Using's Multi-region databases with Elixir's Phoenix Framework

September 24, 2021 · 13 min read

fly phoenix

In this article we’ll be using to host a Phoenix web app that’s connected to multiple Postgres read-replica databases (1 read/write master and a number of read-only replicas). This article assumes that:

Core Documentation

In a sense this article is a combined implementation of:

But since database configuration with a Fly app happens at runtime there were a few things to look out for. If you’re serious about implementing a multi-region database app then you should use read those two articles first.

At the end of the article I’ll talk about benchmarking/performance.


Final code

The final code is here:

0. Table of Contents

1. Deploying a Phoenix app to

Before getting into multi-region databases, we need to setup and deploy a basic Phoenix app to Fly so that we have something to test. I’ll defer to the following links:

The TLDR is:

IMPORTANT: When creating a database…

The docs say to create a database via fly postgres create, but in our case we’ll use the command from the Multi-Region Database docs instead:

fly pg create --name [insert cluster name here] --region [insert region here]

# attach the DB to your app by running the following
# from your app's root folder:
fly pg attach --postgres-app [cluster name here]

The name can be anything but will have to be unique.

For a list of regions, refer to the Region docs.

Creating test data

The fastest way to create scaffolding for test data is to use one of Phoenix’s generators. For this example I’ll be using an example taken straight from the docs. Run the following from the root of your Phoenix project to generate an Account’s context, User schema, HTML templates, etc..

mix phx.gen.html Accounts User users name:string age:integer

Don’t forget to add an endpoint to your router.ex file and migrate your local database.

# router.ex file

resources "/users", UserController

# migrate from console

mix ecto.migrate

We should have everything we need now to generate data from the /users endpoint of your app and/or from IEX console.


  • You should have a Phoenix web app deployed to
  • You should be able to create and save something. If you’re following this example then create some users with names and an age set.

2. Creating multi-region databases

If you ran the command mentioned in the last section (fly pg create --name [insert cluster name here] --region [insert region here]) then you’ve already created a leader DB for writes and a replica for reads.

💡 VERIFY: run the following command to check for the leader and replica:

fly status -a [your db cluster name]

# Example Output:

> fly status -a my-postgres

WARN app flag 'my-postgres' does not match app name in config file 'my-app-name'
? Continue using 'my-postgres' Yes
  Name     = my-postgres
  Owner    = personal
  Version  = 2
  Status   = running
  Hostname =

e9e10050 app  2       nrt    run     running (leader)  3 total, 3 passing 0        23h59m ago
7435e9e5 app  2       nrt    run     running (replica) 3 total, 3 passing 0        23h59m ago

Creating Replicas

For now let’s create a single replica via the following command

fly volumes create pg_data -a [your db cluster name] --size 10 --region [insert region name]

As I’m based out of Tokyo, I created a replica in Chicago:

fly volumes create pg_data -a my-postgres --size 10 --region ord

Important: Add more VM’s. If you don’t then you won’t see the replica you created.

fly scale count 3 -a [your db cluster name]

Why 3? We have one leader (nrt), the local replica of the leader (nrt), and the new region we just created (ord).

💡 VERIFY: Check that you see the new replica when listing VM’s:

> fly status -a [your db cluster name]

# Example output:

  Name     = my-postgres
  Owner    = personal
  Version  = 2
  Status   = running
  Hostname =

f9ed37a0 app  2       ord    run     running (replica) 3 total, 3 passing 0        23h57m ago
e9e10050 app  2       nrt    run     running (leader)  3 total, 3 passing 0        23h59m ago
7435e9e5 app  2       nrt    run     running (replica) 3 total, 3 passing 0        23h59m ago

Connect the cluster to your app

If you haven’t already, attach the cluster to your app by running the following from your app’s root folder:

fly pg attach --postgres-app [your cluster name]

💡 VERIFY: Even though we haven’t connected the read-replicas to our app, we should still be able to read/write from the primary database.

