DockerCLI/docs/extend/plugin_api.md

<!--[metadata]>
+++
title = "Plugins API"
description = "How to write Docker plugins extensions "
keywords = ["API, Usage, plugins, documentation, developer"]
[menu.main]
parent = "engine_extend"
weight=1
+++
<![end-metadata]-->

# Docker Plugin API

Docker plugins are out-of-process extensions which add capabilities to the
Docker Engine.

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](plugins.md).

## What plugins are

A plugin is a process running on the same docker host as the docker daemon,
which registers itself by placing a file in one of the plugin directories described in [Plugin discovery](#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 as `unix:///other.sock`.
* `.json` files are text files containing a full json specification for the plugin.

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:

```json
{
  "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](
http://www.freedesktop.org/software/systemd/man/systemd.unit.html#Before=) 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.

## API design

The Plugin API is RPC-style JSON over HTTP, much like webhooks.

Requests flow *from* the Docker daemon *to* the plugin.  So 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.

## 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.