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Chapter 9
                           The Runtime Environment



                           Michael Adams









                           9.1 Introduction


                           Runtime, with regards to the YAWL environment, refers to the period when the
                           YAWL System is active and executing – the host server is operational and the Engine
                           is running in its servlet container (Apache Tomcat by default), accepting requests
                           from Custom Services and applications to load specifications, start process instances
                           (or cases), check out work items, and so on, generating events and progressing cases
                           as per their control-flow towards conclusion. While much of what occurs during run-
                           time is described in other chapters, most of the internal operations have been hidden.
                           This chapter emphasizes the internal execution mechanisms of the YAWL Engine.




                           9.2 Basic Operations

                           The primary responsibility of the Engine is to execute process instances. However,
                           there is a distinct demarcation between the work performed by the Engine and that
                           handled by custom services. Throughout the life-cycle of a process instance, the
                           Engine will prepare work items (i.e., task instances) for execution, at the appropriate
                           times according to the specified control-flow of the process. But, it is important to
                           realize that:
                             The Engine is not responsible for how the work of a task instance is performed.
                              Each and every task is associated at design time with a chosen Custom Service
                              that will be responsible for performing the work of the task instance. If the asso-
                              ciation of a task with a Custom Service is not explicitly specified at design time,
                              the Resource Service is associated implicitly as a default. The Engine announces
                              the enablement of each task to the specified Custom Service at the designated



                           M. Adams
                           Queensland University of Technology, Brisbane, Australia
                           e-mail: mj.adams@qut.edu.au


                           A.H.M. ter Hofstede et al. (eds.), Modern Business Process Automation,  241
                           DOI 10.1007/978-3-642-03121-2 9, c   Springer-Verlag Berlin Heidelberg 2010
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