github.com/hustcat/docker@v1.3.3-0.20160314103604-901c67a8eeab/docs/extend/plugin_api.md (about)

     1  <!--[metadata]>
     2  +++
     3  title = "Plugins API"
     4  description = "How to write Docker plugins extensions "
     5  keywords = ["API, Usage, plugins, documentation, developer"]
     6  [menu.main]
     7  parent = "engine_extend"
     8  weight=1
     9  +++
    10  <![end-metadata]-->
    11  
    12  # Docker Plugin API
    13  
    14  Docker plugins are out-of-process extensions which add capabilities to the
    15  Docker Engine.
    16  
    17  This page is intended for people who want to develop their own Docker plugin.
    18  If you just want to learn about or use Docker plugins, look
    19  [here](plugins.md).
    20  
    21  ## What plugins are
    22  
    23  A plugin is a process running on the same docker host as the docker daemon,
    24  which registers itself by placing a file in one of the plugin directories described in [Plugin discovery](#plugin-discovery).
    25  
    26  Plugins have human-readable names, which are short, lowercase strings. For
    27  example, `flocker` or `weave`.
    28  
    29  Plugins can run inside or outside containers. Currently running them outside
    30  containers is recommended.
    31  
    32  ## Plugin discovery
    33  
    34  Docker discovers plugins by looking for them in the plugin directory whenever a
    35  user or container tries to use one by name.
    36  
    37  There are three types of files which can be put in the plugin directory.
    38  
    39  * `.sock` files are UNIX domain sockets.
    40  * `.spec` files are text files containing a URL, such as `unix:///other.sock`.
    41  * `.json` files are text files containing a full json specification for the plugin.
    42  
    43  UNIX domain socket files must be located under `/run/docker/plugins`, whereas
    44  spec files can be located either under `/etc/docker/plugins` or `/usr/lib/docker/plugins`.
    45  
    46  The name of the file (excluding the extension) determines the plugin name.
    47  
    48  For example, the `flocker` plugin might create a UNIX socket at
    49  `/run/docker/plugins/flocker.sock`.
    50  
    51  You can define each plugin into a separated subdirectory if you want to isolate definitions from each other.
    52  For example, you can create the `flocker` socket under `/run/docker/plugins/flocker/flocker.sock` and only
    53  mount `/run/docker/plugins/flocker` inside the `flocker` container.
    54  
    55  Docker always searches for unix sockets in `/run/docker/plugins` first. It checks for spec or json files under
    56  `/etc/docker/plugins` and `/usr/lib/docker/plugins` if the socket doesn't exist. The directory scan stops as
    57  soon as it finds the first plugin definition with the given name.
    58  
    59  ### JSON specification
    60  
    61  This is the JSON format for a plugin:
    62  
    63  ```json
    64  {
    65    "Name": "plugin-example",
    66    "Addr": "https://example.com/docker/plugin",
    67    "TLSConfig": {
    68      "InsecureSkipVerify": false,
    69      "CAFile": "/usr/shared/docker/certs/example-ca.pem",
    70      "CertFile": "/usr/shared/docker/certs/example-cert.pem",
    71      "KeyFile": "/usr/shared/docker/certs/example-key.pem",
    72    }
    73  }
    74  ```
    75  
    76  The `TLSConfig` field is optional and TLS will only be verified if this configuration is present.
    77  
    78  ## Plugin lifecycle
    79  
    80  Plugins should be started before Docker, and stopped after Docker.  For
    81  example, when packaging a plugin for a platform which supports `systemd`, you
    82  might use [`systemd` dependencies](
    83  http://www.freedesktop.org/software/systemd/man/systemd.unit.html#Before=) to
    84  manage startup and shutdown order.
    85  
    86  When upgrading a plugin, you should first stop the Docker daemon, upgrade the
    87  plugin, then start Docker again.
    88  
    89  ## Plugin activation
    90  
    91  When a plugin is first referred to -- either by a user referring to it by name
    92  (e.g.  `docker run --volume-driver=foo`) or a container already configured to
    93  use a plugin being started -- Docker looks for the named plugin in the plugin
    94  directory and activates it with a handshake. See Handshake API below.
    95  
    96  Plugins are *not* activated automatically at Docker daemon startup. Rather,
    97  they are activated only lazily, or on-demand, when they are needed.
    98  
    99  ## Systemd socket activation
   100  
   101  Plugins may also be socket activated by `systemd`. The official [Plugins helpers](https://github.com/docker/go-plugins-helpers)
   102  natively supports socket activation. In order for a plugin to be socket activated it needs
   103  a `service` file and a `socket` file.
   104  
   105  The `service` file (for example `/lib/systemd/system/your-plugin.service`):
   106  
   107  ```
   108  [Unit]
   109  Description=Your plugin
   110  Before=docker.service
   111  After=network.target your-plugin.socket
   112  Requires=your-plugin.socket docker.service
   113  
   114  [Service]
   115  ExecStart=/usr/lib/docker/your-plugin
   116  
   117  [Install]
   118  WantedBy=multi-user.target
   119  ```
   120  The `socket` file (for example `/lib/systemd/system/your-plugin.socket`):
   121  ```
   122  [Unit]
   123  Description=Your plugin
   124  
   125  [Socket]
   126  ListenStream=/run/docker/plugins/your-plugin.sock
   127  
   128  [Install]
   129  WantedBy=sockets.target
   130  ```
   131  
   132  This will allow plugins to be actually started when the Docker daemon connects to
   133  the sockets they're listening on (for instance the first time the daemon uses them
   134  or if one of the plugin goes down accidentally).
   135  
   136  ## API design
   137  
   138  The Plugin API is RPC-style JSON over HTTP, much like webhooks.
   139  
   140  Requests flow *from* the Docker daemon *to* the plugin.  So the plugin needs to
   141  implement an HTTP server and bind this to the UNIX socket mentioned in the
   142  "plugin discovery" section.
   143  
   144  All requests are HTTP `POST` requests.
   145  
   146  The API is versioned via an Accept header, which currently is always set to
   147  `application/vnd.docker.plugins.v1+json`.
   148  
   149  ## Handshake API
   150  
   151  Plugins are activated via the following "handshake" API call.
   152  
   153  ### /Plugin.Activate
   154  
   155  **Request:** empty body
   156  
   157  **Response:**
   158  ```
   159  {
   160      "Implements": ["VolumeDriver"]
   161  }
   162  ```
   163  
   164  Responds with a list of Docker subsystems which this plugin implements.
   165  After activation, the plugin will then be sent events from this subsystem.
   166  
   167  Possible values are:
   168   - [`authz`](plugins_authorization.md)
   169   - [`NetworkDriver`](plugins_network.md)
   170   - [`VolumeDriver`](plugins_volume.md)
   171  
   172  
   173  ## Plugin retries
   174  
   175  Attempts to call a method on a plugin are retried with an exponential backoff
   176  for up to 30 seconds. This may help when packaging plugins as containers, since
   177  it gives plugin containers a chance to start up before failing any user
   178  containers which depend on them.
   179  
   180  ## Plugins helpers
   181  
   182  To ease plugins development, we're providing an `sdk` for each kind of plugins
   183  currently supported by Docker at [docker/go-plugins-helpers](https://github.com/docker/go-plugins-helpers).