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visualization and storage), either via readily provided workflow building
blocks or via integration of corresponding functionality into the workflow
system itself.
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Requirement 2.3: hierarchy
A hierarchy concept that allows for encapsulation of sub-workflows into
reusable components is mandatory for scalability [128]. It should be pos-
sible to build new workflows using (entire) existing ones, for instance by
executing several workflows consecutively or by applying sub-workflow
facilities in order to work with different levels of abstraction [166].
Requirement 3: domain modeling support
As sketched above, domain modeling for scientific workflow applications prin-
cipally involves the integration of domain-specific services and the semantic
enrichment of the domain model by definition of domain-specific vocabular-
ies, semantic resource descriptions and constraints. Both aspects should be
supported by a workflow system in order to make it possible to deal with also
the most heterogeneous environments in a homogeneous and user-accessible
way.
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Requirement 3.1: service integration
Workflow systems should obey to an open world assumption for ser-
vices [242], that is, allow for the integration of new services. While the set
of predefined process building blocks is naturally limited, discovery mech-
anisms for popular service providers can increase the number of available
(web) services dynamically. However, it can not be foreseen in general
which kind of local or remote services will be needed. Hence a workflow
management system should enable programmers to extend the library of
workflow building blocks, for instance by providing special adapters to
third-party libraries or arbitrary local and remote services.
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Requirement 3.2: semantic enrichment
Workflow systems should provide the means for defining domain-specific
vocabularies for the workflow applications, for instance via taxonomic or
ontological classifications of the services and data types in the domain.
Semantic annotations of service interfaces in terms of these vocabular-
ies can then provide (both human- and machine-understandable) meta-
information about the resources in the domain that can not only help the
user to search for the right service for a task, but furthermore makes it
possible to automatically find the most suitable tool or select appropriate
alternatives in case that a particular service is temporarily not available.
In addition, it should be possible to formulate constraints that express fur-
ther domain-specific knowledge, such as dependencies between particular
services or general characteristics of the envisaged applications.
Just as the service library of a workflow system should be extensi-
ble, it should also be possible to extend and customize the semantics-
related parts of the domain model according to specific applications. In
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