Service Composition - Guide to Cloud Computing for Business and Technology Managers - page 215

Workflow technology enables developers to describe full intra- or inter-
organizational business processes with dependencies, sequencing selec-
tions, and iteration. It effectively enables the developers to describe the
complex rules for processing in a business process, such as merging,
selection based on field content, and time-based delivery of messages. To
achieve these objectives, workflows are predicated upon the notion of pre-
specified routing paths. Routes define the path taken by the set of objects
making up the workflow. The routes of a workflow may be sequential,
circular, or parallel work routes.
Routing paths can be sequential, parallel, or cyclic:
1. Sequential routing: A segment of a process instance under enact-
ment by a workflow management system in which several activities
are executed in sequence under a single thread of execution is called
sequential routing.
2. Parallel routing: A segment of a process instance under enactment
by a workflow management system where two or more activity
instances are executing in parallel within the workflow, giving rise
to multiple threads of control, is called parallel routing.
3. Condition routing: A point within the workflow where a single
thread of control makes a decision as to which branch to take when
having to select between multiple alternative workflow branches is
known as condition routing.
A split point is a synchronization point within the workflow where a single
thread of control splits into two or more threads that are executed in parallel
within the workflow, allowing multiple activities to be executed simultane-
ously. A join point in the workflow is a synchronization point where two or
more parallel executing activities converge into a single common thread of
control. No split or join points occur during sequential routing. Parallel rout-
ing normally commences with an AND-Split (or split) and concludes with an
AND-Join (or join or rendezvous) point.
Workflow routing includes two more synchronization points: OR-Split
(or conditional routing) and OR-Join (or asynchronous join), which can be
employed by both sequential and parallel routing constructs. A point within
the workflow where two or more alternative activities workflow branches
reconverge to form a single common activity as the next step within the
workflow is known as asynchronous join. It must be noted that as no paral-
lel activity execution has occurred at the join point, no synchronization is
required.
Roles in a workflow define the function of the people or programs involved
in the workflow. A role is a mechanism within a workflow that associates
participants to a collection of workflow activity(ies). The role defines the con-
text in which the user participates in a particular process or activity. The role
often embraces organizational concepts such as structure and relationships,