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Figure 1. SCA component
In comparison, DSpace emphasizes more on
the organization structure of an institution. The
way data is organized in DSpace is intended to
reflect the structure of the organization using the
DSpace system. Each DSpace site is divided into
communities, which can be further divided into
sub-communities reflecting the typical university
structure of college, department, research center,
or laboratory. Communities contain collections,
which are groupings of related content. A collec-
tion may appear in more than one community.
Each collection is composed of items, which are
the basic archival elements of the archive. Each
item is owned by one collection. Items are further
subdivided into named bundles of bit streams.
Unfortunately, these two models cannot be
directly used in Clouds. Therefore, we designed
our model (DRSC model) with the concept of
supporting Cloud Computing Services. The
combination of Cloud Computing and SCA can
give the applications maximum flexibility. If the
cloud services are implemented using SCA, it is
easy to hide the affect on the applications when
change occurs. Based on SCA, we design the
DRSC model to represent the properties, services
and references of a digital resource.
As for the DRSC model, it is an extension of
the traditional digital object model. For a normal
Institutional Repository, an object identifier can
be generated as an auto-incremental number or
some other pre-defined number. For the digital
identifiers in clouds, they should have the unique-
ness property whenever and wherever they are
generated. Appropriately, we choose a Universally
Unique Identifier (UUID) as the identifier because
a UUID is 128 bits long, and can guarantee unique-
ness across space and time. UUID is originally
used in the Apollo Network Computing System
and later in the Open Software Foundation's (OSF)
Distributed Computing Environment (DCE), and
then in Microsoft Windows platforms.
As shown in Figure 1, an SCA component
consists of services, references and properties.
The component provides services to other com-
ponents, which are defined in terms of a business
interface. It references to services provided by
other components, which are also defined in terms
of a business interface. The properties configure
the component behavior.
The service component is recognized through
its services by outside. We define a DRSC object
as the composition of five atomic services (Figure
2): ID, Metadata, Content, Log and Annotation
which will be explained in the following sections.
3.1. Five Atomic Components
3.1.1. Identifier (ID) Component
This is a component with properties of a unique
identifier, registration information and the pointers
to other services for the same digital resource and
is generated as a UUID. The registration informa-
tion includes agency, registration date/time and
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