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Docker is an open-source software container management system. It allows you to create an isolated, self-contained environment to run your application. In this article we will walk thought steps needed to create a Docker image containing a Django application and demonstrate how to run it and access the container.

Benefits of Docker

So why is this Docker thing so popular these days? The basic answer is that it makes your applications portable. Django applications can make use of Python’s virtualenv to create isolated environments, but if some of your apps use Python 2 and some use Python 3, you may need to install a whole slew of additional libraries on your host server.

With Docker on the other hand, each container includes a specific version of the Linux kernel and all other dependencies of your app. You install dependencies such as libjpeg in the container, not on the host, so if your apps need different version of this or any other system libraries, you won’t run into problems.

• An application running in a Docker container is sandboxed and doesn’t have access to the host machine. This limits the potential security implications of a breach if someone were to exploit a bug in your app.
• Docker containers are much lighter then full VMs, because they don’t run the operating system and rely on the host’s kernel. The only processes running in the container are the ones your application requires.
• Once created the same container can be shared among developers and run in your testing and production environments.

You can find more information about Docker in its FAQ pages.

In this article

• Installing Docker
• Creating a Docker image with your Django application
  • Creating an entry-point script which starts Gunicorn
  • Creating a Dockerfile
  • Building a Docker container image
• Running a Docker container with your application
  • Running the container in detached mode
  • Passing additional arguments to Gunicorn
  • Running Django management commands in the container
  • Backing up user media files
  • Writing logs to the Docker host
  • Running the container with custom Django settings

Prerequisites

The following procedure was tested on systems running Debian 7 and Ubuntu 14.04LTS. Everything should also work other Debian-based distributions. If you’re using an RPM-based distro (such as CentOS), you will need to replace the apt-get commands by their yumcounterparts and if you’re using FreeBSD you can install the components from ports. If you don’t have a server to play with, I can recommend the inexpensive VPS servers offered by Digital Ocean.

Installing Docker

A Docker package is available in the Debian 8 repositories, so installing is as simple as:

$ sudo apt-get install docker.io

On Debian 7 and Ubuntu, you will have to run an installation script provided by Docker:

$ sudo apt-get install wget
$ wget -qO- https://get.docker.com/ | sh

If you want to be able to run Docker containers as your user, not only as root, you should add yourself to the group called docker using the following command.

$ sudo usermod -aG docker `whoami`

Once Docker is installed, you can test it by running the following command. This will take a few minutes, so be patient.

$ docker run -i -t ubuntu:14.04 /bin/bash
Unable to find image 'ubuntu:14.04' locally
14.04: Pulling from ubuntu
511136ea3c5a: Pull complete
f3c84ac3a053: Pull complete
a1a958a24818: Pull complete
9fec74352904: Pull complete
d0955f21bf24: Already exists
Digest: sha256:2a214fd5c1c2048ef34bb79b5411efe4aa1e082b53ac1de3191992fe3ec64395
Status: Downloaded newer image for ubuntu:14.04
root@a8fd0ab40b7e:/#

The above command actually downloaded (pulled) a docker container with Ubuntu 14.04 from the official Docker base images repository. After the image was downloaded, Docker fired up the container and started bash inside. Feel free to look around. It looks like a normal Ubuntu system, except that only your bash process is running here:

root@a8fd0ab40b7e:/# ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  18160  1984 ?        Ss   13:07   0:00 /bin/bash
root        23  0.0  0.0  15560  1148 ?        R+   13:10   0:00 ps aux

Type exit or hit Ctrl-D to exit bash and stop the container.

You can list all running Docker containers using the docker ps command. If you add the --allswitch you will also list containers, which were running previously, but are currently closed.

$ docker ps --all
CONTAINER ID        IMAGE          COMMAND        CREATED          STATUS       PORTS    NAMES
a8fd0ab40b7e        ubuntu:14.04   "/bin/bash"    6 minutes ago    Exited (0)            sad_galileo

Creating a Docker image with your Django application

Let’s proceed to create the Docker image for our Django application.

We will need a directory to work in and a copy of our application’s source. Let’s create a directory called dockyard, where we will be making our containers and a subdirectory for the Docker image we are creating.

$ mkdir -p dockyard/hello_django_docker
$ cd dockyard/hello_django_docker

For the purposes of this article I uploaded a sample Django app to Github, so we can grab a copy of the source code using the following command:

$ git clone https://github.com/postrational/hello_django.git

We should now have a working directory containing the source code of our Django application. Please note that I assume that a PIP-compatible requirements.txt file and the Django manage.py script are in the main source code directory named hello_django. The project’s settings.py file is located in the subdirectory hello.

~/dockyard/hello_django_docker    # Our working directory
`-- hello_django                  # Main project source directroy (from repo)
    |-- hello
    |   |-- __init__.py
    |   |-- settings.py           # Project settings
    |   |-- urls.py
    |   `-- wsgi.py               # Project's WSGI start script
    |-- manage.py                 # Django's management command
    |-- project_application_1
    |   `-- (more files...)
    |-- project_application_2
    |   `-- (more files...)
    `-- requirements.txt          # File generated using pip freeze

With the code in place, let’s proceed to create Docker-related files.

