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Working with Objects

Objects are the fundamental component of the Object Storage service. An "object" can be an image, a movie, a file, or anything else that has bits and requires a place to be stored.

Unfortunately, the term object has a very specific meaning in terms of computer programming, and it can be rather confusing to have multiple things carry the same "object" moniker. Thus, php-opencloud refers to an object stored in the object storage service as a DataObject. This is purely to avoid confusion—for example, the built-in PHP function is_object() checks to see whether its argument is a PHP object, not if it's stored in the ObjectStore.

Thus, the complete hierarchy for the object storage service is:

  • OpenStack (or Rackspace) is the parent of
  • ObjectStore service instance, which is the parent of
  • Container, which is the parent of
  • DataObject, which holds the actual object data

The DataObject class and its methods

You create an (empty) DataObject by calling the factory method on a Container object:

$cloud = new OpenStack(...);
$ostore = $cloud->ObjectStore(...);
$container = $ostore->Container('MyContainer');
$obj = $container->DataObject();

Attributes

These are the standard attributes defined on a DataObject:

  • name - name of the object
  • hash - MD5 checksum of the object
  • bytes - size of the object in bytes
  • last_modified - date and time the object was last modified
  • content_type - a Content-Type identifier
  • content_length - content length (should be the same as bytes)

Methods

These are the available methods (examples are below):

  • Url() - the URL of the object in the storage system
  • Create() - creates a new object in the object storage system
  • Update() - updates an existing object
  • Delete() - deletes an object
  • SetData() - sets the content of the object directly (rarely used)
  • SaveToString() - returns the object as a string (rarely used)
  • SaveToFilename() - saves the stored object to a local file

Large objects

It is not uncommon for objects stored in the object storage service to me much large that the available memory on the server. For example, the server may only have a few gigabytes of memory, but a stored video file may be 100GB in size. Thus, the objects' data is typically read from (or written to) files in the local filesystem. This is not always the case, of course, so other methods are provided, but they are marked as (rarely used) in the list above.

What's Next?

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