Cheat sheet & self-check

Pick an answer to see if you got it, and why.

1. Q1. In a Compose project, how does the api service reach the db service?

   1. ABy the db container's IP
   2. BBy the service name db, on the project network Compose creates
   3. COnly through a published port
   4. DThrough localhost
   Show answer

   B. Compose creates a network per project and every service is reachable by its service name through Docker's embedded DNS.

   From lesson 04 · Docker Compose basics

2. Q2. What does `docker compose down` remove by default?

   1. AContainers, networks and named volumes
   2. BContainers and the project's networks; named volumes are kept
   3. COnly containers
   4. DEverything including images
   Show answer

   B. Volumes are kept so your data survives. Add -v to delete them too, and --rmi to remove images.

   From lesson 04 · Docker Compose basics

3. Q3. What is the difference between `docker-compose` and `docker compose`?

   1. AThey are identical binaries
   2. Bdocker-compose is the old standalone v1 tool (no longer maintained); docker compose is the v2 plugin you should use
   3. Cdocker compose is for Swarm only
   4. Ddocker-compose is newer
   Show answer

   B. Compose v1 (Python, docker-compose) reached end of life in 2023. Compose v2 ships as a Docker CLI plugin and is invoked as docker compose.

   From lesson 04 · Docker Compose basics

4. Q4. Where does Compose read ${POSTGRES_VERSION} from when the file says image: postgres:${POSTGRES_VERSION}?

   1. AOnly from the container
   2. BFrom your shell environment or the .env file next to compose.yaml
   3. CFrom daemon.json
   4. DFrom the image labels
   Show answer

   B. Variable substitution happens on your machine when Compose reads the file: shell environment first, then the project's .env file. Check the result with docker compose config.

   From lesson 04 · Docker Compose basics

5. Q5. You changed the api image tag in compose.yaml. What makes the change take effect?

   1. Adocker compose restart api
   2. Bdocker compose up -d (it recreates only the services whose config changed)
   3. CNothing, it reloads automatically
   4. Ddocker compose down -v
   Show answer

   B. restart just restarts the existing container with its old config. up -d compares the desired config with what's running and recreates changed services.

   From lesson 04 · Docker Compose basics

6. Q6. api starts before PostgreSQL accepts connections and crashes. What fixes the ordering?

   1. Adepends_on: [db] alone
   2. BA healthcheck on db and depends_on: db: condition: service_healthy on api
   3. Crestart: always on db
   4. DPutting db first in the file
   Show answer

   B. Plain depends_on waits only until the db container has started, not until Postgres is ready. A healthcheck plus condition: service_healthy waits for readiness.

   From lesson 05 · Compose for real environments

7. Q7. What does the restart policy unless-stopped do?

   1. ANever restarts
   2. BRestarts the container if it exits and when the daemon starts, unless you stopped it yourself
   3. CRestarts only on exit code 0
   4. DRestarts other services too
   Show answer

   B. unless-stopped restarts after crashes and after a host reboot, but respects a manual docker stop.

   From lesson 05 · Compose for real environments

8. Q8. Why use Compose secrets instead of environment: DB_PASSWORD=...?

   1. ASecrets are encrypted in Compose on a single host
   2. BThe value is mounted as a file at /run/secrets/<name> instead of showing up in docker inspect and the process environment
   3. CEnvironment variables don't work with Postgres
   4. DSecrets are faster
   Show answer

   B. Environment variables are visible in docker inspect, in /proc/<pid>/environ and are often logged by crash handlers. File-based secrets keep the value out of all of those; many images accept *_FILE variables for this.

   From lesson 05 · Compose for real environments

9. Q9. You want a pgAdmin container only when debugging, not in normal runs. Which feature fits?

   1. AA separate project
   2. Bprofiles: [debug] on that service
   3. Crestart: no
   4. DA second network
   Show answer

   B. Services with a profile start only when that profile is enabled with --profile debug (or COMPOSE_PROFILES).

   From lesson 05 · Compose for real environments