Dockerizing a Node.js web app

Note: - If you don’t like sudo then see Giving non-root access

The goal of this example is to show you how you can build your own Docker images from a parent image using a Dockerfile . We will do that by making a simple Node.js hello world web application running on CentOS. You can get the full source code athttps://github.com/enokd/docker-node-hello/.

Create Node.js app

First, create a directory src where all the files would live. Then create a package.json file that describes your app and its dependencies:

{
  "name": "docker-centos-hello",
  "private": true,
  "version": "0.0.1",
  "description": "Node.js Hello world app on CentOS using docker",
  "author": "Daniel Gasienica <[email protected]>",
  "dependencies": {
    "express": "3.2.4"
  }
}

Then, create an index.js file that defines a web app using the Express.js framework:

var express = require('express');

// Constants
var PORT = 8080;

// App
var app = express();
app.get('/', function (req, res) {
  res.send('Hello world\n');
});

app.listen(PORT);
console.log('Running on http://localhost:' + PORT);

In the next steps, we’ll look at how you can run this app inside a CentOS container using Docker. First, you’ll need to build a Docker image of your app.

Creating a Dockerfile

Create an empty file called Dockerfile:

touch Dockerfile

Open the Dockerfile in your favorite text editor

Define the parent image you want to use to build your own image on top of. Here, we’ll use CentOS (tag: centos6) available on the Docker Hub:

FROM    centos:centos6

Since we’re building a Node.js app, you’ll have to install Node.js as well as npm on your CentOS image. Node.js is required to run your app and npm is required to install your app’s dependencies defined in package.json. To install the right package for CentOS, we’ll use the instructions from the Node.js wiki:

# Enable Extra Packages for Enterprise Linux (EPEL) for CentOS
RUN     yum install -y epel-release
# Install Node.js and npm
RUN     yum install -y nodejs npm

Install your app dependencies using the npm binary:

# Install app dependencies
COPY package.json /src/package.json
RUN cd /src; npm install --production

To bundle your app’s source code inside the Docker image, use the COPY instruction:

# Bundle app source
COPY . /src

Your app binds to port 8080 so you’ll use the EXPOSE instruction to have it mapped by the docker daemon:

EXPOSE  8080

Last but not least, define the command to run your app using CMD which defines your runtime, i.e. node, and the path to our app, i.e. src/index.js (see the step where we added the source to the container):

CMD ["node", "/src/index.js"]

Your Dockerfile should now look like this:

FROM    centos:centos6

# Enable Extra Packages for Enterprise Linux (EPEL) for CentOS
RUN     yum install -y epel-release
# Install Node.js and npm
RUN     yum install -y nodejs npm

# Install app dependencies
COPY package.json /src/package.json
RUN cd /src; npm install --production

# Bundle app source
COPY . /src

EXPOSE  8080
CMD ["node", "/src/index.js"]

Building your image

Go to the directory that has your Dockerfile and run the following command to build a Docker image. The -t flag lets you tag your image so it’s easier to find later using the docker images command:

$ docker build -t <your username>/centos-node-hello .

Your image will now be listed by Docker:

$ docker images

# Example
REPOSITORY                          TAG        ID              CREATED
centos                              centos6    539c0211cd76    8 weeks ago
<your username>/centos-node-hello   latest     d64d3505b0d2    2 hours ago

Run the image

Running your image with -d runs the container in detached mode, leaving the container running in the background. The -p flag redirects a public port to a private port in the container. Run the image you previously built:

$ docker run -p 49160:8080 -d <your username>/centos-node-hello

Print the output of your app:

# Get container ID
$ docker ps

# Print app output
$ docker logs <container id>

# Example
Running on http://localhost:8080

Test

To test your app, get the port of your app that Docker mapped:

$ docker ps

# Example
ID            IMAGE                                     COMMAND              ...   PORTS
ecce33b30ebf  <your username>/centos-node-hello:latest  node /src/index.js         49160->8080

In the example above, Docker mapped the 8080 port of the container to 49160.

Now you can call your app using curl (install if needed via: sudo apt-get install curl):

$ curl -i localhost:49160

HTTP/1.1 200 OK
X-Powered-By: Express
Content-Type: text/html; charset=utf-8
Content-Length: 12
Date: Sun, 02 Jun 2013 03:53:22 GMT
Connection: keep-alive

Hello world

If you use Docker Machine on OS X, the port is actually mapped to the Docker host VM, and you should use the following command:

$ curl $(docker-machine ip VM_NAME):49160

We hope this tutorial helped you get up and running with Node.js and CentOS on Docker. You can get the full source code at https://github.com/enokd/docker-node-hello/.