get_env("RENDER_EXTERNAL_HOSTNAME") || "localhost", port: 80],

Render populates RENDER_EXTERNAL_HOSTNAME for config/prod.exs.

If you add a custom domain to your Render app, don't forget to change the host to your new domain.

Create a Build Script

We need to run a series of commands to build our app on every push to our Git repo, and we can accomplish this with a build script. Create a script called build.sh at the root of your repo:

   #!/usr/bin/env bash
   # Initial setup
   mix deps.get --only prod
   MIX_ENV=prod mix compile

   # Compile assets
   npm install --prefix ./assets
   npm run deploy --prefix ./assets
   mix phx.digest

   # Remove the existing release directory and build the release
   rm -rf "_build"
   MIX_ENV=prod mix release

Make sure the script is executable before checking it into Git:

   $ chmod a+x build.sh

Build Your Release Locally

Compile your release locally by running ./build.sh. The output should look like this:

   Generated elixir_cluster_demo app
   * assembling elixir_cluster_demo-0.1.0 on MIX_ENV=prod
   * using config/releases.exs to configure the release at runtime
   * skipping elixir.bat for windows (bin/elixir.bat not found in the Elixir installation)
   * skipping iex.bat for windows (bin/iex.bat not found in the Elixir installation)

   Release created at _build/prod/rel/elixir_cluster_demo!

       # To start your system
       _build/prod/rel/elixir_cluster_demo/bin/elixir_cluster_demo start

   Once the release is running:

       # To connect to it remotely
       _build/prod/rel/elixir_cluster_demo/bin/elixir_cluster_demo remote

       # To stop it gracefully (you may also send SIGINT/SIGTERM)
       _build/prod/rel/elixir_cluster_demo/bin/elixir_cluster_demo stop

   To list all commands:

       _build/prod/rel/elixir_cluster_demo/bin/elixir_cluster_demo 

If everything looks good, push your changes to your repo. You can now deploy your app in production! 🎉

Deploying to Render

1. Create a new Web Service on Render, and give Render permission to access your new repo.

2. Use the following values during creation:

   Key	Value
   Environment	Elixir
   Build Command	./build.sh
   Start Command	_build/prod/rel/elixir_cluster_demo/bin/elixir_cluster_demo start

   Under the Advanced section, add the following environment variables:

   Key	Value
   SECRET_KEY_BASE	A sufficiently strong secret. You can generate a secret locally by running mix phx.gen.secret

That's it! Your distributed Elixir web service will be live on your Render URL as soon as the build finishes.

You can add nodes to your cluster by increasing the number of instances for your service under Settings.

You should see the second node on the homepage as soon as the instance update is live.

Congratulations! You've successfully set up distributed Elixir in production, and your cluster will automatically update as nodes are added or removed from it. 🎉