Level 3 exam — Docker · GUIDINEE.COM

Exam — fresh questions

12 questions to validate Docker Compose: compose file and YAML, up/down/logs driving, wired mini-application, variables and profiles, breakage-free troubleshooting.
Answer everything, then click Check: your score and explanations will appear.

3.1 · Recall

1. What does a service declare in a compose.yml?

The office wifi password Image (or build), ports, variables, volumes and network The machine boot time The physical server brand

One service = image + ports + env + volumes + network.

3.1 · Application

2. Idir writes ports: [8080:80] without quotes and the stack fails. Why?

Nginx refuses ports below 9000 Compose requires ports in letters YAML reads 8080:80 as a sexagesimal number: use "8080:80" Port 8080 is forbidden by Docker

Always quote ports in YAML: 80% of errors come from there.

3.2 · Recall

3. What is the difference between down and down -v?

None, they are aliases down keeps volumes, -v also destroys them (reset) down destroys everything, -v does nothing -v simply speeds up the stop

down alone = safe stop; -v = reset wiping volumes.

3.2 · Application

4. Idir edits his code, reruns up -d, but the old code still runs. What to do?

Delete the project's volumes Change all compose ports Reinstall Docker on the machine Relaunch with up -d --build to rebuild the image

Without --build, Compose reuses the existing image.

3.3 · Application

5. Which ports does Idir publish on his api + database + adminer mock-up?

Api and adminer (dev); the database stays internal to the network Only the database, to fill it fast No port: everything via exec All ports of all services, just in case

Minimal surface: only what is needed goes out, the database stays internal.

3.3 · Application

6. Idir's api crashes on the first up then passes on the second. Diagnosis and fix?

The network is saturated: wait for the night The code is wrong: rewrite the api The database starts more slowly: add a retry with delay at startup The image is corrupted: delete everything

depends_on orders without waiting: retry absorbs startup.

3.4 · Recall

7. What does ${PORT:-3000} mean in a compose.yml?

A comment ignored by Compose The PORT variable, or 3000 by default if missing Three ports: variable, default and 3000 Port 3000 minus a PORT variable

Interpolation: PORT value, else the prod-safe default 3000.

3.4 · Application

8. Idir wants adminer in dev but never in prod, without duplicating the file. He uses…

The same service copied twice Adminer published everywhere for simplicity profiles: [dev] + --profile dev (opt-in, lean by default) A sticky note "do not run in prod"

Opt-in profile: absent by default, explicitly enabled in dev.

3.5 · Application

9. "port already allocated" at up: Idir checks and finds an old manual run. He…

Deletes all volumes and restarts Gives up the project and changes business port Changes the (right) container port in the image Moves the (left) host port or cleans the squatter, then up -d

Conflict on the host: left moved or occupant freed.

3.3 · Arbitration

10. For prod, a colleague suggests the URL and password hard-coded in the committed compose. Decision?

Refuse: variables + uncommitted .env (managed secret in prod), unpublished database Accept: it is easier to reread Accept if the password is long Put secrets in the image instead

Never a hard-coded committed secret: variable + managed secret.

3.4 · Arbitration

11. Two copied compose.yml files (dev/prod) diverge and the dev console nearly went to prod. Decision?

Copy dev to prod every Friday Delete prod to simplify Add a third file for staging Merge into one base + -f override + dev profile, versioned .env.example

One base, values per env, differences in overrides.

3.5 · Arbitration

12. Broken stack before a demo: a colleague suggests down -v "to restart clean". Decision?

Refuse and diagnose: ps, service logs, fix the file, targeted up -d Accept: razing is always faster Accept if the demo is in less than an hour Raze then restore the database from memory

Never -v in panic: diagnose first, recreate targeted.

What's next?

Score ≥ 80%: move to Level 4 — registries, healthchecks, advanced networks and volume backup (harden what Compose starts).
Score 50-79%: review the missed sections (YAML and config in 3.1, down cycle in 3.2, names and retry in 3.3, profiles in 3.4, diagnosis in 3.5) then retake the test.
Score < 50%: replay each 48h challenge in order — validated stack, lossless cycle, persisted mini-app, unified base, failure fixed without -v.