Service Composition - Guide to Cloud Computing for Business and Technology Managers - page 225

sequence : The activities contained in a sequence are performed sequen-
tially in lexical order.
if : The if activity represents a choice between multiple branches.
However, exactly one branch is selected.
while : The contained activity of a while loop is executed as long as a
specified predicate evaluates to true.
repeatUntil : The contained activity of a repeatUntil loop is executed
until a specified predicate evaluates to true.
forEach : The activity contained in a forEach loop is performed sequen-
tially or in parallel, controlled by a specified counter variable. This
loop can be terminated prematurely by means of a completion
condition.
pick : The pick activity blocks and waits either for a suitable message to
arrive or for a time out, whichever occurs first.
scope : A container that associates its contained activity with its own
local elements, such as variables, partner links, correlation sets, and
handlers (please see the following).
To handle exceptional situations, WS-BPEL offers four different handlers:
1. catch and catchAll : Fault handlers for dealing with fault situations in a
process. A fault handler can be compared to the catchpart of a try{}…
catch{}-block in programming languages like Java.
2. onEvent and onAlarm : Event handlers for processing unsolicited
inbound messages or timer alarms concurrently to the regular con-
t r ol flow.
3. compensationHandler : A compensation handler undoes the persisted
effects of a successfully completed scope.
4. terminationHandler : A termination handler can be used for custom-
izing a forced scope termination, for example, caused by an external
fault.
In addition to concepts introduced already, there are three more concepts
for communication: partner links, correlation sets, and (variable) properties:
1. PartnerLinks describe the relationship between a process and its ser-
vices. A partner link points to a Web Service interface the process
provides via a myRole attribute. Consequently, a partnerRole attri-
bute points to the Web Service interface that is required from the
partner. A partner link can only have one myRole attribute (inbound
partner), only one partnerRole attribute (outbound partner), and
both attributes (bidirectional partner).