..
      Copyright 2010 OpenStack, LLC
      All Rights Reserved.

      Licensed under the Apache License, Version 2.0 (the "License"); you may
      not use this file except in compliance with the License. You may obtain
      a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
      License for the specific language governing permissions and limitations
      under the License.

Using Glance Programmatically with Glance's Client
==================================================

While it is perfectly acceptable to issue HTTP requests directly to Glance
via its RESTful API, sometimes it is better to be able to access and modify
image resources via a client class that removes some of the complexity and
tedium of dealing with raw HTTP requests.

Glance includes a client class for just this purpose. You can retrieve
metadata about an image, change metadata about an image, remove images, and
of course retrieve an image itself via this client class.

Below are some examples of using Glance's Client class.  We assume that
there is a Glance server running at the address `glance.example.com`
on port `9292`.

Requesting a List of Public VM Images
-------------------------------------

We want to see a list of available virtual machine images that the Glance
server knows about.

Using Glance's Client, we can do this using the following code::

  from glance import client

  c = client.Client("glance.example.com", 9292)

  print c.get_images()


Requesting Detailed Metadata on Public VM Images
------------------------------------------------

We want to see more detailed information on available virtual machine images
that the Glance server knows about.

Using Glance's Client, we can do this using the following code::

  from glance import client

  c = client.Client("glance.example.com", 9292)

  print c.get_images_detailed()


Requesting Detailed Metadata on a Specific Image
------------------------------------------------

We want to see detailed information for a specific virtual machine image
that the Glance server knows about.

We have queried the Glance server for a list of public images and the
data returned includes the `uri` field for each available image. This
`uri` field value contains the exact location needed to get the metadata
for a specific image.

Continuing the example from above, in order to get metadata about the
first public image returned, we can use the following code::

  from glance import client

  c = client.Client("glance.example.com", 9292)

  print c.get_image_meta("http://glance.example.com/images/1")


Retrieving a Virtual Machine Image
----------------------------------

We want to retrieve that actual raw data for a specific virtual machine image
that the Glance server knows about.

We have queried the Glance server for a list of public images and the
data returned includes the `uri` field for each available image. This
`uri` field value contains the exact location needed to get the metadata
for a specific image.

Continuing the example from above, in order to get both the metadata about the
first public image returned and its image data, we can use the following code::

  from glance import client

  c = client.Client("glance.example.com", 9292)

  meta, image_file = c.get_image("http://glance.example.com/images/1")

  print meta

  f = open('some_local_file', 'wb')
  for chunk in image_file:
      f.write(chunk)
  f.close()

Note that the return from Client.get_image() is a tuple of (`metadata`, `file`)
where `metadata` is a mapping of metadata about the image and `file` is a
generator that yields chunks of image data.

Adding a New Virtual Machine Image
----------------------------------

We have created a new virtual machine image in some way (created a
"golden image" or snapshotted/backed up an existing image) and we
wish to do two things:

 * Store the disk image data in Glance
 * Store metadata about this image in Glance

We can do the above two activities in a single call to the Glance client.
Assuming, like in the examples above, that a Glance API server is running
at `glance.example.com`, we issue a call to `glance.client.Client.add_image`.

The method signature is as follows::

  glance.client.Client.add_image(image_meta, image_data=None)

The `image_meta` argument is a mapping containing various image metadata. The
`image_data` argument is the disk image data.

If the data is not yet available, but you would like to reserve a slot in
Glance to hold the image, you can create a 'queued' by omitting the
`image_data` parameter.

The list of metadata that `image_meta` can contain are listed below.

 * `name`

   This key/value is required. Its value should be the name of the image.

   Note that the name of an image *is not unique to a Glance node*. It
   would be an unrealistic expectation of users to know all the unique
   names of all other user's images.

 * `id`

   This key/value is optional. 
   
   When present, Glance will use the supplied identifier for the image.
   If the identifier already exists in that Glance node, then a
   `glance.common.exception.Duplicate` will be raised.

   When this key/value is *not* present, Glance will generate an identifier
   for the image and return this identifier in the response (see below)

 * `store`

   This key/value is optional. Valid values are one of `file` or `swift`

   When present, Glance will attempt to store the disk image data in the
   backing store indicated by the value. If the Glance node does not support
   the backing store, Glance will raise a `glance.common.exception.BadRequest`

   When not present, Glance will store the disk image data in the backing
   store that is marked default. See the configuration option `default_store`
   for more information.

 * `type`

   This key/values is required. Valid values are one of `kernel`, `machine`,
   `raw`, or `ramdisk`.

 * `size`

   This key/value is optional.

   When present, Glance assumes that the expected size of the request body
   will be the value. If the length in bytes of the request body *does not
   match* the value, Glance will raise a `glance.common.exception.BadRequest`

   When not present, Glance will calculate the image's size based on the size
   of the request body.

 * `is_public`

   This key/value is optional.

   When present, Glance converts the value to a boolean value, so "on, 1, true"
   are all true values. When true, the image is marked as a public image,
   meaning that any user may view its metadata and may read the disk image from
   Glance.

   When not present, the image is assumed to be *not public* and specific to
   a user.

 * `properties`

   This key/value is optional.

   When present, the value is assumed to be a mapping of free-form key/value
   attributes to store with the image.

   For example, if the following is the value of the `properties` key in the
   `image_meta` argument::

      {'distro': 'Ubuntu 10.10'}

   Then a key/value pair of "distro"/"Ubuntu 10.10" will be stored with the
   image in Glance.

   There is no limit on the number of free-form key/value attributes that can
   be attached to the image with `properties`.  However, keep in mind that there
   is a 8K limit on the size of all HTTP headers sent in a request and this
   number will effectively limit the number of image properties.

As a complete example, the following code would add a new machine image to
Glance::

  from glance.client import Client

  c = Client("glance.example.com", 9292)

  meta = {'name': 'Ubuntu 10.10 5G',
          'type': 'machine',
          'is_public': True,
          'properties': {'distro': 'Ubuntu 10.10'}}

  new_meta = c.add_image(meta, open('/path/to/image.tar.gz'))

  print 'Stored image. Got identifier: %s' % new_meta['id']
