Docker Compose Debugging Guide: Fixing Real-World Container Issues (Flask + MySQL)
Introduction
After building a 2-tier application using Docker Compose, I realized something important:
Running containers is easy. Debugging them is where real learning begins.
In this blog, I’m not showing a perfect setup.
Instead, I’ll show:
Real problems I faced
Why they happened
How I debugged them step-by-step
This is the kind of knowledge that actually prepares you for real-world DevOps.
🎯 What You Will Learn
How to debug Docker containers
How to identify root cause of failures
Common Docker Compose issues
Practical debugging commands used in real systems
🧠 DEBUGGING MINDSET
Before jumping into commands, always ask:
1. Is the container running?
2. Is the application running inside it?
3. Can containers talk to each other?
4. Is the service accessible externally?
🔧 REAL ISSUES & FIXES
❌ Issue 1: Container Keeps Restarting
Problem:
docker ps
Output:
Restarting (1)
Debug:
docker logs <container>
Cause:
Application crash
Dependency failure
Fix:
Check logs
Fix error inside app
❌ Issue 2: Flask Cannot Connect to MySQL
Error:
Can't connect to server on 'mysql'
Cause:
MySQL not ready yet
Wrong host
Fix:
Use service name → mysql
👉 NOT localhost ❌
❌ Issue 3: Containers Running But App Not Accessible
Debug:
curl localhost:5000
Causes:
Port not exposed
Security group issue
Wrong port mapping
Fix:
Check
-pflagOpen port in AWS
❌ Issue 4: Docker Compose YAML Errors
Error:
Additional property not allowed
Cause:
Wrong indentation
Typo in keys
Fix:
Use correct YAML structure
Validate file before running
❌ Issue 5: Data Not Persisting
Cause:
- No volume attached
Fix:
volumes:
- mysql-data:/var/lib/mysql
❌ Issue 6: Disk Space Full
Debug:
docker system df
Fix:
docker system prune -f
❌ Issue 7: Image Version Issues
Problem:
Using:
image: mysql
Fix:
image: mysql:5.7
👉 Always pin versions
🧰 MOST IMPORTANT DEBUGGING COMMANDS
docker ps
docker logs <container>
docker exec -it <container> bash
docker inspect <container>
docker network ls
docker volume ls
🧠 KEY LEARNINGS
Debugging > coding
Logs tell everything
Containers are isolated systems
Always verify step-by-step
🚀 REAL-WORLD INSIGHT
In production, issues are rarely obvious.
You must: 👉 Observe 👉 Analyze 👉 Fix
This project helped me understand how real systems behave under failure.
🚀 WHAT’S NEXT
Add Nginx (reverse proxy)
Use Gunicorn (production server)
Deploy using ECS / Kubernetes
💡 FINAL THOUGHT
“Anyone can run a container. Engineers debug them.”
If you're learning DevOps: 👉 Break your setup 👉 Fix it 👉 Repeat
That’s the real path to mastery.
🙌 Connect With Me
More real-world DevOps content coming soon 🚀