3. Configure Phoenix/Elixir (but really Ecto)

There are number of ways to configure read-replicas with a Phoenix app, but specifically with we have to pay attention to order of events since DB configuration happens at runtime.

Region helper

First, we need a way of finding the current region so we know which replica-database to use.

Fly automatically sets a FLY_REGION environment variable for any app instance. We could grab it using System.fetch_env("FLY_REGION") but for the sake of keeping things DRY (and wrapping 3rd-party dependancies) let’s create simple helper module.

Create a /fly folder and a region.ex file:

# lib/my_app/fly/region.ex
defmodule MyApp.Fly.Region do
  @moduledoc """
  Helper module for Fly Regions

  [Fly Documenation](

  def current do
    |> case do
      {:ok, region} -> region
      _ -> nil

This module might come in handy later if we want to display the current region somewhere on the site.

Config files

In the runtime.exs file there should already be configuration the project’s Repo module. Setting up a read-replica requires:

Add the following to your runtime.exs file (ignore the MyApp.Repo.Replicas for now):

# runtime.exs

  # Define read-replica regions (not including the primary)
  regions = ["ord"]

  # Create Repo modules for each region
  # Ex:
  # %{
  #    "ord" => MyApp.Repo.Ord
  #  }
  replicas =
    Enum.reduce(regions, %{}, fn region, acc ->
      replica = Module.concat(MyApp.Repo, String.capitalize(region))
      Map.put(acc, region, replica)

  # Replicas Config
  config :my_app, MyApp.Repo.Replicas,
    replicas: replicas

  # Configure each Replica
  for {region, repo} <- replicas do
    # Add region to the hostname
    {_, opts} =
      |> Keyword.get_and_update!(:hostname, fn hostname -> {hostname, "#{region}.#{hostname}"} end)

    # Replica's use the port 5433
    opts = Keyword.replace!(opts, :port, 5433)

    opts =
      opts ++
          socket_options: [:inet6],
          pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")

    config :my_app, repo, opts

The code above configures each read-replica to:

Note: I’ve used Ecto.Repo.Supervisor.parse_url/1 to break down the database url into it’s components (code). Using a third-party method this way isn’t an amazing way of doing things. We might write our own version of this method to make it’s future proof.

Replicas module

We need some methods to find the right replica database. For the sake of keeping things clean I’ve created a /repo folder and Replicas module (ignore compile_replicas/0 for now):

# /lib/my_app/repo/replicas.ex

defmodule MyApp.Repo.Replicas do
  @moduledoc """
  Handles all replica related tasks

  @type region() :: String.t()
  @type replica() :: module()

  @doc """
  Returns a Map of replica module names with
  @spec replicas() :: %{region() => replica()}
  def replicas, do: Application.get_env(:my_app, __MODULE__)[:replicas] || %{}

  @doc """
  Returns a list of regions configured in runtime.exs
  @spec regions() :: [region()]
  def regions, do: Map.keys(replicas())

  @doc """
  Helper method for returning a list of replica modules
  @spec list_replicas() :: [replica()]
  def list_replicas, do: Map.values(replicas())

  @doc """
  Returns a Repo for the current region
  @spec replica() :: module()
  def replica() do
    |> replica()

  @doc """
  Returns a Repo for the a given region, returns primary Repo as a default
  @spec replica(region()) :: module()
  def replica(region) do
    if Enum.member?(regions(), region) do

  @doc """
  Generate Repo modules for each replica
  def compile_replicas do
    for replica <- list_replicas() do
      defmodule replica do
        use Ecto.Repo,
          otp_app: :my_app,
          adapter: Ecto.Adapters.Postgres,
          read_only: true

Then add the following to your app’s Repo module:

# /lib/my_app/repo.ex

defmodule MyApp.Repo do
  use Ecto.Repo,
    otp_app: :my_app,
    adapter: Ecto.Adapters.Postgres

  alias MyApp.Repo.Replicas
  defdelegate replica(), to: Replicas

Supervise our replica Repos

As explained in Ecto’s Replicas and Dynamic Repositories documentation, we need to add the replica Repos to the application supervision tree.

