github.com/phobos182/packer@v0.2.3-0.20130819023704-c84d2aeffc68/website/source/docs/extend/developing-plugins.html.markdown (about)

     1  ---
     2  layout: "docs"
     3  ---
     4  
     5  # Developing Plugins
     6  
     7  This page will document how you can develop your own Packer plugins.
     8  Prior to reading this, it is assumed that you're comfortable with Packer
     9  and also know the [basics of how Plugins work](/docs/extend/plugins.html),
    10  from a user standpoint.
    11  
    12  Packer plugins must be written in [Go](http://golang.org/), so it is also
    13  assumed that you're familiar with the language. This page will not be a
    14  Go language tutorial. Thankfully, if you are familiar with Go, the Go toolchain
    15  makes it extremely easy to develop Packer plugins.
    16  
    17  <div class="alert alert-block">
    18    <strong>Warning!</strong> This is an advanced topic. If you're new to Packer,
    19    we recommend getting a bit more comfortable before you dive into writing
    20    plugins.
    21  </div>
    22  
    23  ## Plugin System Architecture
    24  
    25  Packer has a fairly unique plugin architecture. Instead of loading plugins
    26  directly into a running application, Packer runs each plugin as a
    27  _separate application_. Inter-process communication and RPC is then used
    28  to communicate between the many running Packer processes. Packer core
    29  itself is responsible for orchestrating the processes and handles cleanup.
    30  
    31  The beauty of this is that your plugin can have any dependencies it wants.
    32  Dependencies don't need to line up with what Packer core or any other plugin
    33  uses, because they're completely isolated into the process space of the
    34  plugin itself.
    35  
    36  And, thanks to Go's [interfaces](http://golang.org/doc/effective_go.html#interfaces_and_types),
    37  it doesn't even look like inter-process communication is occuring. You just
    38  use the interfaces like normal, but in fact they're being executed in
    39  a remote process. Pretty cool.
    40  
    41  ## Plugin Development Basics
    42  
    43  Developing a plugin is quite simple. All the various kinds of plugins
    44  have a corresponding interface. The plugin simply needs to implement
    45  this interface and expose it using the Packer plugin package (covered here shortly),
    46  and that's it!
    47  
    48  There are two packages that really matter that every plugin must use.
    49  Other than the following two packages, you're encouraged to use whatever
    50  packages you want. Because plugins are their own processes, there is
    51  no danger of colliding dependencies.
    52  
    53  * `github.com/mitchellh/packer` - Contains all the interfaces that you
    54    have to implement for any given plugin.
    55  
    56  * `github.com/mitchellh/packer/plugin` - Contains the code to serve the
    57    plugin. This handles all the inter-process communication stuff.
    58  
    59  There are two steps involved in creating a plugin:
    60  
    61  1. Implement the desired interface. For example, if you're building a
    62     builder plugin, implement the `packer.Builder` interface.
    63  
    64  2. Serve the interface by calling the appropriate plugin serving method
    65     in your main method. In the case of a builder, this is `plugin.ServeBuilder`.
    66  
    67  A basic example is shown below. In this example, assume the `Builder` struct
    68  implements the `packer.Builder` interface:
    69  
    70  <pre class="prettyprint">
    71  import (
    72    "github.com/mitchellh/packer/plugin"
    73  )
    74  
    75  // Assume this implements packer.Builder
    76  type Builder struct{}
    77  
    78  func main() {
    79  	plugin.ServeBuilder(new(Builder))
    80  }
    81  </pre>
    82  
    83  **That's it!** `plugin.ServeBuilder` handles all the nitty gritty of
    84  communicating with Packer core and serving your builder over RPC. It
    85  can't get much easier than that.
    86  
    87  Next, just build your plugin like a normal Go application, using `go build`
    88  or however you please. The resulting binary is the plugin that can be
    89  installed using standard installation procedures.
    90  
    91  The specifics of how to implement each type of interface are covered
    92  in the relevant subsections available in the navigation to the left.
    93  
    94  <div class="alert alert-warn alert-block">
    95  <strong>Lock your dependencies.</strong> Unfortunately, Go's dependency
    96  management story is fairly sad. There are various unofficial methods out
    97  there for locking dependencies, and using one of them is highly recomended
    98  since the Packer codebase will continue to improve, potentially breaking
    99  APIs along the way until there is a stable release. By locking your dependencies,
   100  your plugins will continue to work with the version of Packer you lock to.
   101  </div>
   102  
   103  ## Logging and Debugging
   104  
   105  Plugins can use the standard Go `log` package to log. Anything logged
   106  using this will be available in the Packer log files automatically.
   107  The Packer log is visible on stderr when the `PACKER_LOG` environmental
   108  is set.
   109  
   110  Packer will prefix any logs from plugins with the path to that plugin
   111  to make it identifiable where the logs come from. Some example logs are
   112  shown below:
   113  
   114  ```
   115  2013/06/10 21:44:43 ui: Available commands are:
   116  2013/06/10 21:44:43 Loading command: build
   117  2013/06/10 21:44:43 packer-command-build: 2013/06/10 21:44:43 Plugin minimum port: 10000
   118  2013/06/10 21:44:43 packer-command-build: 2013/06/10 21:44:43 Plugin maximum port: 25000
   119  2013/06/10 21:44:43 packer-command-build: 2013/06/10 21:44:43 Plugin address: :10000
   120  ```
   121  
   122  As you can see, the log messages from the "build" command plugin are
   123  prefixed with "packer-command-build". Log output is _extremely_ helpful
   124  in debugging issues and you're encouraged to be as verbose as you need to
   125  be in order for the logs to be helpful.
   126  
   127  ## Plugin Development Tips
   128  
   129  Here are some tips for developing plugins, often answering common questions
   130  or concerns.
   131  
   132  ### Naming Conventions
   133  
   134  It is standard practice to name the resulting plugin application
   135  in the format of `packer-TYPE-NAME`. For example, if you're building a
   136  new builder for CustomCloud, it would be standard practice to name the
   137  resulting plugin `packer-builder-custom-cloud`. This naming convention
   138  helps users identify the purpose of a plugin.
   139  
   140  ### Testing Plugins
   141  
   142  While developing plugins, you can configure your Packer configuration
   143  to point directly to the compiled plugin in order to test it. For example,
   144  building the CustomCloud plugin, I may configure packer like so:
   145  
   146  <pre class="prettyprint">
   147  {
   148    "builders": {
   149      "custom-cloud": "/an/absolute/path/to/packer-builder-custom-cloud"
   150    }
   151  }
   152  </pre>
   153  
   154  This would configure Packer to have the "custom-cloud" plugin, and execute
   155  the binary that I am building during development. This is extremely useful
   156  during development.
   157  
   158  ### Distributing Plugins
   159  
   160  It is recommended you use a tool like [goxc](https://github.com/laher/goxc)
   161  in order to cross-compile your plugin for every platform that Packer supports,
   162  since Go applications are platform-specific. goxc will allow you to build
   163  for every platform from your own computer.