:title: Running a Node.js app on CentOS
:description: Installing and running a Node.js app on CentOS
:keywords: docker, example, package installation, node, centos

.. _nodejs_web_app:

Node.js Web App
===============

.. include:: example_header.inc

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 at
https://github.com/gasi/docker-node-hello.

Create Node.js app
++++++++++++++++++

First, create a ``package.json`` file that describes your app and its
dependencies:

.. code-block:: json

    {
      "name": "docker-centos-hello",
      "private": true,
      "version": "0.0.1",
      "description": "Node.js Hello World app on CentOS using docker",
      "author": "Daniel Gasienica <daniel@gasienica.ch>",
      "dependencies": {
        "express": "3.2.4"
      }
    }

Then, create an ``index.js`` file that defines a web app using the
`Express.js <http://expressjs.com/>`_ framework:

.. code-block:: javascript

    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``:

.. code-block:: bash

    touch Dockerfile

Open the ``Dockerfile`` in your favorite text editor and add the following line
that defines the version of Docker the image requires to build
(this example uses Docker 0.3.4):

.. code-block:: bash

    # DOCKER-VERSION 0.3.4

Next, define the parent image you want to use to build your own image on top of.
Here, we’ll use `CentOS <https://index.docker.io/_/centos/>`_ (tag: ``6.4``)
available on the `Docker index`_:

.. code-block:: bash

    FROM    centos:6.4

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 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`_:

.. code-block:: bash

    # Enable EPEL for Node.js
    RUN     rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
    # Install Node.js and npm
    RUN     yum install -y npm

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

.. code-block:: bash

    # Bundle app source
    ADD . /src

Install your app dependencies using the ``npm`` binary:

.. code-block:: bash

    # Install app dependencies
    RUN cd /src; npm install

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

.. code-block:: bash

    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):

.. code-block:: bash

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

Your ``Dockerfile`` should now look like this:

.. code-block:: bash


    # DOCKER-VERSION 0.3.4
    FROM    centos:6.4

    # Enable EPEL for Node.js
    RUN     rpm -Uvh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
    # Install Node.js and npm
    RUN     yum install -y npm

    # Bundle app source
    ADD . /src
    # Install app dependencies
    RUN cd /src; npm install

    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 let’s you tag your
image so it’s easier to find later using the ``docker images``
command:

.. code-block:: bash

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

Your image will now be listed by Docker:

.. code-block:: bash

    sudo docker images

    > # Example
    > REPOSITORY                 TAG       ID              CREATED
    > centos                     6.4       539c0211cd76    8 weeks ago
    > gasi/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:

.. code-block:: bash

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

Print the output of your app:

.. code-block:: bash

    # Get container ID
    sudo docker ps

    # Print app output
    sudo docker logs <container id>

    > # Example
    > Running on http://localhost:8080


Test
++++

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

.. code-block:: bash

    sudo docker ps

    > # Example
    > ID            IMAGE                          COMMAND              ...   PORTS
    > ecce33b30ebf  gasi/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``):

.. code-block:: bash

    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

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/gasi/docker-node-hello.

Continue to :ref:`running_redis_service`.


.. _Node.js wiki: https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager#rhelcentosscientific-linux-6
.. _docker index: https://index.docker.io/