In Ecto’s example, each replica module was generate inside the main Repo module, but we need something that works with our runtime configuration (believe me, I tried all sorts of ways to get this to work). Otherwise when our Application’s supervision tree starts it won’t find any of our generated replica modules (because they don’t exist yet). This is where MyApp.Repo.Replicas.compile_replicas/0 becomes useful.

In your app’s application.ex create the modules first, then supervise them. The start function should look like the following:

defmodule MyApp.Application do

  def start(_type, _args) do
	  # Create our replica modules first

    children =
        # Start the Ecto repository
        # Start the Telemetry supervisor
        # Start the PubSub system
        {Phoenix.PubSub, name: MyApp.PubSub},
        # Start the Endpoint (http/https)
        # Start a worker by calling: MyApp.Worker.start_link(arg)
        # {MyApp.Worker, arg}
      ] ++ MyApp.Repo.Replicas.list_replicas()

    # See
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)


4. Using a read-replica in code

If you’ve been following along and created an Accounts context then we can adjust our GET methods to use a read-only replica database. Adjust list_users and get_user to the following:

# /lib/my_app/accounts.ex

def list_users do

def get_user!(id), do: Repo.replica().get!(User, id)

That’s it! Your app will now read from the read-replica if it’s region matches one of your replicas.

5. Performance/Benchmarking

For benchmarking I used benchee.

I’m based in Tokyo (local region: nrt).

You’ll want to set up SSH with Fly so that you can remote console into your app. Here are some relevant docs.

fly ssh console


app/bin/my_app remote

Once you’ve got an IEx console then you can run the following:

%{"nrt" => MyApp.Repo}
|> Map.merge(MyApp.Repo.Replicas.replicas())
|> {region, repo} ->
   {region, fn -> repo.all(MyApp.Accounts.User) end}
|> Enum.into(%{})
|>; nil;

# Note: the ; nil; is just to quell console output of the return value.

Here’s a comparison of regions when connecting from Japan:

Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 0 ns
parallel: 1
inputs: none specified
Estimated total run time: 35 s

Benchmarking ams...
Benchmarking atl...
Benchmarking nrt...
Benchmarking ord...
Benchmarking syd...

Name           ips        average  deviation         median         99th %
nrt         580.10        1.72 ms    ±32.92%        1.61 ms        3.23 ms
ord           7.23      138.27 ms    ±25.76%      127.82 ms      256.94 ms
syd           6.49      154.09 ms    ±29.89%      137.27 ms      277.29 ms
atl           5.13      194.94 ms    ±32.01%      168.88 ms      339.00 ms
ams           3.56      280.98 ms    ±36.08%      219.86 ms      440.07 ms

nrt         580.10
ord           7.23 - 80.21x slower +136.55 ms
syd           6.49 - 89.39x slower +152.37 ms
atl           5.13 - 113.09x slower +193.22 ms
ams           3.56 - 163.00x slower +279.25 ms

We can see that connecting to a Japan replica (nrt) is the fastest based on my current location, whereas Amsterdam is the slowest (ams).

6. Improvements/Optimizations

Repos Supervision tree

In this example we’re supervising our main Repo and replica Repos from the top level Application. It would probably make more sense to create a Repos supervision tree that monitors our Repo and replica Repo modules. Then add the Repos supervision tree to our Application supervision tree.

This way we’d then have the opportunity to specify a restart strategy specifically for Repos (probably still one for one). It also just keeps our Application module clean.

Module Complilation

Calling a function like compile_replicas from the Application module is not my favourite solution and I’m sure there is a more appropriate solution.

7. Conclusion

There’s still tons of room for improvement, but this article should serve as a proof-of-concept that can be turned into a library eventually.

Let’s check the goals:

Yes. We only have to change the “region” list in runtime.exs.

Yes. Our local and test environments continue to use a single database.

Should you use a multi-region database setup? It really depends on your use case.

Good luck!