Workflow Execution Net Based On Formalized Pattern Analysis

The core issue in workflow application development domain is the design and implementation of workflow engine. Above all, the workflow engine must be a computational system, and then it is possible to become a service provider that provides workflow enactment service to external business application environment. And workflow computation model is the foundation for this computational capability.Workflow computation model is a specialized model other than workflow definition models. Some specific qualities are required: formalized and accurate semantics, precise and determinate computational behavior, as well as integrated and adequate workflow expressiveness. Unfortunately, neither the Petri-Net and its derived methodologies which is dominant in academia, nor the typical workflow modeling technologies which is popular in industry (i.e. EPC, XPDL, BPEL, and JPDL) can fulfill all the computational requirements.The objective of the dissertation is to construct a workflow computation model provided with formal semantics, determinate computational behavior, and adequate expressiveness. As the result, A new workflow computation model, WE-Net (Workflow Execution Net), is proposed by introducing extended semantics, structural elements and behavioral algorithms into classical Petri-Nets on the basis of formalized analysis to workflow patterns.The main research issues of dissertation are:1. A rule-centric and event-driven workflow conceptual model system is founded The model system is integrated and consistent which completely covers all the key aspects in workflow domain: concept, instance, component, and dynamic behavior. And the system establishes a unified model foundation to followed research.2. The formal analysis and definition to Process Transition Patterns and Task Execution Patterns are accomplishedThese definitions promote the theory of classical workflow patterns from an intuitional and experiential pattern set to a unified and formalized pattern system. The research objective of process transition patterns includes 2 parts: one is the execution order or flow relationship of a collection of tasks, and another is the instantiation rule of each single task. The formal definitions of 3 elementary patterns (split, join, and multiple instances) are proposed on the basis of detailed analysis to their essential principle. The essence of split pattern could be described as a control matrix. The complexity of join pattern lies in 2 aspects: the polymorphism of join preconditions in logical, as well as the stochastic and over-sufficient possibility of join events in temporal. The essence of join computation is the processing to a stochastic temporal sequence of reachable join events. And the essence of multiple instances pattern could be transformed to 2 considerations: why multiple instances are required and how the instances are generated or initiated.The research objective of task execution patterns is the dynamic interactions between workflow engine and resources during the progress of task execution. A task-centric interaction model is proposed from an interactive perspective which decomposes the task execution problem into 2 independent sub areas: task assignment problem and task execution state transition problem. The decomposition provides a better separation to problem domain than nowadays research in workflow resource patterns. A hierarchical state model of work item is proposed which provide an extensible modeling solution to task execution state transition problem. And 3 categories of 15 task assignment patterns are identified on the basis of structural analysis to workflow task assignment problem. The categories are: assignment process control patterns, candidate resources definition patterns, and final assignment strategy patterns. The formal definition of workflow task assignment rules is also proposed as a combination of these orthogonal patterns.3. A new workflow computation model WE-Net (workflow execution net) is proposed WE-Net synthesizes extensions to classical Petri-Net and formalization of pattern analysis that thoroughly enables the modeling and implementation of many complicated workflow patterns: join, multiple instances, running control to workflow instances, workflow task assignment, and nested processes.The extensions to Petri-Net in WE-Net theory could be summarized into 3 levels: Firstly, a formal-semantic and reactive workflow computation model is established by transforming and deriving the semantics of basic elements (Condition, Action, and Edge) with specific constraints. Secondly, advanced join patterns are supported by specifying Action-Fire algorithm and Condition-Reachability-Detection algorithm. Finally, a direct conceptual mapping to workflow runtime objects (process instance, task instance, and work item) is setup by introducing standard sub-nets which enables the modeling of instance-level patterns (multiple instances, running control, task assignment, and nested processes).At the end of dissertation, a brief introduction to OpenJet workflow engine is described, and 4 groups of workflow modeling examples are described to illustrate and verify the computational capability of WE-Net: running control to workflow instances, modeling support to all the 20 classical workflow patterns, dynamic evolution and hierarchical association of WE-Nets at runtime, and modeling support to complicated workflow in actual business context.