World map

 

BM

World D3: Lockers & Phone Lines

Lockers: named volumes

You will learn: to keep data in a volume, a locker outside the container.

Checked with Docker Engine 29 on 31 Oct 2026

The problem

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • dbrunning
    from postgres:18-alpine no name → /var/lib/postgresql

Frozen packs (images)

  • postgres:18-alpine9 layers · 433MB · in use

Lockers (volumes)

  • 26e7b8e4309c… (no name)24 files · used by db

Kancil swaps the database box for a fresh one. Then: ERROR: relation "orders" does not exist. Plain words: the new database has no orders table.

Watch

A volume: A locker outside the lunchbox, so data stays when the container is gone. is a locker outside the lunchbox. Docker keeps it until you delete it. Tap the pieces to build a database with a locker.

Tap the pieces in the right order.

…

Try it

Start db with the pgdata locker. Then make an orders table.

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • No containers yet

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB
  • nginx:1.30-alpine8 layers · 93.5MB
  • postgres:18-alpine9 layers · 433MB

Predict

Guess first, then run it. Which file is still there?

Where does each file live? Guess what stays, then run it.

$ docker run -d --name kedai -v kedai-data:/data nginx:1.30-alpine

Question 1 of 2

Lunchbox: container kedai

  • Inside the lunchbox (writable layer)/app/today.txt

Outside the lunchbox

  • Locker kedai-data (volume)/data/orders.db

Next, you run:

Throw the container away and start a new one.

$ docker rm -f kedai$ docker run -d --name kedai -v kedai-data:/data nginx:1.30-alpine

Pick Stays or Gone for every file first.

Try it

Throw db away. Start it again with the same locker. Then look at the orders.

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • dbrunning
    from postgres:18-alpine pgdata → /var/lib/postgresql

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB
  • postgres:18-alpine9 layers · 433MB · in use

Lockers (volumes)

  • pgdata24 files · used by db

Check

Which command deletes the orders for good?

You did it!

You can now keep data in a locker that outlives the container.

Go deeper (optional)

Real error: volume is in use. Plain words: remove the box first. Without -v, Postgres gets a nameless locker. A new box gets a new one, so docker volume ls collects leftovers.

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • dbrunning
    from postgres:18-alpine pgdata → /var/lib/postgresql

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB
  • postgres:18-alpine9 layers · 433MB · in use

Lockers (volumes)

  • pgdata24 files · used by db

Real rep (optional), on your own Docker:

Real mission

  1. docker volume create practice-data
  2. docker run --rm -v practice-data:/data alpine:3.22 sh -c "echo hi > /data/note.txt"
  3. docker run --rm -v practice-data:/data alpine:3.22 cat /data/note.txt
  4. docker volume rm practice-data

0 of 4 steps done · Your ticks are saved.