The operation principle of traditional workflow management system is that each new case is instantiated by the corresponding workflow process definition, that is, a new workflow instance is created for each case. According to the corresponding workflow process definition, the workflow engine will calculate which activities should be activated for this case. For each active activity, a work item is generated and placed in the "Work Folder" of each user with the corresponding role. Users select work items from their work folders and start to perform corresponding activities, and so on. Although a work item can appear in the work folders of multiple users, only one user performs the corresponding activities. When a work item is selected, the workflow management system will start related applications and monitor the execution results of the corresponding activities. It should be pointed out that users can only see the work items in their work folders, and when selecting a work item, they can only get information related to the execution of the corresponding activities [2 ~ 4].
2. The workflow management of conceptual engineering project based on case handling can be regarded as a task, which consists of many processes and activities. However, unlike manufacturing and other industrial sectors, the engineering construction process is highly complex, and this complexity can be divided into two aspects: weak structure and variability. Just as about 90% of engineering construction information is unstructured document information, most of the processing processes in engineering construction belong to unstructured or weakly structured work processes. For the support of these unstructured or weakly structured processes, it is impossible to adopt traditional workflow management technology. At the same time, there are some highly structured management processes in the field of engineering construction, such as design changes, engineering claims, bidding and procurement. Although these management processes are few in number, they are quite important. Some studies have pointed out that 85% of the construction problems are related to the technology and have little to do with the products, so how to realize the automation of management workflow in the process of engineering construction is still of great significance. However, it must be noted that these management workflows are variable to some extent, and rely heavily on traditional workflow management technologies with fixed prior process definitions, which cannot provide effective support for them. In fact, many researchers pointed out that due to the lack of flexibility, the traditional workflow management technology often ended in failure in engineering practice. The key reason for the inflexibility of traditional workflow management technology is that the path is the only mechanism to drive the workflow, that is, the work flows from one working folder to another based on a pre-fixed causal relationship. Therefore, the final process model is either too simple or too complex and opaque. In view of the above reasons, in recent years, some scholars have put forward the so-called case-handling system, advocating a fundamental ideological change: workflow should not be driven by predetermined paths, but by cases. Traditional workflow management technology focuses on "what should be done" in a workflow process, while case handling technology focuses on "what can be done" to achieve business goals. As a new workflow management method, case handling technology provides new possibilities for supporting flexible and knowledge-intensive business processes. In fact, the application of case handling principle has achieved great success in some projects of a large Dutch construction company named Hai Jie Mans. Simply put, a case is an example of a workflow process, and it is the object that workflow participants need to deal with. In the field of engineering construction, examples can be specific design change process, specific engineering claim process and specific bidding procurement process. If the case is regarded as a product made by executing the workflow process (the product of the construction management process is information), what really drives the workflow process is the characteristics of the product. By paying attention to the characteristics of products, the traditional push-oriented path (from one working folder to another) can be transformed into a pull-oriented mechanism (centered on the data object about a case). In order to further illustrate the workflow management method based on case handling, the corresponding object model is proposed through Unified Modeling Language (UML) (Figure 2).
3. Process definition of project workflow management based on case processing For project workflow management based on case processing, process definition is also needed. The traditional construction technology is considered to be separated from each other. Without information system, information is isolated, forming an "information island"; After the application of information system, a certain workflow has been formed; However, it is necessary to apply the idea of process management to integrate and optimize the workflow of information system, that is, to describe the process logic of engineering project workflow on the basis of BPR tools. The process model generated by process definition is the basis of the whole workflow management system. Many development platforms of workflow management systems provide visual process modeling tools, which enable users to model the actual business process in an intuitive way, and the established process model can be directly supported by the system. The methods of process modeling include activity network graph, directed graph, integrated definition method (IDE F3) and Petri net, among which Petri net process modeling method has attracted the most attention from academic circles in recent years [5, 6]. The simplified Petri net model is used to model the task management process. In the general task management process, the team leader first asks a member of the team to complete a task. Team members check the task requirements according to their own abilities and various constraints, and then send a reply to the team leader. If a team member thinks that the task cannot be completed, the team leader needs to find other suitable team members. If the team members confirm their ability to complete the task, the team leader will describe the task in detail and send it to the team members. When a team member doesn't understand the detailed description of the task, he can ask questions until the task is understood and executed. For the task results submitted by team members, the team leader compares them with the original task status statement. If approved, submit the work results. Otherwise, the team leader returns the task to the team members again (Figure 3).
4. Workflow management system architecture based on case processing Through the analysis in the previous section, Figure 4 gives the workflow management system architecture based on case processing, which is basically consistent with the reference model proposed by the Workflow Management Alliance [7]. The logical design of the system includes process definition, user role assignment, data processing design, form definition, case authorization and assignment, etc. The workflow engine in the workflow execution service is the core of the whole system, which is mainly responsible for the execution of workflow process instances, the state control of case activities, the maintenance of user case list and the access to external resources. The management monitoring tool monitors and manages the status of process instances during operation. Workflow engine can access database resources such as process data, user information and document information through agents. Client applications provide users with a way to handle tasks that require human intervention during the running of a process instance. Called application refers to an external application (such as document management module) that processes application data when a process instance is running. Several WAPI (workflow application picture interface) in the figure depend on the determined development platform. According to this architecture, it can be realized through the Flow2 Mark workflow development platform in Lotus Domino/ Notes.
5. Case Figure 5 shows the interface of engineering project workflow management system based on case handling. The upper window of the workspace is the process currently being executed or viewed, which may contain subprocesses. The window at the bottom left shows the activities and subprocesses in the current process accordingly. The window on the right below is information such as forms and documents related to the current process. As can be seen from the figure, the current process of the system is the change of design scheme, including three activities: registering the sub-process of design scheme change requirements and modifying design scheme, reviewing new design scheme and filing and distributing. For this interface, it should be noted that: ① The status of activities and subprocesses can be to-do, in progress, completed, skipped, redone, etc. For example, Zhang San (assuming that he is a designer) has no executive role in approving the new design scheme, so he can skip this activity; ② The opened form should indicate which data is mandatory and which is restricted. For example, the "agree or disagree" in the design scheme review form should be mandatory data.
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