Introduction - User-Level Workflow Design: A Bioinformatics Perspective - page 20

particular, the user himself should be given the possibility to bring in
his expert knowledge, for instance by classifying services and formulating
constraints.

Requirement 4: semantics-based service composition support

Workflow systems should exploit the available semantic information about
the services to help the user in finding valid and adequate service combina-
tions. Approaches can principally be distinguished by whether they aim at
discovering single suitable services or finding appropriate service sequences:

• Requirement 4.1: service discovery

Service discovery techniques provide means for identifying single services
according to some criterion. In the basic case, the criterion can be a search
term or an ontological classification. More sophisticated is the discovery
of services based on their input and output data types, for instance for
identifying the services that match with other services in the workflow.

• Requirement 4.2: automatic workflow composition

(Semi-) automatic workflow composition techniques aim at finding se-
quences of services according to some higher-level specification. Therefore,
it should be possible to express the (global) intents and goals of the work-
flow at an abstract, semantic level that does not deal with the technical
aspects of the concrete workflow realization. Typically, this specification
is based on the input and output data types of the intended sequence.
Ideally, however, the specification should allow for a fine-tuning of the
intents beyond a mere input/output description of the workflow.
Furthermore, a mechanism is required that translates the abstract work-
flow description into a concrete workflow realization automatically. This
involves the identification of the required services and their composition
and configuration in order to form an executable workflow.

Note that automatic workflow composition techniques can often directly
be used for service discovery as described above by restricting the search to
service sequences of length 1.

Requirement 5: workflow validation and verification

Workflow systems should support the validation and verification of the work-
flow model with respect to both their static properties and their runtime be-
havior:

• Requirement 5.1: static properties

Syntax and type checks at the local level as well as model checking tech-
niques that consider the workflow model as a whole may prevent miscon-
figuration of workflow building blocks at modeling time.