Challenge - Volume

Run a database with a volume for persistent data.

Challenge: a safe update with a volume

To move a database image to a new version, the container is removed and recreated with the same volume. But a backup is taken first; if something goes wrong you go back to the backup:

# 1. backup
docker run --rm -v pgdata:/veri:ro -v "$PWD":/yedek alpine tar czf /yedek/pgdata.tar.gz -C /veri .
# 2. replace
docker rm -f db
docker run -d --name db -e POSTGRES_PASSWORD=gizli -v pgdata:/var/lib/postgresql/data postgres:16.4
# 3. check
docker logs --tail 5 db

Exercise Back up

Write the command that, in an alpine container deleted when done, mounts pgdata read-only at /veri and the current folder ("$PWD") at /yedek, and runs tar czf /yedek/pgdata.tar.gz -C /veri .; then show the created file with ls -lh pgdata.tar.gz.

Show example solution

docker run --rm -v pgdata:/veri:ro -v "$PWD":/yedek alpine tar czf /yedek/pgdata.tar.gz -C /veri .
ls -lh pgdata.tar.gz

Exercise Upgrade the version

Write three commands: force-remove the db container; start a container named db from postgres:16.4 in the background with POSTGRES_PASSWORD=gizli and the pgdata volume mounted at /var/lib/postgresql/data; show the last 5 log lines.

Show example solution

docker rm -f db
docker run -d --name db -e POSTGRES_PASSWORD=gizli -v pgdata:/var/lib/postgresql/data postgres:16.4
docker logs --tail 5 db

Test yourself

When the new container mounts the same pgdata volume, what happens if POSTGRES_PASSWORD is changed?
- Nothing; the variable is only used for first setup on an empty data directory
- The password is updated
- The data is deleted
- The container doesn't start
Show answer

Nothing; the variable is only used for first setup on an empty data directory — In an existing database, the password is changed with SQL (ALTER USER).

Extract the backup back into the volume.

docker run --rm -v pgdata:/veri -v "$PWD":/yedek alpine tar ____ /yedek/pgdata.tar.gz -C /veri

Show answer

docker run --rm -v pgdata:/veri -v "$PWD":/yedek alpine tar xzf /yedek/pgdata.tar.gz -C /veri

Match.

tar czf

Take a backup
tar xzf

Restore
docker rm -f db

Remove the old container
postgres:16.4

New version

Next lesson: Challenge - Compose

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