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Running Phoenix application in docker-compose with ease

We will setup a #Phoenix application inside a #Docker container and run it on development environment.

Creating the application

Starting from scratch, let's create our Phoenix application

mix phx.new easy_compose

When asked to install dependencies, say no (n).
Since we are going to run the application inside a container we won't need its dependencies on our machine. 😉

The Dockerfile - building the application image

Now let's build the Dockerfile. We are going to use the elixir:1.10.4-alpine image.
In a container we will need:

Some system dependencies:

apk add --no-cache build-base npm git python inotify-tools

Hex and Rebar:

mix do local.hex, local.rebar

Install our dependencies listed on mix.exs:

mix do deps.get, deps.compile

Compile our app, that lives in lib/

mix compile

And install node dependencies

npm install --prefix ./assets

After running all these commands we are able to mix phx.server inside a docker container. Let's put it all in a Dockerfile and try.
cd into your easy_compose/ folder and create the Dockerfile

FROM elixir:1.10.4-alpine

# install build dependencies
RUN apk add --no-cache build-base npm git python inotify-tools

# prepare build dir
WORKDIR /app

# install hex and rebar
RUN mix do local.hex --force, local.rebar --force

# install mix dependencies
COPY mix.exs mix.lock ./
COPY config config
RUN mix do deps.get, deps.compile

# compile app
COPY lib lib
RUN mix compile

# build assets
COPY assets/package.json assets/package-lock.json ./assets/
RUN npm install --prefix ./assets --progress=false --no-audit --loglevel=error

# run server
CMD ["mix", "phx.server"]

This is our first time running this application so we better run mix deps.get in order to generate the mix.lock file, which was included in the Dockerfile. The same for npm, we gotta run npm install --package-lock-only --prefix ./assets to get the node file package-lock.json. And we are ready to build our docker image.

docker build --tag easy_compose .

Now if you run docker image ls you should see our new image.

REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE
easy_compose        latest              b19489228953        About a minute ago   449MB

If we try to just start a container from this image: docker run --rm easy_compose we should see an error because we still have no database configured. We will configure it inside the docker-compose. Let's start with it.

Creating the docker-compose

The first structure we can think of is as follows:
Create a file called docker-compose.yml containing these lines.

version: '3.7'

services:
  web:
    build: .
    volumes:
      - .:/app
    ports:
      - 4000:4000
    depends_on:
      - db
  db:
    image: postgres:12
    volumes:
      - postgres_data:/var/lib/postgresql/data/
volumes:
  postgres_data:

And try to run the containers: docker-compose up
Sadly this won't work. 😟 , CRTL + C to stop the containers and run docker-compose down to remove the stopped containers.

First, the database container was started with no user password. Since we are in development environment we shouldn't care about database passwords, so we pass the following environment variable: POSTGRES_HOST_AUTH_METHOD=trust in order to allow container start without a password.

Second problem was our application was trying to compile again, it seems like it could't find the _build/ and deps/ folder and that's because after we built our image, docker-compose is mounting our machine folder easy_compose/ entirely to the container working directory app/, but this ends up erasing all artifacts generated at building stage. To prevent this behaviour we gotta add more volumes to the web service. This way our container can access his _build/ and deps/ inside our machine /var/lib/docker/volumes/ rather than the easy_compose/ folder.
After all the changes we should get the following docker-compose.yml

version: '3.7'

services:
  web:
    build: .
    volumes:
      - .:/app
      - /app/_build
      - /app/deps
      - /app/priv/static
      - /app/assets/node_modules
    ports:
      - 4000:4000
    depends_on:
      - db
  db:
    image: postgres:12
    environment:
      - POSTGRES_HOST_AUTH_METHOD=trust
    volumes:
      - postgres_data:/var/lib/postgresql/data/
volumes:
  postgres_data:

If we try to run again we will see that the database is started but we still got an error

web_1  | [error] Postgrex.Protocol (#PID<0.367.0>) failed to connect: ** (DBConnection.ConnectionError) tcp connect (localhost:5432): connection refused - :econnrefused

It's because we han't configured our application to fetch our database. We can do this by going to config/dev.exs and changing the hostname from localhost to our service name, which is db.

# Configure your database
config :easy_compose, EasyCompose.Repo,
  username: "postgres",
  password: "postgres",
  database: "easy_compose_dev",
  hostname: "db",
  show_sensitive_data_on_connection_error: true,
  pool_size: 10

Stop docker-compose, CRTL + C, make the changes (in config/dev.exs) and start it again, docker-compose up.
Now we got another error:

db_1   | 2020-08-27 12:36:11.978 UTC [43] FATAL:  database "easy_compose_dev" does not exist
web_1  | [error] Postgrex.Protocol (#PID<0.449.0>) failed to connect: ** (Postgrex.Error) FATAL 3D000 (invalid_catalog_name) database "easy_compose_dev" does not exist

But this is just because we didn't created the database. Stop the container once again and now run

docker-compose run web mix ecto.create

And start docker-compose again

docker-compose up

No errors this time! 🎉🎉🎉
Go to http://127.0.0.1:4000 and you have a working phoenix application on docker-compose. 😎