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/engine/extend/

Develop and use a plugin with the managed plugin system

API, Usage, plugins, documentation, developer

Managed plugin system

Docker Engine managed plugin system

Docker Engine's plugins system allows you to install, start, stop, and remove plugins using Docker Engine. This mechanism is currently only available for volume drivers, but more plugin driver types will be available in future releases.

For information about the legacy plugin system available in Docker Engine 1.12 and earlier, see Understand legacy Docker Engine plugins.

Installing and using a plugin

Plugins are distributed as Docker images and can be hosted on Docker Hub or on a private registry.

To install a plugin, use the docker plugin install command, which pulls the plugin from Docker hub or your private registry, prompts you to grant permissions or capabilities if necessary, and enables the plugin.

To check the status of installed plugins, use the docker plugin ls command. Plugins that start successfully are listed as enabled in the output.

After a plugin is installed, you can use it as an option for another Docker operation, such as creating a volume.

In the following example, you install the sshfs plugin, verify that it is enabled, and use it to create a volume.

Install the sshfs plugin.

$ docker plugin install vieux/sshfs

Plugin "vieux/sshfs" is requesting the following privileges:
- network: [host]
- capabilities: [CAP_SYS_ADMIN]
Do you grant the above permissions? [y/N] y

vieux/sshfs

The plugin requests 2 privileges:

It needs access to the host network.
It needs the CAP_SYS_ADMIN capability, which allows the plugin to run the mount command.

Check that the plugin is enabled in the output of docker plugin ls.

$ docker plugin ls

NAME                TAG                 ENABLED
vieux/sshfs         latest              true

Create a volume using the plugin. This example mounts the /remote directory on host 1.2.3.4 into a volume named sshvolume. This volume can now be mounted into containers.
```
$ docker volume create \
  -d vieux/sshfs \
  --name sshvolume \
  -o sshcmd=user@1.2.3.4:/remote

sshvolume
```

Verify that the volume was created successfully.

$ docker volume ls

DRIVER              NAME
vieux/sshfs         sshvolume

Start a container that uses the volume sshvolume.

$ docker run -v sshvolume:/data busybox ls /data

<content of /remote on machine 1.2.3.4>

To disable a plugin, use the docker plugin disable command. To completely remove it, use the docker plugin remove command. For other available commands and options, see the command line reference.

Developing a plugin

Currently, there are no CLI commands available to help you develop a plugin. This is expected to change in a future release. The manual process for creating plugins is described in this section.

Plugin location and files

Plugins are stored in /var/lib/docker/plugins. The plugins.json file lists each plugin's configuration, and each plugin is stored in a directory with a unique identifier.

# ls -la /var/lib/docker/plugins
total 20
drwx------  4 root root 4096 Aug  8 18:03 .
drwx--x--x 12 root root 4096 Aug  8 17:53 ..
drwxr-xr-x  3 root root 4096 Aug  8 17:56 cd851ce43a403
-rw-------  1 root root 2107 Aug  8 18:03 plugins.json

Format of plugins.json

The plugins.json is an inventory of all installed plugins. This example shows a plugins.json with a single plugin installed.

# cat plugins.json
{
  "cd851ce43a403": {
    "plugin": {
      "Config": {
        "Args": {
          "Value": null,
          "Settable": null,
          "Description": "",
          "Name": ""
        },
        "Env": null,
        "Devices": null,
        "Mounts": null,
        "Capabilities": [
          "CAP_SYS_ADMIN"
        ],
        "Description": "sshFS plugin for Docker",
        "Documentation": "https://docs.docker.com/engine/extend/plugins/",
        "Interface": {
          "Socket": "sshfs.sock",
          "Types": [
            "docker.volumedriver/1.0"
          ]
        },
        "Entrypoint": [
          "/go/bin/docker-volume-sshfs"
        ],
        "Workdir": "",
        "User": {},
        "Network": {
          "Type": "host"
        }
      },
      "Config": {
        "Devices": null,
        "Args": null,
        "Env": [],
        "Mounts": []
      },
      "Active": true,
      "Tag": "latest",
      "Name": "vieux/sshfs",
      "Id": "cd851ce43a403"
    }
  }
}

Contents of a plugin directory

Each directory within /var/lib/docker/plugins/ contains a rootfs directory and two JSON files.

# ls -la /var/lib/docker/plugins/cd851ce43a403
total 12
drwx------ 19 root root 4096 Aug  8 17:56 rootfs
-rw-r--r--  1 root root   50 Aug  8 17:56 plugin-settings.json
-rw-------  1 root root  347 Aug  8 17:56 config.json

The rootfs directory

The rootfs directory represents the root filesystem of the plugin. In this example, it was created from a Dockerfile:

Note: The /run/docker/plugins directory is mandatory inside of the plugin's filesystem for docker to communicate with the plugin.

$ git clone https://github.com/vieux/docker-volume-sshfs
$ cd docker-volume-sshfs
$ docker build -t rootfs .
$ id=$(docker create rootfs true) # id was cd851ce43a403 when the image was created
$ sudo mkdir -p /var/lib/docker/plugins/$id/rootfs
$ sudo docker export "$id" | sudo tar -x -C /var/lib/docker/plugins/$id/rootfs
$ sudo chgrp -R docker /var/lib/docker/plugins/
$ docker rm -vf "$id"
$ docker rmi rootfs

The config.json and plugin-settings.json files

The config.json file describes the plugin. The plugin-settings.json file contains runtime parameters and is only required if your plugin has runtime parameters. See the Plugins Config reference.

Consider the following config.json file.

{
	"description": "sshFS plugin for Docker",
	"documentation": "https://docs.docker.com/engine/extend/plugins/",
	"entrypoint": ["/go/bin/docker-volume-sshfs"],
	"network": {
		   "type": "host"
		   },
	"interface" : {
		   "types": ["docker.volumedriver/1.0"],
		   "socket": "sshfs.sock"
	},
	"capabilities": ["CAP_SYS_ADMIN"]
}

This plugin is a volume driver. It requires a host network and the CAP_SYS_ADMIN capability. It depends upon the /go/bin/docker-volume-sshfs entrypoint and uses the /run/docker/plugins/sshfs.sock socket to communicate with Docker Engine.

Consider the following plugin-settings.json file.

{
  "Devices": null,
  "Args": null,
  "Env": [],
  "Mounts": []
}

This plugin has no runtime parameters.

Each of these JSON files is included as part of plugins.json, as you can see by looking back at the example above. After a plugin is installed, config.json is read-only, but plugin-settings.json is read-write, and includes all runtime configuration options for the plugin.

Creating the plugin

Follow these steps to create a plugin:

Choose a name for the plugin. Plugin name uses the same format as images, for example: <repo_name>/<name>.
Create a rootfs and export it to /var/lib/docker/plugins/$id/rootfs using docker export. See The rootfs directory for an example of creating a rootfs.
Create a config.json file in /var/lib/docker/plugins/$id/.
Create a plugin-settings.json file if needed.
Create or add a section to /var/lib/docker/plugins/plugins.json. Use <user>/<name> as “Name” and $id as “Id”.
Restart the Docker Engine service.
Run docker plugin ls.
- If your plugin is enabled, you can push it to the registry.
- If the plugin is not listed or is disabled, something went wrong. Check the daemon logs for errors.
If you are not already logged in, use docker login to authenticate against the registry so that you can push to it.
Run docker plugin push <repo_name>/<name> to push the plugin.

The plugin can now be used by any user with access to your registry.

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