Charming with Docker

You have a Docker container and you heard about Juju. Juju can deploy your Docker container to any cloud. This document will outline the best practices for using Juju to deploy Docker images.

First things first

This document assumes you already know about Docker and how to create, pull and use application containers. Juju may be a new concept so you should get familiar with the technology, there is more information on installation and configuration of Juju software on This document will detail some of the higher level concepts of Juju.

Reactive and layered charms


Another software paradigm is reactive programming. Do something when the state or conditions are correct. Juju offers the charms.reactive package to allow charms to be written in the reactive paradigm. In charms.reactive code execution is controlled by boolean logic. You can define when the conditions are right, run this code, or when something is not set, run different code or do nothing at all.


The idea of charm layers is to combine objects or data into more complex objects or data. When applied to Charms, layers allow you to extend or build off other charms to make more complex or useful charms. The layer.yaml file in the root directory of the charm controls what layer(s) will be imported.

Reactive Charms

The docker charm makes use of the charms.reactive python framework. The code for the docker layer can be found in the reactive/ folder in the root charm directory.

Building Charms

The docker layer makes use of the Charm Layers concept building off the base charm and creating its own layer of added functionality.

Start with Docker, and build your own

layer-docker charm

The layer-docker charm can be found on at: This charm encapsulates installation and lifecycle management of the Docker daemon, emitting events such as docker.available, and forthcoming support for the plugins that Docker is growing. This charm is designed to be a base for other docker based charms.

This can be achieved by creating a new charm directory, and placing the following directives in your layer.yaml

includes: ['layer:docker']

When you run charm build the resulting charm will contain all of the logic to install and upgrade docker. Freeing you to focus on delivering your application layer and focus on how to do that.

A guided example

In the first section we illustrated the workflow. The resulting charm will be a simple method to serve static content with NGinx, including a relation to a load balancer charm.

Charm authoring using reactive paradigm

The charms.reactive is a python module to bring the reactive programming pattern to Juju charms. We call this "reactive" because code can be executed when certain conditions exist, as if the code is reacting to those conditions. For the most complete information on charms.reactive go to

The Big Picture Decomposed

We're going to be dissecting each section of the layers. To give you a top-down view of what we'll be examining, the following illustration will provide the "big picture" view of the end product as we start to decompose each of the layers and accompanying code.

Charm Layers Decomposed diagram

layer-docker charm

An example of a charm using the reactive pattern is the layer-docker charm. It also uses the compose workflow and can serve as the base for other Docker charms. This document will focus on the reactive parts of the layer-docker charm. You can read more about layers in the +building a charm with layers documentation.

├── composer.yaml
├── metadata.yaml
├── reactive
│   └──
└── scripts

The reactive directory

Making a charm "reactive" adds two directories (reactive and hooks/relations) to the basic charm structure. The layer-docker charm only has a reactive directory because it does not add (or provide) any new relations.

Inside the reactive directory there is a file The reactive framework goes into the reactive directory and finds the file.

def install():
    hookenv.status_set('maintenance', 'Installing Docker and AUFS')
    charm_path = path(os.environ['CHARM_DIR'])
    install_script_path = charm_path/'scripts/'
    hookenv.status_set('active', 'Docker Installed')

The file contains a single function named "install" which has a decorator @hook indicating this function should be called on the install event. The reactive framework handles hooks in a special way. All the hooks of the same name will be run in a non-deterministic order, so generally only one layer should implement a hook for best results.

This install hook creates a path to an install script and calls the script to install Docker. After Docker installs it calls the reactive set_state method. This method puts the "docker.available" state in the reactive framework so that other layers know that Docker is installed and available.

The layer-docker charm is a great example of how small and focused a charm can be when it uses reactive and compose concepts.


States can be thought of as persistent events. The code using the reactive framework can set and remove states. Other code known as handlers can use the boolean logic to run when the state or combination of states is correct. States may be useful to other layers so it is very important to document in the what states are set or removed in this layer.


