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postgres.md

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In this guide, you’ll practice creating and using volumes to persist data created by a Postgres container. When the database runs, it stores files into the /var/lib/postgresql/data directory. By attaching the volume here, you will be able to restart the container multiple times while keeping the data.

Start a container using the Postgres image with the following command:

docker run --name=db -e POSTGRES_PASSWORD=secret -d -v postgres_data:/var/lib/postgresql/data postgres

This will start the database in the background, configure it with a password, and attach a volume to the directory PostgreSQL will persist the database files.

Connect to the database by using the following command:

docker exec -ti db psql -U postgres

In the PostgreSQL command line, run the following to create a database table and insert two records:

CREATE TABLE tasks (
    id SERIAL PRIMARY KEY,
    description VARCHAR(100)
);
INSERT INTO tasks (description) VALUES ('Finish work'), ('Have fun');

Verify the data is in the database by running the following in the PostgreSQL command line:

SELECT * FROM tasks;

You should get output that looks like the following:

 id | description
----+-------------
  1 | Finish work
  2 | Have fun
(2 rows)

Exit out of the PostgreSQL shell by running the following command:

\q

Stop and remove the database container. Remember that, even though the container has been deleted, the data is persisted in the postgres_data volume.

docker stop db
docker rm db

Start a new container by running the following command, attaching the same volume with the persisted data:

docker run --name=new-db -d -v postgres_data:/var/lib/postgresql/data postgres

You might have noticed that the POSTGRES_PASSWORD environment variable has been omitted. That’s because that variable is only used when bootstrapping a new database.

Verify the database still has the records by running the following command:

docker exec -ti new-db psql -U postgres -c "SELECT * FROM tasks"

Remove volumes

Before removing a volume, it must not be attached to any containers. If you haven’t removed the previous container, do so with the following command (the -f will stop the container first and then remove it):

docker rm -f new-db

There are a few methods to remove volumes, including the following:

Use the docker volume rm command:

docker volume rm postgres_data

Use the docker volume prune command to remove all unused volumes:

docker volume prune