Connecting to Postgres from a Separate Container

Connecting to Postgres

As you become more and more comfortable with using Compose, you will find that you trust it to do what you need. A quick docker-compose ps is probably all you need to verify that a service is running. However, since running services like Postgres inside a container is still fairly new for us, let’s take the extra step of manually connecting to it from a different container, just like we did with redis.

Starting the Postgres client

As was the case with redis, the postgres image comes preinstalled with psql, the Postgres client. This means we can piggyback on our new database service in order to run a one-off container, based on the postgres image. However, instead of using the default command for the image, which starts the Postgres server, we run a command to start the Postgres client.

Command

We can do this by running the following command:

$ docker-compose run --rm database psql -U postgres -h database

Command explanation

Here we are saying, “Start a new, throwaway container (run --rm) for the database service and run the command psql -u postgres -h database inside it.” This command starts the Postgres client and tells it to connect to the hostname database with the postgres user. We are relying on the fact that Compose magically sets up a network for our application with DNS configured so that the hostname database will reach the container running our database service.

Why use run --rm?

We could have used exec instead of run --rm, which would have avoided starting a new container and instead would have executed the command on the database container that is already running. However, we deliberately didn’t because we wanted the extra verification of connecting from a different container.

When you run this command, you will be prompted to enter a password. You need to enter some-long-secure-password, the password we set in our docker-compose.yml file. This should be accepted and will take you to the psql prompt. When you are ready, you can quit the psql client as follows by typing this:

\q <Enter>

🍀 Practice

Let’s try doing it ourselves.

1. Click the Run button.

2. Start a new, throwaway container connecting the database service.

3. Quit the psql client.