This chapter is a challenge. One project, ~/signup, is a Flask app and a Postgres database run with Compose. It is broken in several ways. Each lab fixes part of it with skills from chapters 1-8. The lessons say where to look, not what to type.

The first step is a survey. Change nothing until you know which service fails and why.

1. Read     README.md, compose.yaml, Dockerfile, .env     what should run
2. State    docker compose ps -a                          what does run
3. Logs     docker compose logs [SERVICE]                 why it stopped
4. Config   docker compose config                         what Compose resolved
5. Save     docker compose logs > file                    evidence for later

What each command answers

Read the state

cd ~/signup
docker compose ps -a

This is the target, the stack as it looks once this chapter is done:

NAME           IMAGE         COMMAND                  SERVICE   CREATED         STATUS                   PORTS
signup-db-1    postgres:16   "docker-entrypoint.s…"   db        4 seconds ago   Up 4 seconds (healthy)   5432/tcp
signup-web-1   signup-web    "python app.py"          web       2 seconds ago   Up 2 seconds             0.0.0.0:8000->5000/tcp

For a stopped container, STATUS reads Exited (N) with its exit code. COMMAND is the command the container ran. PORTS is empty for a container that is not running, so read the intended ports from compose.yaml.

Read the logs

Only the last lines of a crashed service usually matter. The first error is the one to fix. A later error may only appear once the first is gone.

docker compose logs web
docker compose logs db | tail -5

Compare config with code

Wrong values often sit in two files that disagree. Note each pair: