Fly Postgres

Helps take advantage of geographically distributed Elixir applications using Ecto and PostgreSQL in a primary/replica configuration on

Online Documentation

Mark Ericksen's ElixirConf 2021 presentation explaining what this library is for and the problems it helps solve.


If available in Hex, the package can be installed by adding fly_postgres to your list of dependencies in mix.exs:

def deps do
    {:fly_postgres, "~> 0.1.0"}

Note that fly_postgres depends on fly_rpc so it will be pulled in as well. The configuration section below includes the relevant parts for fly_rpc.



This assumes your project already has an Ecto.Repo. To start using the Fly.Repo, here are the changes to make.

For a project named "MyApp", change it from this...

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

To something like this...

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

  # Dynamically configure the database url based on runtime environment.
  def init(_type, config) do
    {:ok, Keyword.put(config, :url, Fly.Postgres.database_url())}

defmodule MyApp.Repo do
  use Fly.Repo, local_repo: MyApp.Repo.Local

This renames your existing repo to "move it out of the way" and adds a new repo to the same file. The new repo uses the Fly.Repo and links back to your project's Ecto.Repo. The new repo has the same name as your original Ecto.Repo, so your application will be referring to it now when talking to the database.

The other change was to add the init function to your Ecto.Repo. This dynamically configures your Ecto.Repo to connect to the primary (writable) database when your application is running in the primary region. When your application is not in the primary region, it is configured to connect to the local read-only replica. The replica is like a fast local cache of all your data. This means you Ecto.Repo is configured to talk to it's "local" database.

The Fly.Repo performs all read operations like all, one, and get_by directly on the local replica. Other modifying functions like insert, update, and delete are performed on the primary database through proxy calls to a node in your Elixir cluster running in the primary region. That ability is provided by the fly_rpc library.

Config Files

In your config/config.exs, add something like the following:

# Configure database repository
config :fly_postgres, :local_repo, MyApp.Repo.Local

This helps the library to know which repo to use when talking to the database to ensure the needed replications have completed.

Production Config

In either config/prod.exs or config/runtime.exs, instruct the library to rewrite the DATABASE_URL used when connecting to the database. This takes into account which region is your primary region and attempts to connect to the primary or the replica accordingly.

# Instruct Fly.Postgres to operate in "prod" mode and do DB URL rewrites
config :fly_postgres, :rewrite_db_url, true

Without this setting, the database URL will not be used. This works great for dev and test environments which don't expect or depend on a DATABASE_URL ENV setting.

If this is missing in production, it results in database connection failure.

Releases and Migrations

Assuming you are using a custom "Release" module like this one in the HelloElixir demo project to execute your migrations in a special release task, then you may need to explicitly load the :fly_postgres config so it will be available to connect to the database.

The load_app function can be changed like this:

  defp load_app do

Once the configuration is loaded, the database migrations can be run as expected. Without this step, your deployment will fail when running migrations because the database connection settings will be wrong.

Repo References

The goal with using this repo wrapper, is to leave all of your application code and business logic unchanged. However, there are a few places that need to be updated to make it work smoothly.

The following examples are places in your project code that need reference your actual Ecto.Repo. Following the above example, it should point to MyApp.Repo.Local.

  • test_helper.exs files make references like this Ecto.Adapters.SQL.Sandbox.mode(MyApp.Repo.Local, :manual)
  • data_case.exs files start the repo using Ecto.Adapters.SQL.Sandbox.start_owner! calls.
  • channel_case.exs need to start your local repo.
  • conn_case.exs need to start your local repo.
  • config/config.exs needs to identify your local repo module. Ex: ecto_repos: [MyApp.Repo.Local]
  • config/dev.exs, config/test.exs, config/runtime.exs - any special repo configuration should refer to your local repo.

With these project plumbing changes, you application code can stay largely untouched!

Primary Region

If your application is deployed to multiple regions, the instances (or nodes) must be clustered together.

Through ENV configuration, you can to tell the app which region is the "primary" region.


This example configuration says that the Sydney Australia region is the "primary" region. This is where the primary postgres database is created and where our application has fast write access to it.



There are two entries to add to your application supervision tree.

