# 🚀 From “Why is nothing working?” to “Now I Understand Everything” — My Docker Compose + MongoDB + Volume Learning Journey

Prajwal P — Tue, 14 Apr 2026 12:53:58 +0000

``## 📌 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

Docker #DockerCompose #DockerVolumes #DevOps #MongoDB #AWS #NodeJS #Debugging #LearningByDoing #30DaysofAWSTerraform

DEV Community: Prajwal P