Docker Networking, Volumes & Health Checks — Challenge

The goal#

Achieve the same outcome as Docker Networking, Volumes & Health Checks, from an empty starting point, without the steps.

The application connects fine on your machine and fails in the pipeline with 'connection refused'. It starts before the database is ready, and everything it wrote last night is gone.

These three problems are the same three you will meet again as Services, PersistentVolumes and readiness probes — which is why this lab sits before Kubernetes.

What must be true when you are done#

• The application reaches PostgreSQL and Redis by service name, with no IP addresses anywhere.
• Database data survives docker compose down and returns after up.
• The application waits for the database to be ready, not merely started.
• You can explain why two containers on different networks cannot see each other.

Rules#

• Do not open the guided lab until you are finished, or until the same problem has held you up for 20 minutes.
• Documentation is allowed and encouraged.
• Verify every criterion with a command whose output you can read.

If you get stuck#

1. What did you expect, exactly?
2. What happened instead — the error text, not a paraphrase?
3. Which layer is that error from?
4. What is the smallest command that proves the layer below is fine?