Elixir Ecto has excellent documentation for how to use read-only replica databases, but because I'm so dense it took me a bit of trial and error to figure out where all the changes suggested by the documentation should go in my own app. Here's a concrete example of what I had to change for my conventional Mix + Phoenix application.
(The docs describe how to add N different replicas dynamically — MyApp.Repo.Replica1, MyApp.Repo.Replica2, etc — but since I only have to worry about a single endpoint for my read replicas, I simplified things and just used a single, static MyApp.Repo.Replica instance in my Elixir configuration and code.)
Mix Environment Helper
To allow my app to determine whether it was compiled and is running with a test, dev, or prod configuration, I added a config_env setting to my app, and set it to the value of the Mix.env/0 function at compile time:
# config/config.exs
config :my_app,
config_env: Mix.env(),
ecto_repos: [MyApp.Repo]
end
Note that with Elixir 1.11 and newer, you can instead use Config.config_env/0 in place of Mix.env/0:
# config/config.exs
config :my_app,
config_env: Config.config_env(),
ecto_repos: [MyApp.Repo]
end
And in my root MyApp module, I added a helper function to access this config_env setting:
# lib/my_app.ex
defmodule MyApp do
def config_env, do: Application.get_env(:my_app, :config_env)
end
This means that I can call MyApp.config_env/0 at runtime in various places in my app's code, and get the Mix.env/0 value with which the app was compiled (like :test, :dev, or :prod).
Replica Module
To my existing lib/my_app/repo.ex file (which already contained the MyApp.Repo module), I added the definition for my new MyApp.Repo.Replica module, like so:
# lib/my_app/repo.ex
defmodule MyApp.Repo do
use Ecto.Repo,
otp_app: :my_app,
adapter: Ecto.Adapters.Postgres
def replica, do: MyApp.Repo.Replica
end
defmodule MyApp.Repo.Replica do
use Ecto.Repo,
otp_app: :my_app,
adapter: Ecto.Adapters.Postgres,
default_dynamic_repo: if(MyApp.config_env() != :test, do: MyApp.Repo.Replica, else: MyApp.Repo),
read_only: true
end
The default_dynamic_repo option in the MyApp.Repo.Replica module uses the config_env helper I added above to set up the module to use the primary MyApp.Repo's own connection pool for the read replica in the test environment, as recommended by the Ecto docs. This way the replica instance will just delegate to the primary repo instance for all of its read operations in the test environment, but will still enforce its own read-only setting. Also, this way I don't have to configure any test-env-specific settings for the read replica in my config/test.exs file (nor do I need to start up another child process for the replica, as we'll see in the next section).
Application Module
In non-test environments, the new read replica module does need to be started as a child process, alongside the primary repo. So I modified the start/2 function in my application module to start it:
# lib/my_app/application.ex
defmodule MyApp.Application do
use Application
def start(_type, _args) do
# don't start separate readonly repo in test mode
repos =
if MyApp.config_env() != :test do
[MyApp.Repo, MyApp.Repo.Replica]
else
[MyApp.Repo]
end
children =
repos ++
[
MyApp.Repo,
MyAppWeb.Endpoint
]
opts = [strategy: :one_for_one, name: MyApp.Supervisor]
Supervisor.start_link(children, opts)
end
end
Dev Config
For my dev environment configuration, I updated my config/dev.exs file to simply duplicate the configuration of the primary MyApp.Reop for the MyApp.Repo.Replica (creating a separate connection pool to the same database as the primary for the replica):
# config/dev.exs
config :my_app, MyApp.Repo,
for repo <- [MyApp.Repo, MyApp.Repo.Replica] do
config :my_app, repo,
username: "myusername",
password: "mypassword",
database: "mydatabase",
hostname: "localhost",
show_sensitive_data_on_connection_error: true,
pool_size: 10
end
Prod Config
For the prod environment configuration, I updated my config/releases.exs file to use a similar configuration as the primary for the replica, but have it instead pull the replica hostname from a different environment variable (DB_READONLY in this case):
# config/releases.exs
config :my_app, MyApp.Repo,
ssl: true,
username: System.get_env("DB_USERNAME"),
password: System.get_env("DB_PASSWORD"),
database: System.get_env("DB_DATABASE"),
hostname: System.get_env("DB_HOSTNAME"),
pool_size: String.to_integer(System.get_env("DB_POOLSIZE") || "10")
config :my_app, MyApp.Repo.Replica,
ssl: true,
username: System.get_env("DB_USERNAME"),
password: System.get_env("DB_PASSWORD"),
database: System.get_env("DB_DATABASE"),
hostname: System.get_env("DB_READONLY"),
pool_size: String.to_integer(System.get_env("DB_POOLSIZE") || "10")
Using the Replica
With all the above in place, everywhere in my Elixir code that I want to query a read replica instead the primary database, I can just replace MyApp.Repo with MyApp.Repo.replica():
# lib/my_app/users.ex
import Ecto.Query
alias MyApp.Repo
alias MyApp.Users.User
def list_usernames do
from(u in User, select: u.username)
|> Repo.replica().all()
end
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