The layer-docker-nginx charm adds the Nginx HTTP server docker image to the layer-docker charm by using charm compose and also uses the reactive framework. The layer-docker-nginx charm can be found on github.

The reactive directory

Inside the reactive directory of the layer-docker-nginx charm is a file that contains all the code for this charm.

├── assets
│   ├── index.html
│   └── jujuanddocker.png
├── composer.yaml
├── config.yaml
├── copyright
├── metadata.yaml
├── reactive
│   └──

Standard hook function

The file contains a "config-changed" hook that is called when someone changes a configuration value in Juju. This function uses charmhelpers to determine if the port has changed and the container needs to be recycled.

Reactive functions

The file contains several functions that make use of the reactive framework. The @when and @when_not are charms.reactive decorators. The decorated functions are only run if the conditions match the current state. The functions described here are specific to the nginx workload, but the concepts can be extended to the other Docker worklodas to write your own charm with the layer-docker charm as the base.


The layer-docker charm sets the "docker.available" state after installing and configuring Docker. The install_nginx function is decorated with @when('docker.available') meaning that the code will run after Docker is installed and configured. The install_nginx function sets the state "nginx.available" when it is complete.

def install_nginx():
    Default to only pulling the image once. A forced upgrade of the image is
    planned later. Updating on every run may not be desirable as it can leave
    the service in an inconsistent state.
    if reactive.is_state('nginx.available'):
    hookenv.status_set('maintenance', 'Pulling Nginx image')
    check_call(['docker', 'pull', 'nginx'])


This function handles running the nginx container which many Docker charms will have to do. The run_container function is decorated with two decorators @when('nginx.available', 'docker.available') and @when_not('nginx.started'). The @when decorator indicates the desired states and both must be set before the run_container function is executed. The @when_not decorator indicates the state that must not be active for this function to run. Since the run_container function sets the "nginx.started" state this ensures the container is not started over and over again.

@when('nginx.available', 'docker.available')
def run_container(webroot=None):
    Wrapper method to launch a docker container under the direction of Juju,
    and provide feedback/notifications to the end user.
    if not webroot:
        webroot = config['webroot']
    # Run the nginx docker container.
    run_command = [
    hookenv.status_set('active', 'Nginx container started')


The stop_container function is decorated with @when('nginx.stop', 'docker.available'). The "nginx.stop" state is an indication to stop the container in which case it will set the "nginx.stopped" state. The decorator @when_not('nginx.stopped') protects this function from being called repeatedly. Note the reactive.remove_state('nginx.stop') and reactive.set_state('nginx.stopped')calls to assert a stopped state.

@when('nginx.stop', 'docker.available')
def stop_container():
    Stop the NGinx application container, remove it, and prepare for launching
    of another application container so long as all the config values are
    appropriately set.
    hookenv.status_set('maintenance', 'Stopping Nginx container')
    # make this cleaner
        check_call(['docker', 'kill', 'docker-nginx'])
        check_call(['docker', 'rm', 'docker-nginx'])
    hookenv.status_set('waiting', 'Nginx container stopped')


The configure_website_port is a function that handles the http relationship, specifically the port. This function is decorated with @when('nginx.started', 'website.available') to only run when Nginx is started and the website is available.

@when('nginx.started', 'website.available')
def configure_website_port(http):
    Relationship context, used in tandem with the http relation stub to provide
    an ip address (default to private-address) and set the port for the
    relationship data
    serve_port = config['port']
    hookenv.status_set('active', '')

Fully Assembled Diagram

Charm artifact composed diagram

Now write your own compose and reactive charm

The layer-docker charm was designed to be a base layer for other charms that want to run Docker style application containers. By using the reactive framework the charms that you write can be very small and concentrate on the application or applications that your charm provides. Use the compose workflow and reactive framework to create a new charm with your Docker image very similar to the layer-docker-nginx charm.