Create an entry-point script

A Docker container can use an script as the default command which will be fired when the container is run. In our case we will use the following script as the entry point.

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#!/bin/bash python manage.py migrate                  # Apply database migrations python manage.py collectstatic --noinput  # Collect static files

# Prepare log files and start outputting logs to stdout touch /srv/logs/gunicorn.log touch /srv/logs/access.log
tail -n 0 -f /srv/logs/*.log &

# Start Gunicorn processes echo Starting Gunicorn. exec gunicorn hello.wsgi:application \
    --name hello_django \
    --bind 0.0.0.0:8000 \
    --workers 3 \
    --log-level=info \
    --log-file=/srv/logs/gunicorn.log \
    --access-logfile=/srv/logs/access.log \
    "$@"

The above entry point script does a few things:

• starts by running a few management commands, to apply the changes made in the application source code
• proceeds to create two log files in /srv/logs/ and to run the tail -f command which will output the logs to the console
• finally is starts gunicorn which will serve our Django application
• the "$@" notation in the last line will allow you to pass additional arguments to gunicornwhen you start the container.

Save the script as docker-entrypoint.sh and change its permissions to make it executable:

$ chmod u+x docker-entrypoint.sh

Create the Dockerfile

We will now make the container definition file named Dockerfile. Create the file and give it the following content.

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############################################################ # Dockerfile to run a Django-based web application
# Based on an Ubuntu Image ############################################################ 
# Set the base image to use to Ubuntu FROM ubuntu:14.04 
# Set the file maintainer (your name - the file's author) MAINTAINER Michal Karzynski 
# Set env variables used in this Dockerfile (add a unique prefix, such as DOCKYARD) # Local directory with project source ENV DOCKYARD_SRC=hello_django # Directory in container for all project files ENV DOCKYARD_SRVHOME=/srv # Directory in container for project source files ENV DOCKYARD_SRVPROJ=/srv/hello_django 
# Update the default application repository sources list RUN apt-get update && apt-get -y upgrade
RUN apt-get install -y python python-pip 
# Create application subdirectories WORKDIR $DOCKYARD_SRVHOME
RUN mkdir media static logs
VOLUME ["$DOCKYARD_SRVHOME/media/", "$DOCKYARD_SRVHOME/logs/"] 
# Copy application source code to SRCDIR COPY $DOCKYARD_SRC $DOCKYARD_SRVPROJ 
# Install Python dependencies RUN pip install -r $DOCKYARD_SRVPROJ/requirements.txt 
# Port to expose EXPOSE 8000 
# Copy entrypoint script into the image WORKDIR $DOCKYARD_SRVPROJ
COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]

Dockerfile specifies the steps needed to create our container image:

1. Use Ubuntu 14.04 as the base image of our container. Docker will download the image named ubuntu:14.04 and all subsequent commands will be executed inside of a running container with this base OS.
2. We set some environment variables in our container using the ENV commands. These variables can be used later in the Dockerfile, but will also be available in the environment of all programs executed in the container. For this reason we use a prefix (DOCKYARD), so that our variables don’t accidentally override anything else.
3. We run apt-get to install any system tools and libraries we may need.
4. We prepare all directories our application will use in the /srv/ directory of our container. Using the VOLUME command we make some of these directories available to other containers. This will come in handy later, see “Backing up user media files”.
5. Next we use the COPY command to copy the source code of our app into the container.
6. We use the requirements.txt file from the source code to install Python dependencies.
7. We use the EXPOSE command to make the Gunicorn port (8000) accessible outside of our container.
8. Finally we copy over docker-entrypoint.sh and define it as the script which should execute when the container is started.

You may want to refer to the Dockerfile documentation for more information about the available commands.

Build the Docker container image

All the pieces are now in place. Let’s just review to make sure the files we created are in the right spot:

~/dockyard/hello_django_docker
|-- docker-entrypoint.sh         # We added the executable entry script here
|-- Dockerfile                   # And the Dockerfile here
`-- hello_django
    |-- hello
    |   |-- __init__.py
    |   |-- settings.py
    |   |-- urls.py
    |   `-- wsgi.py
    |-- manage.py
    |-- project_application_1
    |-- project_application_2
    `-- requirements.txt

We can now build the Docker container image. I will call the image michal/hello_django.

$ docker build -t michal/hello_django ~/dockyard/hello_django_docker
Sending build context to Docker daemon 80.38 kB
Sending build context to Docker daemon
Step 0 : FROM ubuntu:14.04
(...)
Successfully built 03c7aeb70a09

You will see output of many commands as the container is put together. At the end you should be able to see you newly created image when running the docker images command:

$ docker images
REPOSITORY            TAG      IMAGE ID       CREATED         VIRTUAL SIZE
michal/hello_django   latest   17a441b8bdbd   2 seconds ago   394.6 MB

Running a Docker container with your application

Now that the container image is created, we can use it to start a container.

