Charm Layer for Docker

This repository contains the Charm layer for docker, which can be used as a base layer for other Charms that use docker. Please refer to the Charm Layers documentation.

Usage

In a layer that wants to use docker, the integration can be as simple as placing the following in the layer.yaml file:

includes: ['layer:docker']

From here, you simply amend any hooks/reactive patterns you require to deliver and manage the lifecycle of your applications docker image. Refer to the documentation on how to write a layer.

Once you have the code written to manage your container(s) you need to assemble the charm from the layers. To do this run the charm build command from the layer directory you just created.

charm build

Now you should be able to deploy the assembled charm.

juju deploy ./<series>/<charm-name>

Where series it the code name for Ubuntu releases, such as "trusty" or "xenial" and charm-name is what you named the layer in metadata.yaml.

States

The docker layer raises a few synthetic events:

• docker.ready

• docker.available

docker.ready

When docker.ready is set, this event is before we signify to other layers that we are ready to start workloads, which should allow for docker extensions to be installed free of disrupting active workloads.

For example, installing SDN support and getting the daemon configured for TCP connections.

@when('docker.ready')
def start_flannel_networking():
    # do something here

docker.available

When docker.available is set, the daemon is considered fully configured and ready to accept workloads.

@when('docker.available')
def start_my_workload():
    # do something with docker

Layer Options

skip-install

Skips the installation of docker and raises the docker.available state. This is particularly useful when programming subordinate charms from layer-docker, where you know docker is already installed on the host. This allows you to skip the potential 90 second install routine, and go straight to deploying your application.

Docker Compose

This layer installs the 'docker-compose' python package from pypi. So once the Docker layer is installed you have the ability to use Docker Compose functionality such as control files, and logging.

Memory Accounting

The charm supports altering the GRUB2 options enabling CGROUPS and memory accounting. Changing this value will reboot the host, and any running workloads are at the mercy of the charm author inheriting from this charm. Please use --restart=always on your container runs that need to be persistent.

charms.docker

This layer also includes a wheelhouse of charms.docker a python library to make charming with docker, and configuring the docker daemon much easier, and syntactically enjoyable. For more information about this library see the charms.docker project.

Credit

This charm contains a slightly modified copy of the script contained at: https://get.docker.io

The modifications were raising status messages for Juju so its apparent what is happening to the end user following along with Juju's extended status feature.