 Petri net
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WS-BPEL, for short BPEL,
is an OASIS orchestration language for specifying actions within
Web service business processes. BPEL is therefore an orchestration
language in the sense that it is used to define the composition
of services from a local viewpoint, describing the individual
behaviour of each participant. BPEL activities
can be basic or structured:.
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- Basic activities are those which describe the elemental
steps of the process behaviour, such as the assignment of
variables (assign), empty action (empty),
time delay (wait), invoke a service (invoke) and receive
a message (receive), reply to a client (reply),
and throw an exception (throw). We also have an action to terminate
the process execution at any moment (exit).
For technical reasons we have also included a barred form of
reply action, which is used when a service invocation
expects a reply, in order to implement the synchronization with
the reply action from the server.
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- Structured activities encode control-flow logic in a nested
way. The considered structured activities are the following:
a sequence of activities, separated by a semicolon,
the parallel composition, represented by two parallel bars (|),
the conditional repetitive behaviour (while),
and a timed extension of the receive activity, which allows to
receive different types of messages with a time-out associated
(pick).
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In addition, WSRF is a resource specification language
developed by OASIS and some of the most pioneering computer companies,
whose purpose is to define a generic framework for modelling Web services
with stateful resources, as well as the relationships among these services
in a Grid/Cloud environment. This specification contains the following parts:.
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- WS-ResourceProperties: There is a precise specification
to define WS-Resource properties, which
defines the properties of the associated resource
(disk size, processor capacity,...). Furthermore,
a WSDL file must be provided in order to
facilitate the allowed resource operations.
Among the operations allowed by the standard are GetResourceProperty
and SetResourceProperty, which are used to manipulate
the resource property values.
- WS-ResourceLifetime: The WSRF specification does not
provide a standard way to create resources. However,
resources have an associated lifetime, which means that once
this time has elapsed, the resource is considered to be destroyed,
and the subscribers are correspondingly notified.
We have then included, for completeness, an operation to create
resources, createResource, in which the initial value
of the resource, its lifetime and the activity that must
be launched upon its destruction are indicated. We also have
an operation in order to modify the current resource lifetime,
setTimeout.
- WS-Notification: Clients can subscribe to WSRF
resources in order to be notified about some topics (resource
conditions). We therefore include the subscribe operator
for a customer to subscribe to a resource, indicating the
condition under which it must be notified, and the activity that
must be executed upon that event.
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