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    # ...

    children = [
      # Start the RPC server
      {Fly.RPC, []},
      # Start the Ecto repository
      # Start the tracker after your DB.
      {Fly.Postgres.LSN.Tracker, []},

    # ...

The following changes were made:


Local Development

If you get an error like (ArgumentError) could not fetch environment variable "PRIMARY_REGION" because it is not set then see the README docs in fly_rpc for details on setting up your local development environment.

Automatic Usage

Normal calls like MyApp.Repo.all(User) are performed on the local replica repo. They are unchanged and work exactly as you'd expect.

Calls that modify the database like "insert, update, and delete", are performed through an RPC (Remote Procedure Call) in your application running in the primary region.

In order for this to work, your application must be clustered together and configured to identify which region is the "primary" region. Additionally, your application needs to be deployed to multiple regions. There must be a deployment in the primary region as well.

A call to MyApp.Repo.insert(changeset) will be proxied to perform the insert in the primary region. If the function is already running in the primary region, it just executes normally locally. If the function is running in a non-primary region, it makes a RPC execution to run on the primary. Additionally, it gets the Postgres LSN (Log Sequence Number) for the database after making the change. The calling function then blocks, waits for the async database replication process to complete, and continues on once the data modification has replayed on the local replica.

In this way, it becomes seamless for you and your code! You get the benefits of being globally distributed and running closer to your users without re-designing your application!

By default, a Repo function that modifies the database is proxied to the server and it waits for the data to be replicated locally before continuing. Passing the await: false option instructs the proxy code to not wait for replication. This is helpful when you only need the function result or the data is not immediately needed locally.

MyApp.Repo.insert(changeset, await: false)

MyApp.Repo.update(changeset, await: false)

MyApp.Repo.delete(item, await: false)

Explicit RPC Usage

When business logic code makes a number of changes or does some back and forth with the database, the "Automatic Usage" will be too slow. An example is looping though a list and performing a database insert on each iteration. Waiting for the insert to complete and be locally replicated before performing the next iteration could be very slow.

For those cases, execute the function that does all the database work but do it in the primary region where it is physically close to the database.

Fly.Postgres.rpc_and_wait(MyModule, :do_complex_work, [arg1, arg2])

The function will be executed in the primary region and it blocks locally until any relevant database changes are replicated locally.

Explicit RPC but don't Wait for Replication

Sometimes you might not want to wait for DB replication. Perhaps it's a fire-and-forget or the function result is enough.

For this case, you can use the fly_rpc library directly.

Fly.rpc_primary(MyModule, :do_work, [arg1, arg2])

This is a convenience function which is equivalent to the following:

Fly.RPC.rpc_region(:primary, MyModule, :do_work, [arg1, arg2])

This also works when modifying the database too.

LSN Polling

The library polls the local database for what point in the replication process it has gotten to. It uses the LSN (Log Sequence Number) to determine that. Using this information, a process making changes against the primary database can request to be notified once the LSN it cares about has been replicated. This enables blocking operations that pause and wait for replication to complete.

The active polling only happens once a process has requested to be notified. When there are no pending requests, there is no active polling. Also, there can be many active pending requests and still there will be only 1 polling process. So each waiting process isn't polling the database itself.

The polling design scales well and doesn't perform work when there is nothing to track.

Backup Regions

By default, defines some "backup regions" where your app may be deployed. This can happen even during a normal deploy where the app instance created for running database migrations could come up in a backup region.

The fly_postgres library makes an important assumption: that the app instance is always running in a region with either the primary or replica database.

To make your deployments reliably end up in a desired region, we'll disable the backup regions. Or rather, explicitly set which regions should be use as backup regions.

To see you current set of backup regions:

fly regions backup list

If you want to serve 2 regions like lax and syd, then you can the backup regions like this:

fly regions backup lax syd

This makes the backup regions only use the desired regions.

Ensuring a Deployment in your Primary Region

Currently it is possible to be configured for two regions, scale your app to 2 instances and end up with both app instances in the same region! Clearly, when one app needs to be running in the primary region in order to receive the RPC calls, it's really important that it consistently be there!

A config option lets us have more control over that.

fly scale count 2 --max-per-region 1

Using the --max-per-region 1 ensures each region will not get an unbalanced number like 2 in one place.

If your scale count matches up with your desired number of regions, then they will be evenly distributed.