Clustering Your Application

Elixir and the BEAM have the incredible ability to be clustered together and processes can pass messages seamlessly to each other between nodes. makes clustering easy! This extra (and totally optional) portion of the guide walks you through clustering your Elixir application.

There are 3 parts to getting clustering quickly setup on

  • Naming the Node(s)
  • Installing and using dns_cluster
  • Scaling our application to multiple VMs

Naming the Node(s)

To make clustering easier, we want our Elixir nodes to be named using our Fly application name and the IPv6 address assigned to the node. Then later, our cluster can form up using DNS settings based on this naming scheme.

If you’ve run fly launch already then you should have a rel folder with a rel/ file already.

If not, run this command from your Elixir application:

mix release.init

Then check the generated rel/ file and add ensure the lines looks similar:

# configure node for distributed erlang with IPV6 support
export ERL_AFLAGS="-proto_dist inet6_tcp"
export ECTO_IPV6="true"
export DNS_CLUSTER_QUERY="${FLY_APP_NAME}.internal"

This names our Elixir node’s name (aka RELEASE_NODE) using the Fly application name, the Docker image reference value, and the internal IPv6 address. Make sure to deploy after making this change!

fly deploy

Adding dns_cluster

The Phoenix library dns_cluster helps here.

The dns_cluster library lets you easily setup Erlang Clustering using DNS, and has built in DNS support!

After installing dns_cluster, add it to the application like this:

defmodule HelloElixir.Application do
  use Application

  def start(_type, _args) do
    children = [
      # ...
      # setup for clustering
      {Phoenix.PubSub, ...},
      {DNSCluster, query: Application.get_env(:hello_elixir, :dns_cluster_query) || :ignore},

    # ...

  # ...

Our next step is to add the dns_cluster_query configuration to the file config/runtime.exs.

  config :hello_elixir, dns_cluster_query: System.get_env("DNS_CLUSTER_QUERY")

And finally add a key to our fly.toml file:

  DNS_CLUSTER_QUERY = "hello-elixir.internal"

REMEMBER: Deploy your updated app so the clustering code is available, with fly deploy.

This configures dns_cluster to look for other deployed apps using the same $FLY_APP_NAME on the .internal private network.

This assumes that your rel/ file is configured to name your Elixir node using the $FLY_APP_NAME. We did this earlier in the “Naming Your Elixir Node” section.

Before this app can be clustered, we need more than one VM. We’ll do that next!

Running multiple VMs

There are two ways to run multiple VMs.

  1. Scale our application to have multiple Fly Machines in one region.
  2. Add a Machine to another region (multiple regions).

Both approaches are valid and our Elixir application doesn’t change at all for the approach you choose!

Let’s first start with a baseline of our single deployment.

fly status
app     6e82dd00f75687  20      sea     started 1 total, 1 passing      2023-03-16T22:01:45Z

Scaling in a single region

Let’s scale up to 2 VMs in our current region.

fly scale count 2

Checking on the status we can see what happened.

fly status
app     5683d474b4658e  20      sea     started 1 total, 1 passing      2023-06-16T01:49:36Z
app     6e82dd00f75687  20      sea     started 1 total, 1 passing      2023-03-16T22:01:45Z

We now have two VMs in the same region! That was easy.

Let’s make sure they are clustered together. We can check the logs:

fly logs
app[5683d474b4658e] sea [info] 21:50:21.924 [info] [libcluster:fly6pn] connected to :"fly-elixir@fdaa:0:1da8:a7b:ac2:f901:4bf7:2"

But that’s not as rewarding as seeing it from inside a node. From an IEx shell, we can ask the node we’re connected to, what other nodes it can see.

fly ssh console --pty -C "/app/bin/hello_elixir remote"
iex(fly-elixir@fdaa:0:1da8:a7b:ac2:f901:4bf7:2)1> Node.list

I included the IEx prompt because it shows the IP address of the node I’m connected to. Then getting the Node.list returns the other node. Our two VMs are connected and clustered!

Scaling to multiple regions makes it super easy to run VMs of your applications physically closer to your users. Through the magic of DNS, users are directed to the nearest region where your application is located.

Starting back from our baseline of a single VM running in sea which is Seattle, Washington (US), I’ll add the region ewr which is Parsippany, NJ (US). I can do this by cloning the existing Fly Machine into my desired region:

fly machine clone 6e82dd00f75687 --region ewr

Now our status shows we have two Machines spread across 2 regions! This puts a VM on both coasts of the US.

fly status
app     0e2869ea63d486  20      ewr     started 1 total, 1 passing      2023-06-16T01:56:19Z
app     6e82dd00f75687  20      sea     started 1 total, 1 passing      2023-03-16T22:01:45Z

Let’s ensure they are clustered together.

fly ssh console --pty -C "/app/bin/hello_elixir remote"
iex(fly-elixir@fdaa:0:1da8:a7b:ac2:cdf6:c422:2)1> Node.list

We have two VMs of our application deployed to the West and East coasts of the North American continent and they are clustered together! Our users will automatically be directed to the server nearest them. That is so cool!

Before two Elixir nodes can cluster together, they must share a secret cookie. The cookie itself isn’t meant to be a super secret encryption key or anything like that, it’s designed to let us create multiple sets of small clusters on the same network that don’t all just connect together. Different cookies means different clusters. For instance, only the nodes that all use the cookie “abc” will connect together.

For us, this means that in order for my_remote node to connect to the cluster on Fly, we need to share the same cookie value used in production.

When we build a mix release, it generates a long random string for the cookie value. When we re-run the mix release command, it keeps the same cookie value. That is, when we don’t run it in Docker. The Dockerfile we’re using is building a fresh release every time we run it. That’s kind of the point of a Docker container. So our cookie value is being randomly generated every time we deploy. This means after every deploy, we would have to figure out what the new cookie value is so our local node can use it.

The easiest solution here is to specify the value to use for our cookie. One that we will know outside of the build and that won’t keep changing on us.

If we read the Mix.Tasks.Release docs, in the :cookie section we learn that if we provide an ENV named RELEASE_COOKIE, it will be used. If that ENV is not found, it falls back to the randomly generated one.

To do this, we can create the cookie we want and store it in our fly.toml file like this:

  RELEASE_COOKIE = "my-app-cookie"

Also from the docs, we can generate the cookie string to use with this Elixir command:


After deploying the application, we can verify that the cookie is being used by getting an IEx shell into our running server and issuing the following command:


This shows the cookie being used at runtime.

With a known and unchanging cookie deployed in our application, we are ready for the next step!