$ docker run --publish=8001:8000 michal/hello_django:latest

This command starts a new container from the michal/hello_django image.

Once the container is started in this way, you should be able to navigate to port 8001 of the Docker host and see the famous Django start page declaring that “It worked!”.

Visit your docker host in a browser (use the IP or domain of you machine): http://docker.host:8001

You can stop the container by hitting Ctrl-C in the terminal.

Running the container in detached mode

Starting and stopping a container as we did above is useful for debugging, but in most other cases you will want to start the container without attaching it to a terminal session.

Use the --detach=true argument when starting the container.

$ docker run --name=hello_django \
    --detach=true \
    --restart=always \
    --publish=8001:8000 \
    michal/hello_django:latest
81512acac0e4875a218587737ea31ce09aae746e4a8248461e16ab601bb1b0aa

Note, that we specified the name of the container (--name=hello_django). We also specified that the container should always be restarted if the process inside stops or crashes (--restart=always).

You can now check that the container is running by listing all running Docker containers using the docker ps command.

$ docker ps
CONTAINER ID        IMAGE                        COMMAND                CREATED             STATUS              PORTS                    NAMES
2c7cbc6fd5b8        michal/hello_django:latest   "/docker-entrypoint.   3 seconds ago       Up 3 seconds        0.0.0.0:8001->8000/tcp   hello_django

You can also follow the logs which are being output by the processes running in the container using the docker logs command.

$ docker logs -f hello_django

You can stop and restart the container:

$ docker stop hello_django
$ docker start hello_django
$ docker restart hello_django

And you can delete the container when you’re done with it.

$ docker stop hello_django
$ docker rm hello_django

Passing additional arguments to Gunicorn

Docker will pass any arguments specified after the name of the image to the command which starts the container. In our case those arguments will be passed to the docker-entrypoint.shscript, which in turn will pass them to the gunicorn command which it starts.

If we want to change the number of Gunicorn worker processes running in the container, we just need to add the --workers argument to the end of docker run.

$ docker run \
    michal/hello_django:latest \
    --workers 5

Backing up user media files

As we noted earlier, the VOLUME command in the Dockerfile made some directories accessible from other containers. If you would like to make a backup up of files stored in the /srv/media/directory, you can start another container using the --volumes-from argument. The volume directories will be accessible in the newly stared container.

$ docker run --rm -i -t --volumes-from=hello_django ubuntu:14.04 /bin/bash
root@ed95f0967489:/# cd /srv/media/
root@d0198a264b3a:/srv/media# apt-get install -y ssh-client
root@d0198a264b3a:/srv/media# scp -r * user@remote-host:~/path/to/backup

You can find more information about managing volumes in containers in the docs.

Writing logs to a directory on the Docker host

You can also mount a Docker host machine directory as a volume inside the container, using the --volume argument.

$ sudo mkdir -p /var/log/webapps/hello
$ docker run --name=hello_django \
    --detach=true \
    --restart=always \
    --publish=8001:8000 \
    --volume=/var/log/webapps/hello:/srv/logs \
    michal/hello_django:latest \
    --workers 5
$ tail -f /var/log/webapps/hello/*.log

You can find more information about managing volumes in containers in the docs.

Using custom Django settings

In many cases you will want to use different settings when running your Django application in development, during testing and in production.

In order to do this, create a new settings file on the Docker host. Let’s assume you save it in the directory /etc/webapps/hello_django/local_settings.py. In the file import all of your project’s default settings and override only what’s needed.

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from hello.settings import *
DATABASES = {
    # Databases for this instance of the container
}

You can then use the --volume argument to mount the single file local_settings.py into your container and use the --env argument to set the DJANGO_SETTINGS_MODULE environment variable to the new settings module.

$ docker run --name=hello_django \
    --detach=true \
    --restart=always \
    --publish=8001:8000 \
    --env="DJANGO_SETTINGS_MODULE=hello.local_settings" \
    --volume=/etc/webapps/hello_django/local_settings.py:/srv/hello_django/hello/local_settings.py \
    michal/hello_django:latest \
    --workers 5

Running Django management commands in the container

You will probably want to run some Django management commands on the Docker host. In order to do this you can start another container from the same image and specify another command which will be started instead of the entrypoint script. If you use /bin/bash, you will arrive at a shell in the container. From here you can execute Django’s manage.py commands.

$ docker run --rm -i -t --entrypoint=/bin/bash michal/hello_django:latest
root@ac0073a6bb9c:/srv/hello_django# ./manage.py createsuperuser
Username (leave blank to use 'root'): michal
Email address: michal@docker.image
Password:
Password (again):
Superuser created successfully.

Thanks for reading. If you find any issues with this article of have any other ideas, leave a comment below.