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Docker


Overview

The Docker driver is a hypervisor driver for Openstack Nova Compute. It was introduced with the Havana release, but lives out-of-tree for Icehouse and Juno. Being out-of-tree has allowed the driver to reach maturity and feature-parity faster than would be possible should it have remained in-tree. It is expected the driver will return to mainline Nova in the Kilo release.

Docker is an open-source engine which automates the deployment of applications as highly portable, self-sufficient containers which are independent of hardware, language, framework, packaging system and hosting provider.

Docker provides management of Linux containers with a high level API providing a lightweight solution that runs processes in isolation. It provides a way to automate software deployment in a secure and repeatable environment. A Docker container includes a software component along with all of its dependencies - binaries, libraries, configuration files, scripts, virtualenvs, jars, gems, tarballs, etc. Docker can be run on any x64 Linux kernel supporting cgroups and aufs.

Docker is a way of managing multiple containers on a single machine. However used behind Nova makes it much more powerful since it’s then possible to manage several hosts, which in turn manage hundreds of containers. The current Docker project aims for full OpenStack compatibility.

Containers don't aim to be a replacement for VMs, they are complementary in the sense that they are better for specific use cases.


What unique advantages Docker bring over other containers technologies?

Docker takes advantage of containers and filesystem technologies in a high-level which are not generic enough to be managed by libvirt.

  • Process-level API: Docker can collect the standard outputs and inputs of the process running in each container for logging or direct interaction, it allows blocking on a container until it exits, setting its environment, and other process-oriented primitives which don’t fit well in libvirt’s abstraction.
  • Advanced change control at the filesystem level: Every change made on the filesystem is managed through a set of layers which can be snapshotted, rolled back, diff-ed etc.
  • Image portability: The state of any docker container can be optionally committed as an image and shared through a central image registry. Docker images are designed to be portable across infrastructures, so they are a great building block for hybrid cloud scenarios.
  • Build facility: docker can automate the assembly of a container from an application’s source code. This gives developers an easy way to deploy payloads to an OpenStack cluster as part of their development workflow.


How does the Nova hypervisor work under the hood?

The Nova driver embeds a tiny HTTP client which talks with the Docker internal Rest API through a unix socket. It uses the HTTP API to control containers and fetch information about them.

The driver will fetch images from the OpenStack Image Service (Glance) and load them into the Docker filesystem. Images may be placed in Glance by exporting them from Docker using the 'docker save' command.

Older versions of this driver required running a private docker-registry, which would proxy to Glance. This is no longer required.

Docker-under-the-hood.png

Configure OpenStack to enable Docker

Installing Docker for OpenStack

The first requirement is to install Docker on your compute hosts.

In order for Nova to communicate with Docker over its local socket, add nova to the docker group and restart the compute service to pick up the change:

usermod -G docker nova
service openstack-nova-compute restart

You will also need to install the driver:

pip install -e git+https://github.com/stackforge/nova-docker#egg=novadocker

You should then install the required modules

cd src/novadocker/
python setup.py install

You may optionally choose to create operating-system packages for this, or use another appropriate installation method for your deployment.

Nova configuration

Nova needs to be configured to use the Docker virt driver.

Edit the configuration file /etc/nova/nova.conf according to the following options:

[DEFAULT]
compute_driver = novadocker.virt.docker.DockerDriver

Create the directory /etc/nova/rootwrap.d, if it does not already exist, and inside that directory create a file "docker.filters" with the following content:

# nova-rootwrap command filters for setting up network in the docker driver
# This file should be owned by (and only-writeable by) the root user

[Filters]
# nova/virt/docker/driver.py: 'ln', '-sf', '/var/run/netns/.*'
ln: CommandFilter, /bin/ln, root

Glance configuration

Glance needs to be configured to support the "docker" container format. It's important to leave the default ones in order to not break an existing glance install.

[DEFAULT]
container_formats = ami,ari,aki,bare,ovf,docker

Configure DevStack to Use Docker

Using Docker hypervisor through DevStack replaces all manual configuration needed above.

Before running DevStack's stack.sh script, configure the following options in the "localrc" file:

VIRT_DRIVER=docker

Follow the instructions in README.rst

Finally, run stack.sh from devstack directory:

$ ./stack.sh

Configure DevStack to Use Docker (alt. method)

Using the Docker hypervisor via DevStack replaces all manual configuration needed above.

Install Docker, then install Devstack and run stack.sh

Once stack.sh completes, run unstack.sh from the devstack directory

Install nova-docker:

git clone https://git.openstack.org/stackforge/nova-docker /opt/stack/nova-docker
cd /opt/stack/nova-docker
sudo python setup.py install

Prepare DevStack:

export INSTALLDIR={Devstack_Parent_Dir}
cd /opt/stack/nova-docker
./contrib/devstack/prepare_devstack.sh
cat localrc >> local.conf

Run stack.sh from devstack directory:

$ ./stack.sh

It may be necessary to install a Docker filter as well:

sudo cp /opt/stack/nova-docker/etc/nova/rootwrap.d/docker.filters \
        /etc/nova/rootwrap.d/

How to use it

Once you configured Nova to use the docker driver, the flow is the same as any other driver.

