Fly.io is amazing, especially for Phoenix projects. Currently, I have 3 phoenix projects hosted on fly.io(since they have a very generous free tier plan). You can also use fully-managed Redis instance in fly, it's pretty easy to setup.

Recently, I had to connect a personal phoenix project to redis. Aaaand it wasn't easy! I had to do a lot of debugging, had to go through various github issues to figure everything out.

At the end, I was able to connect my phoenix app to redis 🎉. I'm putting down how I did it in this blog!

Things to consider when setting up Redis

When setting up a Redis instance using flyctl(Fly's command line tool), make sure that the redis instance is in same organization as the Phoenix app. Note down the private URL of the redis instance you get after setup(alternatively you can see the private URL using flyctl redis status <redis-instance-name>).

The private URL should be something like this: redis://default:<password>@fly-<redis-instance-name>.upstash.io

This URL can be broken into different components such as host: fly-<redis-instance-name>.upstash.io, the password: <password>. By default you can connect to this redis instance through port 6379(unless configured otherwise).

Now that we have a redis instance and its URL, lets see how to connect it to Phoenix.

Phoenix + Redix + Redis

I used the redix library as redis driver for elixir. To configure Redix, we just need to pass some args to start_link function. Here's start_link function for the redis instance.

{:ok, conn} = Redix.start_link(host: "fly-<redis-instance-name>.upstash.io", password: "<password>", host: 6379, socket_opts: [:inet6])

Notice the socket_opts: [:inet6] option, it is necessary to provide this. I'm not exactly sure why, it has something to do with fly using IPv6 for their IP address.

Once deployed, Redix will be able to connect to the redis instance. This won't work locally since the URL is private, meaning your application will only be able to access the redis instance when running inside flyio's server.

That's it. Your phoenix application should have access to redis.

Aside: Running redix under supervision tree

Most of the time, instead of using start_link manually, you would want to start Redix automatically under a supervision tree.

For this, you just need to put redix under you application child spec.

defmodule Application do
# ...other stuff

  def start(_, _) do
    children = [
      {Redix, [host: "fly-<redis-instance-name>.upstash.io", password: "<password>", host: 6379, socket_opts: [:inet6]]} # <- Starting redix under supervision tree
    ]

    # ...other stuff
  end

# ...other stuff
end

Aside: Putting redix config as enviroment variables

It's not good to put redix configuration directly into start_link, you would want to put this configuration into an enviroment variable.

For this, first we need to use Application.fetch_env!/2 to fetch the configuration.

Here's an example running redix under supervision tree:

defmodule Application do
# ...other stuff

  def start(_, _) do
    children = [
      {Redix, Application.fetch_env!(:redix, :config)} # <- Using Application.fetch_env to get redix configs
    ]

    # ...other stuff
  end

# ...other stuff
end

Application.fetch_env looks for configuration defined in Config files(such as config/dev.exs, config/prod.exs, config/runtime.exs, etc.).

Application.fetch_env!(:redix, :config) will look for a config named redix and get the config value defined in it. Lets define a config inside config/runtime.exs.

# ...other stuff

if config_env() == :prod do # <- NOTE: we are putting config inside this if expression
  # ...other stuff
  config :redix,
    config: [
      name: :redix,
      host: System.get_env("REDIS_HOST"), # <- Get host from env
      password: System.get_env("REDIS_PASSWORD"), # <- Get password from env
      port: 6379,
      socket_opts: [:inet6]
    ]
end

Now you just need to define REDIS_HOST and REDIS_PASSWORD inside fly.io, and everything should work as expected.

These were the things I learned when connecting Phoenix to redis in fly.io.

That's all for this blog. See you on the next one! 👋