Containerizing a Node.js and Express App¶

Let's see how we can containerize a Node.js and Express application.

Step 1: Create Application Using Node.js and Express¶

The first step is to create an application using Node.js and Express framework.

Create server.js and package.json files as follows:

server.js package.json

const express = require('express')
const app = express()
var os = require('os');

const PORT = process.env.PORT || 5000;

// Returns the hostname, version and other app details
app.get('/', function (_req, res) {
let host = os.hostname();
data = {
    Host: host,
    Version: "v1"
}
res.status(200).json(data);
});

app.get('/health', function(_req, res) {
res.status(200).send('Healthy');
});

app.get('/random', function(_req, res) {
randomNumber = Math.floor(Math.random() * 10) + 1;
console.log(`The random number generated is: ${randomNumber}`);
res.status(200).send(randomNumber.toString());
});

app.listen(PORT, () => {
console.log(`Server is running on port ${PORT}`);
});

{
    "dependencies": {
        "express": "^4.18.2"
    }
}

Here's what your folder structure should look like:

|-- my-folder
│   |-- package.json
│   |-- server.js

The above Express app has the following routes:

GET /
GET /health
GET /random

What does each route return?

GET / returns a JSON object containing Host and Version.
GET /health returns the health status.
GET /random returns a random integer between 1 and 10

Step 2: Test the Application Locally¶

Go to the folder containing package.json and run the following commands:

# Install npm packages
npm install

# Start the express server
node server.js

Note

You must have Node.js installed on your local machine to run the above commands.

If there are no errors, your Express application will begin serving requests on the specified port, which in our case is 5000.

Verify if the endpoints are working as expected:

curl localhost:5000
curl localhost:5000/health
curl localhost:5000/random

You can also access the endpoints mentioned above directly from your browser.

Note

When you run npm install, it generates a package-lock.json file to record and lock down the specific versions of installed packages and their dependencies. This ensures consistency in package versions across different environments and collaborations.

Step 3: Create Dockerfile¶

Now, let's containerize our application. To do that, we first need to create a Dockerfile with instructions on how to build the image.

Create the Dockerfile as follows:

Dockerfile

FROM node:18

# Create app directory
WORKDIR /usr/src/app

COPY package*.json ./

RUN npm install

# Bundle app source
COPY . .

EXPOSE 5000

CMD [ "node", "server.js" ]

Brief explanation of each command in Dockerfile

FROM node:18: Specifies the base image as Node.js version 18, which serves as the foundation for your image.
WORKDIR /usr/src/app: Sets the working directory inside the container to /usr/src/app.
COPY package*.json ./: Copies the package.json and package-lock.json (if present) from the local directory into the container's working directory.
RUN npm install: Runs the npm install command inside the container to install the application's dependencies.
COPY . .: Copies the contents of the local directory (your Node.js application code) into the container's working directory.
EXPOSE 5000: Informs Docker that the container will listen on port 5000, though it doesn't actually publish the port to the host.
CMD [ "node", "server.js" ]: Specifies the command that will be executed when the container starts. In this case, it runs your Node.js application using the server.js script.

These steps are essential for building a Docker image for your Node.js application, including setting up the environment, copying code and dependencies, and defining how to run the application within the container.

Here's what your folder structure should look like at this point:

|-- my-folder
│   |-- package.json
│   |-- server.js
|   |-- Dockerfile

Step 4: Build the Docker Image¶

With the application code and Dockerfile prepared, we are now ready to build the Docker image for our application.

Build the Docker image as follows:

# Command template
docker build -t <image-name>:<image-tag> <path-to-Dockerfile>

# Actual command
docker build -t my-node-express-image:v1 .

The above command will build an image named my-node-express-image with tag v1.

Step 5: Verify the Image¶

List images and verify if the newly created image is present:

docker images

Look for the image name and the tag we specified while building the image.

Step 6: Run a Container From the Image¶

Now that we have the image ready, let's create a container from it and test the application.

docker run -d --name my-node-express-container -p 5001:5000 my-node-express-image:v1

What does the above command do?

It runs a container named my-node-express-container in detached mode, using the image my-node-express-image:v1.
Additionally, it exposes port 5000 of the container to port 5001 on the host

Step 7: Test the Application¶

Test the application from within the container:

Start a shell session inside the container

docker exec -it my-node-express-container bash

Access the endpoints

curl localhost:5000
curl localhost:5000/health
curl localhost:5000/random

Test the application from host computer (Outside the container):

Open any browser on your host computer and hit the following endpoints to verify if port mapping is working as expected.
```
localhost:5001
localhost:5001/health
localhost:5001/random
```

Step 8: View Container Logs¶

# Command template
docker logs -f <container-id/container-name>

# Actual command
docker logs -f my-node-express-container

Step 9: Clean Up¶

# Stop the container
docker stop my-node-express-container

# Remove the container
docker rm my-node-express-container