Architecture of Process Execution Environments:

business process
management system capable of controlling the enactment of business processes
based on business process models;

The architecture
model contains the Business Process Environment, a Business Process Modelling
subsystem, a Business Process Model Repository, a Process Engine, and
a set of Service Providers. The roles of these constituents of the architecture
model are characterized as follows.
� Business Process Modelling: The business process modelling subsystem
is used for creating business process models, containing information on
activities, their operations, and the structure of the business process. This
architecture subsystem can be realized by business process modelling tools.
� Business Process Environment: The business process environment triggers
the instantiation and enactment of process instances based on process
models.
� Business Process Model Repository: The business process model repository
holds business process models that are created by the business process
modelling component.
� Process Engine: The process engine is responsible for instantiating and
controlling the execution of business processes. It is the core component
of a business process management system. This component is triggered by
the business process environment. It uses process models to instantiate and
control the enactment of process instances. To execute a particular activity
instance, it calls entities that act as providers of the required functionality.
In a service-oriented architecture, service providers are called to execute
individual services that realize business process activities.
� Service Providers: Service providers host application services that realize
business process activities. In the architecture model, service providers
represent an abstract entity that subsumes not only Web service providers
but also knowledge workers that realize particular activities in business
processes. The organizational and technical information that the process
engine needs in order to determine and access the service provider is also
stored in the business process model repository.These components of the architectural model control the enactment of process
instances. To capture the distributed nature of business process executions,
the components and the service providers are represented by agents that communicate
by sending and receiving messages, i.e., the agents do not share
memory, but are distributed.Gateways are nodes in a process model that are used to guide the process
flow. Therefore, for each gateway node the process engine needs to perform
some action. This work that the process engine conducts is represented by
a gateway instance, just as the work defined by an activity model is represented
by an activity instance. A property of gateway instances is that the
process engine executes them, whereas activity instances are executed by service
providers, requiring nonlocal communication.
As will be detailed in the next chapter for more complex workflow patterns,
control flow patterns restrict the ordering of execution events for activities
involved in a business process. For instance, an AnalyzeOrder activity can
only be started after the initial event has occurred, and a SimpleCheck activity
can only be started after the exclusive or gateway has completed, and so on.
The execution semantics of a process instance based on a process model is
described by restrictions on the events and their ordering during the execution
of process instances.