11. Docker Volumes Mapping

🎯 Objective

Create and test an Nginx container on App Server 3 in Stratos DC using Docker volume mapping.

Requirements:

Final mapping:

Host                         Container

/opt/data      ───────────►  /tmp
    │                         │
    └── sample.txt            └── sample.txt

🧠 Concept

A Docker volume/bind mount allows a directory from the Docker host to be mounted inside a container.

Without a mount:

Docker Host                 Container

/opt/data                   /tmp
    │                         │
    └── sample.txt            └── Independent storage

With the required mapping:

Docker Host                 Container

/opt/data  ───────────────► /tmp
    │                         │
    └── sample.txt ────────► sample.txt

Changes made inside the mounted directory are reflected on the host because both paths refer to the same mounted storage.

🔍 Step 1: Check Docker

Check the running containers:

docker ps

You can also check whether the Nginx image already exists:

docker image ls

📥 Step 2: Pull the Nginx Image

Pull the latest Nginx image:

docker pull nginx:latest

You can also use:

docker pull nginx

When no tag is specified, Docker uses latest by default.

Verify:

docker image ls nginx

You should see something similar to:

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
nginx        latest    xxxxxxxxxxxx   ...           ...

📄 Step 3: Verify the Source File

The task states that sample.txt already exists under:

/tmp

Check it:

ls -l /tmp/sample.txt

Optionally inspect its contents:

cat /tmp/sample.txt

📂 Step 4: Create the Host Directory

The required host directory is:

/opt/data

Make sure it exists:

sudo mkdir -p /opt/data

Verify:

ls -ld /opt/data

📋 Step 5: Copy sample.txt

Copy the existing file:

sudo cp /tmp/sample.txt /opt/data/

Verify:

ls -l /opt/data

You should see:

sample.txt

Check the copied file:

cat /opt/data/sample.txt

The host now looks like:

/opt/
 └── data/
      └── sample.txt

🚀 Step 6: Create the games Container

Create the container using the Nginx image:

docker run -d \
  --name games \
  -v /opt/data:/tmp \
  nginx:latest

If Docker requires elevated privileges:

sudo docker run -d \
  --name games \
  -v /opt/data:/tmp \
  nginx:latest

🧩 Command Breakdown

Detached Mode

-d

Runs the container in the background.

This is important because the requirement says the container must remain in a running state.

Container Name

--name games

Sets the container name to:

games

Volume Mapping

-v /opt/data:/tmp

The syntax is:

-v <host_path>:<container_path>

Therefore:

Host Path      → /opt/data
Container Path → /tmp

The mapping becomes:

/opt/data:/tmp

Docker Image

nginx:latest

Creates the container from the Nginx image.

🧠 How the Volume Mapping Works

On the host:

/opt/data/
    │
    └── sample.txt

The directory is mounted into:

Container
   │
   └── /tmp/
         │
         └── sample.txt

So:

App Server 3
│
├── /tmp/sample.txt
│
│      cp
│      ↓
│
└── /opt/data/
       │
       └── sample.txt
              │
              │ Docker bind mount
              ▼
        games container
              │
              └── /tmp/sample.txt

🔍 Step 7: Verify the Container Is Running

Run:

docker ps

You should see:

NAMES
games

with a running status such as:

Up ...

You can also specifically check:

docker ps --filter name=games

The container must remain running to satisfy the task.

🔎 Step 8: Verify the Mount

Inspect the container:

docker inspect games

Look for the Mounts section.

It should show information equivalent to:

Source      → /opt/data
Destination → /tmp

This confirms:

/opt/data
    ↓
Docker Mount
    ↓
/tmp

📄 Step 9: Verify sample.txt Inside the Container

There is no need to manually enter the container.

Use docker exec:

docker exec games ls -l /tmp

You should see:

sample.txt

Then:

docker exec games cat /tmp/sample.txt

The content should match:

cat /opt/data/sample.txt

This proves that the volume mapping works.

🧪 Understanding the Persistence

Because /tmp inside the container is mapped to /opt/data on the host:

Container
/tmp/sample.txt
      │
      │ Same mounted storage
      ▼
Host
/opt/data/sample.txt

For example, if you create:

docker exec games touch /tmp/test.txt

the host should also see:

ls /opt/data

sample.txt
test.txt

This demonstrates that data in the mounted path is stored through the host directory rather than only in the container's writable layer.

🆚 Host Directory vs Container Directory

For this task:

The file first moves:

/tmp/sample.txt
      │
      │ cp
      ▼
/opt/data/sample.txt

Then Docker exposes it inside the container:

/opt/data/sample.txt
      │
      │ -v /opt/data:/tmp
      ▼
/tmp/sample.txt

⚠️ Important Notes

⚙️ Complete Workflow

Pull Nginx:

docker pull nginx:latest

Check the source file:

ls -l /tmp/sample.txt

Create the host directory:

sudo mkdir -p /opt/data

Copy the file:

sudo cp /tmp/sample.txt /opt/data/

Verify:

ls -l /opt/data

Create the container:

docker run -d \
  --name games \
  -v /opt/data:/tmp \
  nginx:latest

Check the container:

docker ps

Verify the mounted file:

docker exec games ls -l /tmp

docker exec games cat /tmp/sample.txt

Inspect the mount:

docker inspect games

🧪 Validation Checklist

📌 Summary

Pull the image:

docker pull nginx:latest

Prepare the host volume:

sudo mkdir -p /opt/data
sudo cp /tmp/sample.txt /opt/data/

Create the container:

docker run -d \
  --name games \
  -v /opt/data:/tmp \
  nginx:latest

Verify:

docker ps
docker exec games ls -l /tmp
docker exec games cat /tmp/sample.txt

The complete flow is:

App Server 3
│
├── /tmp/sample.txt
│       │
│       │ cp
│       ▼
│
├── /opt/data/
│       └── sample.txt
│              │
│              │ -v /opt/data:/tmp
│              ▼
│
└── Docker
      │
      └── games
            │
            └── /tmp/
                  └── sample.txt ✅

The key command is:

docker run -d --name games -v /opt/data:/tmp nginx:latest

> Docker bind mounts map a host directory into a container. Here, /opt/data on App Server 3 is mounted at /tmp inside the games container, allowing sample.txt to be accessed through both locations while keeping the Nginx container running.