Docker Plugin API
Docker plugins are out-of-process extensions which add capabilities to the Docker Engine.
This document describes the Docker Engine plugin API. To view information on plugins managed by Docker Engine, refer to Docker Engine plugin system.
This page is intended for people who want to develop their own Docker plugin. If you just want to learn about or use Docker plugins, look here.
What plugins are
A plugin is a process running on the same or a different host as the Docker daemon, which registers itself by placing a file on the daemon host in one of the plugin directories described in Plugin discovery.
Plugins have human-readable names, which are short, lowercase strings. For
example, flocker
or weave
.
Plugins can run inside or outside containers. Currently running them outside containers is recommended.
Plugin discovery
Docker discovers plugins by looking for them in the plugin directory whenever a user or container tries to use one by name.
There are three types of files which can be put in the plugin directory.
.sock
files are Unix domain sockets..spec
files are text files containing a URL, such asunix:///other.sock
ortcp://localhost:8080
..json
files are text files containing a full json specification for the plugin.
Plugins with Unix domain socket files must run on the same host as the Docker daemon.
Plugins with .spec
or .json
files can run on a different host if you specify a remote URL.
Unix domain socket files must be located under /run/docker/plugins
, whereas
spec files can be located either under /etc/docker/plugins
or /usr/lib/docker/plugins
.
The name of the file (excluding the extension) determines the plugin name.
For example, the flocker
plugin might create a Unix socket at
/run/docker/plugins/flocker.sock
.
You can define each plugin into a separated subdirectory if you want to isolate definitions from each other.
For example, you can create the flocker
socket under /run/docker/plugins/flocker/flocker.sock
and only
mount /run/docker/plugins/flocker
inside the flocker
container.
Docker always searches for Unix sockets in /run/docker/plugins
first. It checks for spec or json files under
/etc/docker/plugins
and /usr/lib/docker/plugins
if the socket doesn't exist. The directory scan stops as
soon as it finds the first plugin definition with the given name.
JSON specification
This is the JSON format for a plugin:
{
"Name": "plugin-example",
"Addr": "https://example.com/docker/plugin",
"TLSConfig": {
"InsecureSkipVerify": false,
"CAFile": "/usr/shared/docker/certs/example-ca.pem",
"CertFile": "/usr/shared/docker/certs/example-cert.pem",
"KeyFile": "/usr/shared/docker/certs/example-key.pem"
}
}
The TLSConfig
field is optional and TLS will only be verified if this configuration is present.
Plugin lifecycle
Plugins should be started before Docker, and stopped after Docker. For
example, when packaging a plugin for a platform which supports systemd
, you
might use
systemd
dependencies to
manage startup and shutdown order.
When upgrading a plugin, you should first stop the Docker daemon, upgrade the plugin, then start Docker again.
Plugin activation
When a plugin is first referred to -- either by a user referring to it by name
(e.g. docker run --volume-driver=foo
) or a container already configured to
use a plugin being started -- Docker looks for the named plugin in the plugin
directory and activates it with a handshake. See Handshake API below.
Plugins are not activated automatically at Docker daemon startup. Rather, they are activated only lazily, or on-demand, when they are needed.
Systemd socket activation
Plugins may also be socket activated by systemd
. The official
Plugins helpers
natively supports socket activation. In order for a plugin to be socket activated it needs
a service
file and a socket
file.
The service
file (for example /lib/systemd/system/your-plugin.service
):
[Unit]
Description=Your plugin
Before=docker.service
After=network.target your-plugin.socket
Requires=your-plugin.socket docker.service
[Service]
ExecStart=/usr/lib/docker/your-plugin
[Install]
WantedBy=multi-user.target
The socket
file (for example /lib/systemd/system/your-plugin.socket
):
[Unit]
Description=Your plugin
[Socket]
ListenStream=/run/docker/plugins/your-plugin.sock
[Install]
WantedBy=sockets.target
This will allow plugins to be actually started when the Docker daemon connects to the sockets they're listening on (for instance the first time the daemon uses them or if one of the plugin goes down accidentally).
API design
The Plugin API is RPC-style JSON over HTTP, much like webhooks.
Requests flow from the Docker daemon to the plugin. The plugin needs to implement an HTTP server and bind this to the Unix socket mentioned in the "plugin discovery" section.
All requests are HTTP POST
requests.
The API is versioned via an Accept header, which currently is always set to
application/vnd.docker.plugins.v1+json
.
Handshake API
Plugins are activated via the following "handshake" API call.
/Plugin.Activate
Request: empty body
Response:
{
"Implements": ["VolumeDriver"]
}
Responds with a list of Docker subsystems which this plugin implements. After activation, the plugin will then be sent events from this subsystem.
Possible values are:
Plugin retries
Attempts to call a method on a plugin are retried with an exponential backoff for up to 30 seconds. This may help when packaging plugins as containers, since it gives plugin containers a chance to start up before failing any user containers which depend on them.
Plugins helpers
To ease plugins development, we're providing an sdk
for each kind of plugins
currently supported by Docker at
docker/go-plugins-helpers.