Docker Volumes with Docker Compose

Let's now look at using Docker Volumes when working with Docker Compose. We will use the same Node.js application example.

Problem We Had Earlier

We noticed earlier that when we stop and restart our MongoDB container, all data in the MongoDB database is lost.

To solve this, we will now add Docker Volumes to our mongodb.yaml file to bring persistence.

Updated Docker Compose File

version: '3.8'

services:
  mongo:
    image: mongo
    ports:
      - 27017:27017
    volumes:
      - /Users/your-username/Desktop/data:/data/db
    environment:
      MONGO_INITDB_ROOT_USERNAME: admin
      MONGO_INITDB_ROOT_PASSWORD: qwerty

  mongo-express:
    image: mongo-express
    ports:
      - 8081:8081
    environment:
      ME_CONFIG_MONGODB_ADMINUSERNAME: admin
      ME_CONFIG_MONGODB_ADMINPASSWORD: qwerty
      ME_CONFIG_MONGODB_URL: mongodb://admin:qwerty@mongo:27017/

Important Concept — MongoDB Data Directory

According to MongoDB documentation:

/data/db

This is the container directory path where MongoDB stores its data.

We map it to our host machine directory:

/Users/your-username/Desktop/data

So now:

Host Machine ↔ MongoDB Container Storage

How Volume Mapping Works

/Desktop/data (Host Machine)
        ↕
/data/db (Mongo Container)

Running Docker Compose

docker compose -f mongodb.yaml up -d

What Happens Internally?

• Docker creates containers
• Docker mounts the host directory to /data/db
• MongoDB starts using this mapped storage
• Any data written is stored on the host machine

Verifying Volume Setup

After running the command:

• Check your Desktop folder

/Desktop/data

You will see files automatically created by MongoDB.

This confirms:

• Volume is successfully mounted
• Data is being persisted outside the container

Testing Persistence

Step 1 — Open Mongo Express

Visit:

http://localhost:8081

Then:

• Create database → college-db
• Create collection → users
• Add a document

Example:

{
  "email": "john@yahoo.in",
  "username": "JohnDoe",
  "password": "secret"
}

Step 2 — Stop and Remove Containers

docker compose -f mongodb.yaml down

This will:

• Stop all containers
• Remove containers
• Remove Docker network

But ❗ data remains intact

Step 3 — Restart Containers

docker compose -f mongodb.yaml up -d

Step 4 — Verify Data Again

Visit:

http://localhost:8081

Now you will see:

• college-db still exists
• users collection still exists
• Previously added document is still present

Final Result

Even after:

• Stopping containers
• Removing containers
• Recreating containers

The data is still available.

Why This Works

Because data is stored outside the container in a Docker Volume (bind mount):

Container → /data/db → Host Machine Folder

So the container can be destroyed, but the data remains safe.

Key Takeaway

Docker Volumes make databases persistent and production-ready.

Without volumes:

• Data is temporary ❌

With volumes:

• Data survives restarts and deletions ✅

Summary

• MongoDB stores data in /data/db
• We map it to a host directory using volumes
• Docker Compose supports volumes directly
• Data persists even after docker compose down
• This is essential for real-world database applications