``## 📌 Introduction
This is not just a blog.
This is a complete journey of how I:
- Deployed a multi-container application on EC2
- Debugged Docker networking issues
- Fixed backend and frontend problems
- Understood MongoDB behavior
- And finally learned Docker volumes deeply
I didn’t learn this from theory.
I learned it by:
`text id="k5l1tt"
breaking things → debugging → understanding → fixing
`
🏗️ Project Architecture
My setup had 3 containers:
- Node.js App (Frontend + Backend)
- MongoDB Database
- Mongo Express (DB UI)
All managed using Docker Compose.
⚙️ Step 1: Initial Setup
I ran:
`id="g3f8t0"
docker compose up -d
`
Everything started successfully.
When I opened:
`id="v34txs"
http://<EC2-IP>:3000
`
👉 UI loaded
👉 But no data was showing from MongoDB ❌
❗ Problem 1: No Data in UI
`text id="x0k3qv"
Data from MongoDB:
`
👉 Empty
🔍 Debugging Step 1: Browser Network Tab
I checked DevTools → Network
`id="zk6v0l"
/fetch-data → FAILED ❌
`
❗ Problem 2: Wrong API URL
Frontend code:
`id="s2o64r"
fetch("http://localhost:3000/fetch-data")
`
👉 Wrong ❌
Why?
- Browser → localhost = my laptop
- App → running on EC2
✅ Fix 1: Use Relative Path
`id="kz8yqf"
fetch("/fetch-data")
`
🔍 Debugging Step 2: API Response
Now API returned:
`id="r1w9cz"
200 OK
`
But response:
`id="2l6c9c"
{}
`
❗ Problem 3: MongoDB Empty
I checked MongoDB:
`id="s1vxz1"
show dbs
use my-db
show collections
`
👉 No collections ❌
✅ Fix 2: Insert Data
`id="7x6pkn"
db.my-collection.insertOne({
data: "some dynamic data loaded from DB"
})
`
🔍 Debugging Step 3: Still Not Working
Even after inserting data:
👉 UI still empty ❌
❗ Problem 4: Backend Not Fetching Data
Backend was not:
- Selecting DB properly
- Querying collection
- Returning response
✅ Fix 3: Backend Logic
`js id="v9u5ah"
const db = client.db("my-db");
db.collection("my-collection").findOne({}, function (err, result) {
res.json({
data: result.data
});
});
`
🔍 Debugging Step 4: Still Empty UI
Now:
- API working ✅
- DB has data ✅
- Backend correct ✅
👉 UI still empty ❌
❗ Problem 5: Frontend Bug
Mistake:
`id="n3x7ml"
fetch('/fetch-data');
const jsonResponse = await response.json();
`
👉 response undefined ❌
✅ Fix 4: Correct Fetch
`js id="9z1qj8"
const response = await fetch('/fetch-data');
const jsonResponse = await response.json();
`
❗ Problem 6: DOM Not Loaded
`id="3u7q3x"
document.getElementById('dynamic')
`
👉 ran too early ❌
✅ Fix 5: window.onload
`js id="y5u2p0"
window.onload = async function () {
const response = await fetch('/fetch-data');
const jsonResponse = await response.json();
document.getElementById('dynamic').textContent = jsonResponse.data;
};
`
❗ Problem 7: Container Not Updating
Changes not reflecting ❌
✅ Fix 6: Rebuild + Recreate
`id="0j6yxn"
docker compose down
docker compose up --build --force-recreate -d
`
🎉 Final Result
`id="l8m1n7"
Data from MongoDB: some dynamic data loaded from DB
`
💾 The Biggest Learning — Docker Volumes
After everything worked, I discovered a critical issue.
When I ran:
`id="w5n6t2"
docker compose down -v
`
👉 My database data was gone 💥
❗ Why Data Was Lost
Because:
- Containers are temporary
- Data inside container is NOT persistent
🔥 What Is a Docker Volume?
A Docker volume is persistent storage mounted directly into a container.
🧠 My Initial Wrong Thinking
“Volume is just a link”
❌ WRONG
✅ Correct Understanding
Volume is directly mounted into container
📦 MongoDB Data Path
`id="d7n9rs"
/data/db
`
🔗 What Docker Does
`yaml id="3c5y7a"
volumes:
- mongo-data:/data/db
`
👉 Docker:
`text id="9n4qkz"
mounts volume → /data/db
`
📊 Visual Flow
`id="y0h4b8"
[ Container ]
│
▼
/data/db
│
▼
[ Docker Volume ]
│
▼
[ Host Storage ]
`
🔁 What Happens on Recreate
`id="5v1q6k"
docker compose down
docker compose up
`
- Container deleted ❌
- Volume remains ✅
- New container created ✅
- Volume mounted again ✅
👉 MongoDB reads existing data ✅
💡 Real-world Analogy
- Container = Laptop 💻
- Volume = External Hard Disk 💾
⚠️ Critical Command
`id="3m9q8x"
docker compose down -v
`
👉 Deletes volume ❌
👉 Deletes data 💥
✅ Safe Command
`id="n8k2z7"
docker compose down
`
👉 Keeps data ✅
🧠 Final Understanding
- Container = temporary
- Volume = persistent
- Data lives in volume
- Container just uses it
🎯 Final DevOps Insight
“Containers are ephemeral, but data must be persistent. Docker volumes ensure data survives container lifecycle.”
🚀 What I Learned
This project taught me:
- Docker Compose networking
- Service name vs localhost
- EC2 deployment
- MongoDB authentication
- Backend API debugging
- Frontend async handling
- Docker rebuild vs recreate
- Docker volumes (most important 🔥)
🧠 Interview Answer
“I deployed a multi-container app using Docker Compose on EC2, debugged networking and application issues, and implemented Docker volumes to ensure persistent database storage across container restarts.”
📢 Connect with me
- LinkedIn: https://www.linkedin.com/in/prajwalputtaraju/
- GitHub: https://github.com/Prajwal8651