$ glance image-list
+-------------------------------+---------------------------------+-------------+------------------+----------+--------+
| ID                            | Name                            | Disk Format | Container Format | Size     | Status |
+-------------------------------+---------------------------------+-------------+------------------+----------+--------+
| f5049d8b-93cf-49ab-af56-e7... | cirros-0.3.1-x86_64-uec         | ami         | ami              | 25165824 | active |
| 0f1ec86c-157f-4f22-9889-c0... | cirros-0.3.1-x86_64-uec-kernel  | aki         | aki              | 4955792  | active |
| 03a54807-2e35-4864-a337-45... | cirros-0.3.1-x86_64-uec-ramdisk | ari         | ari              | 3714968  | active |
| 77083f3c-d320-46e3-bcba-0c... | docker-busybox:latest           | raw         | docker           | 2271596  | active |
+-------------------------------+---------------------------------+-------------+------------------+----------+--------+

Only images with a "docker" container format will be bootable. The image contains basically a tarball of the container filesystem.

It's recommended to add new images to Glance by using Docker. For instance, here is how you can fetch images from the public registry and push them back to Glance in order to boot a Nova instance with it:

 $ docker search hipache
Found 3 results matching your query ("hipache")
NAME                             DESCRIPTION
samalba/hipache                  https://github.com/dotcloud/hipache

Then, pull the image and push it to Glance:

 $ docker pull samalba/hipache
 $ docker save samalba/hipache | glance image-create --is-public=True --container-format=docker --disk-format=raw --name samalba/hipache

NOTE: The name you provide to glance must match the name by which the image is known to docker.

 $ glance image-list
+-------------------------------+---------------------------------+-------------+------------------+----------+--------+
| ID                            | Name                            | Disk Format | Container Format | Size     | Status |
+-------------------------------+---------------------------------+-------------+------------------+----------+--------+
| f5049d8b-93cf-49ab-af56-e7... | cirros-0.3.1-x86_64-uec         | ami         | ami              | 25165824 | active |
| 0f1ec86c-157f-4f22-9889-c0... | cirros-0.3.1-x86_64-uec-kernel  | aki         | aki              | 4955792  | active |
| 03a54807-2e35-4864-a337-45... | cirros-0.3.1-x86_64-uec-ramdisk | ari         | ari              | 3714968  | active |
| 77083f3c-d320-46e3-bcba-0c... | docker-busybox:latest           | raw         | docker           | 2271596  | active |
| 998f52ba-fe03-46b0-b5a6-4b... | samalba/hipache               | raw         | docker           | 486      | active |
+-------------------------------+---------------------------------+-------------+------------------+----------+--------+

You can obviously boot instances from nova cli:

 $ nova boot --image "samalba/hipache" --flavor m1.tiny test
+--------------------------------------+--------------------------------------+
| Property                             | Value                                |
+--------------------------------------+--------------------------------------+
| OS-EXT-STS:task_state                | scheduling                           |
| image                                | samalba/hipache                |
| OS-EXT-STS:vm_state                  | building                             |
| OS-EXT-SRV-ATTR:instance_name        | instance-0000002d                    |
| OS-SRV-USG:launched_at               | None                                 |
| flavor                               | m1.micro                             |
| id                                   | 31086c50-f937-4f80-9790-045096ecb32c |
| security_groups                      | [{u'name': u'default'}]              |
| user_id                              | 1a3eed38d1344e869dd019b3636db12b     |
| OS-DCF:diskConfig                    | MANUAL                               |
| accessIPv4                           |                                      |
| accessIPv6                           |                                      |
| progress                             | 0                                    |
| OS-EXT-STS:power_state               | 0                                    |
| OS-EXT-AZ:availability_zone          | nova                                 |
| config_drive                         |                                      |
| status                               | BUILD                                |
| updated                              | 2013-08-25T00:22:32Z                 |
| hostId                               |                                      |
| OS-EXT-SRV-ATTR:host                 | None                                 |
| OS-SRV-USG:terminated_at             | None                                 |
| key_name                             | None                                 |
| OS-EXT-SRV-ATTR:hypervisor_hostname  | None                                 |
| name                                 | test                                 |
| adminPass                            | QwczSPAAT6Mm                         |
| tenant_id                            | 183a9b7ed7c6465f97387458d693ca4c     |
| created                              | 2013-08-25T00:22:31Z                 |
| os-extended-volumes:volumes_attached | []                                   |
| metadata                             | {}                                   |
+--------------------------------------+--------------------------------------+

Once the instance is booted:

$ nova list
+--------------------------------------+------+--------+------------+-------------+------------------+
| ID                                   | Name | Status | Task State | Power State | Networks         |
+--------------------------------------+------+--------+------------+-------------+------------------+
| 31086c50-f937-4f80-9790-045096ecb32c | test | ACTIVE | None       | Running     | private=10.0.0.2 |
+--------------------------------------+------+--------+------------+-------------+------------------+

You can also see the corresponding container on docker:

$ docker ps
docker ps
ID              IMAGE                                  COMMAND      CREATED             STATUS          PORTS
f337c7fec5ff    samalba/hipache              sh           10 seconds ago      Up 10 seconds

The command used here is the one configured in the image. Each container image can have a command configured for the run. The driver does not usually override this. You can image booting an apache2 instance, it will start the apache process if the image is authored properly via a Dockerfile.

Resources

Community

We have a Nova Subteam and involvement of various contributors may be verified via Github's contributors page.

The Docker team is also involved with the more generic and highly-overlapping efforts of the Nova Containers Sub-team.

We are available on IRC on Freenode in #nova-docker. The containers team may be found in #openstack-containers.