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

     1  ---
     2  layout: "docs"
     3  ---
     4  
     5  # Custom Provisioner Development
     6  
     7  Provisioners are the components of Packer that install and configure
     8  software into a running machine prior to turning that machine into an
     9  image. An example of a provisioner is the [shell provisioner](/docs/provisioners/shell.html),
    10  which runs shell scripts within the machines.
    11  
    12  Prior to reading this page, it is assumed you have read the page on
    13  [plugin development basics](/docs/extend/developing-plugins.html).
    14  
    15  Provisioner plugins implement the `packer.Provisioner` interface and
    16  are served using the `plugin.ServeProvisioner` function.
    17  
    18  <div class="alert alert-block">
    19    <strong>Warning!</strong> This is an advanced topic. If you're new to Packer,
    20    we recommend getting a bit more comfortable before you dive into writing
    21    plugins.
    22  </div>
    23  
    24  
    25  ## The Interface
    26  
    27  The interface that must be implemented for a provisioner is the
    28  `packer.Provisioner` interface. It is reproduced below for easy reference.
    29  The actual interface in the source code contains some basic documentation as well explaining
    30  what each method should do.
    31  
    32  
    33  <pre class="prettyprint">
    34  type Provisioner interface {
    35  	Prepare(...interface{}) error
    36  	Provision(Ui, Communicator) error
    37  }
    38  </pre>
    39  
    40  ### The "Prepare" Method
    41  
    42  The `Prepare` method for each provisioner is called prior to any runs with
    43  the configuration that was given in the template. This is passed in as
    44  an array of `interface{}` types, but is generally `map[string]interface{}`. The prepare
    45  method is responsible for translating this configuration into an internal
    46  structure, validating it, and returning any errors.
    47  
    48  For multiple parameters, they should be merged together into the final
    49  configuration, with later parameters overwriting any previous configuration.
    50  The exact semantics of the merge are left to the builder author.
    51  
    52  For decoding the `interface{}` into a meaningful structure, the
    53  [mapstructure](https://github.com/mitchellh/mapstructure) library is recommended.
    54  Mapstructure will take an `interface{}` and decode it into an arbitrarily
    55  complex struct. If there are any errors, it generates very human friendly
    56  errors that can be returned directly from the prepare method.
    57  
    58  While it is not actively enforced, **no side effects** should occur from
    59  running the `Prepare` method. Specifically, don't create files, don't launch
    60  virtual machines, etc. Prepare's purpose is solely to configure the builder
    61  and validate the configuration.
    62  
    63  The `Prepare` method is called very early in the build process so that
    64  errors may be displayed to the user before anything actually happens.
    65  
    66  ### The "Provision" Method
    67  
    68  The `Provision` method is called when a machine is running and ready
    69  to be provisioned. The provisioner should do its real work here.
    70  
    71  The method takes two parameters: a `packer.Ui` and a `packer.Communicator`.
    72  The UI can be used to communicate with the user what is going on. The
    73  communicator is used to communicate with the running machine, and is
    74  guaranteed to be connected at this point.
    75  
    76  The provision method should not return until provisioning is complete.
    77  
    78  ## Using the Communicator
    79  
    80  The `packer.Communicator` parameter and interface is used to communicate
    81  with running machine. The machine may be local (in a virtual machine or
    82  container of some sort) or it may be remote (in a cloud). The communicator
    83  interface abstracts this away so that communication is the same overall.
    84  
    85  The documentation around the [code itself](https://github.com/mitchellh/packer/blob/master/packer/communicator.go)
    86  is really great as an overview of how to use the interface. You should begin
    87  by reading this. Once you have read it, you can see some example usage below:
    88  
    89  <pre class="prettyprint">
    90  // Build the remote command.
    91  var cmd packer.RemoteCmd
    92  cmd.Command = "echo foo"
    93  
    94  // We care about stdout, so lets collect that into a buffer. Since
    95  // we don't set stderr, that will just be discarded.
    96  var stdout bytes.Buffer
    97  cmd.Stdout = &stdout
    98  
    99  // Start the command
   100  if err := comm.Start(&cmd); err != nil {
   101    panic(err)
   102  }
   103  
   104  // Wait for it to complete
   105  cmd.Wait()
   106  
   107  // Read the stdout!
   108  fmt.Printf("Command output: %s", stdout.String())
   109  </pre>