⚡ Flask + Database in Docker | Debugging Networking Issues

🎯 Why Docker Networking Matters

Every DevOps engineer eventually faces this:

The app container is running.
The database container is running.
But the app just can’t connect.

Instead of “Hello World,” you see an ugly connection refused error.

This isn’t just a local issue — it’s one of the most common production outages caused by container networking mistakes.

Knowing how Docker networking works — and how to debug it — separates engineers who panic from those who solve outages in minutes.

▶️ What You’ll Learn in This Video

In this step-by-step hands-on demo, I break down: https://youtu.be/PaUmQzT0ocs

📌 Basics of Container Networking

• Why localhost fails inside containers

• How containers are isolated by default

📌 Hands-On Setup

• Running Postgres in Docker

• Updating Flask app to connect with psycopg2

• Running both containers and hitting the ❌ error

📌 Debugging

• Using docker logs to confirm the connection issue

• Understanding why localhost only points to the same container

📌 The Fix

• Creating a Docker network with docker network create

• Running both Flask + Postgres in the same network

• Using the container name as hostname (flask-db)

📌 Best Practices

• Never use localhost between containers

• Always create user-defined networks

• Use container names as hostnames

• For production, use docker-compose or Kubernetes

👉 Watch the full video here: [YouTube Link]
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🛠 Takeaway Example Command

❓ How do you run Flask + Postgres in the same Docker network?

✅ Answer:

docker network create flask-net

docker run -d --name flask-db --network flask-net \
  -e POSTGRES_USER=flaskuser \
  -e POSTGRES_PASSWORD=flaskpass \
  -e POSTGRES_DB=flaskdb \
  postgres:14

docker run -d --name flask-v3 --network flask-net -p 5000:5000 flask-app:v3

💡 Inside this network, you simply set host="flask-db" in Flask.
Docker’s internal DNS automatically resolves the container name — no IPs needed.

💡 Why This Guide Stands Out

Real-world focus → I don’t just explain networking; I simulate the localhost outage.
Practical → Flask + Postgres connected exactly like in real projects.
Debug-driven → Logs + container isolation explained step by step.

This isn’t theory — it’s production-grade troubleshooting made reproducible.

👋 Final Note

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Every week I share real DevOps outages, interview prep, and hands-on labs you can reproduce — so you’ll never be caught off guard in production.

— Arbaz
📺 YouTube: Learn with DevOps Engineer
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