Web Services Architecture, choreography architecture

Text reviewed and agreed during Web Services Architecture meeting, in
Boston, November 14-16, 2002. For inclusion in section 3.2

David Burdett
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Choreography

A single web service is limited to accepting a request for information or
the execution of a process and providing the information or process results
in return. Richer functionality can be provided when multiple web services
are used together. 

Choreographies define how multiple web services are used together,
specifying the linkages and usage patterns involved. The linkages between
Web Services consist of interactions between those services implemented by
sending messages between those Web Services, for example by invoking
operations as defined in WSDL.

A choreography definition describes the sequence and conditions which
control how the interactions occur. Successful execution of a choreography
should result in the completion of some useful function, for example: the
placement of an order, information about its delivery and eventual payment.

Example uses of choreography definitions include:
·	Service Composition. Creating a new web service with its own new
service definition by specifying in a choreography, the sequence of
interactions between two or more web services that were used to create that
new web service. This will facilitate the easy composition of new web
services from other web services that already exist
·	Workflow. The sequence in which processes, implemented using web
services, are executed within a business to support a business function.
This will enable reuse of workflow definitions within a business or in
another business
·	Linking businesses through Web Services. Specification of the
sequence of interactions between web services, operated by different
businesses, that support a useful function such as order placement. This
will support interoperable eCommerce between businesses that operate their
own independently managed web services.

A standard choreography definition language provides significant benefits:
·	Complements WSDL. WSDL only defines the service interface. It does
not define the relationships between operations or services. Choreography
definitions specify the sequence and conditions for combining multiple web
services
·	Standardized choreographies. If businesses, organizations,
individuals and vertical industries groups define "standard" choreographies
to be used within their community, then the costs of implementing solutions
that conform to that choreography are greatly reduced.