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7 The Architecture 207

Fig. 7.1 The YAWL system architecture: the YAWL services in the business logic layer access the
data layer and interact with users via the presentation layer

execution of work items according to a workflow specification. Every other service
must first establish a valid session with the Engine before further communication
can proceed. The Engine, on receiving a connection request from a service will, if
the service is deemed to be authorized, pass back a session handle token. That ses-
sion handle is then passed as a parameter to the Engine for all subsequent requests.
A request that is accompanied by a missing or invalid session handle will be rejected
by the Engine. Thus, only authorized services may communicate with the Engine.
A session handle expires after a certain period of inactivity. By default, this allowed
period of inactivity is set to 60 minutes but it can be configured differently.
A number of services communicate with the Engine to extend the environment
with functionality related to the management of individual tasks. To understand
these task-related services, it is useful to recall the concept of a work item.Awork
item is a runtime instantiation of a task defined in a workflow specification together
with its associated data, and is instantiated from its task definition when control-
flow reaches the task during execution of the process instance. When a work item
is instantiated, it is said to be enabled. Each enabled work item is handled (or pro-
cessed) in some way either by a task-related service that was associated with the
task at design time, or, if no such service association is defined, its handling defaults
to the resourcing process described below.