A Petri net semantics for the PLC language Instruction List

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模糊 Ｐ ｔｉ网是基 于模糊产生式规则知识库系统 ｅｒ 的 良好的建模工具【 ，可 将知识的表达 与推理 融为一 １ 】
ｌａｎｉ ｆＦｕ ｚ ｅｒ ｔｂ ｓ ｄ ｏ ｈ ｅ ｕｔ ｅ ｄ ａ ｋ ｎ ｍｅ ｙ ＦＢ Ｐ ｅ ｒ ｎｇ ｏ ｚ Ｐ ｔｉ ｙ Ｎｅ ａ ｅ ｎ ｔ ｅ ｒ ｓ ｌ ｆ ｅ ｂ ｃ ， ａ ｌ Ｆ Ｎ．Ｆｒ ｔ ， ｈ ｓａｇ ｒｔ ｍ ａ ｒ ｄ ｓ ｉｓｌ ｔ ｉ ｌｏ ｉ ｙ ｈ ｌｙｅ ｅ
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业务流程管理BPM的认识(精)1第3页language such as provided by the Workflow Management Coalition are not precise enough.As always,it is preferable to identify any problems in software before it is actually deployed.In the case of Business Process Models this is especially im-portant as they may involve core business and/or complex business transactions.To reduce the risk of costly corrections,a thorough analysis of a Business Pro-cess Model can be beneficial.Analysis of Business Process Models can also be used to investigate ways of improving processes(e.g.reducing their cost.Formal languages may have associated analysis techniques which can be used for inves-tigating properties of specifications.These techniques can then be relied upon to provide i nsight into the behavior and characteristics of a Business Process Model specified in such a language.In[1]three reasons are stated arguing the benefits of the use of Petri nets for the specification of workflows.The reasons brought forward are the fact that Petri nets are formal,have associated analysis techniques,and are state-based rather than event-based.The development of Woflan(see e.g.[25]demonstrates that workflows specified as workflow nets[2],a subclass of Petri nets,can be analyzed in order to determine whether they are e.g.deadlock free.In the context of UML activity diagrams,tool support forverification is discussed in[15].Through the notion of place,Petri nets provide natural support for modeling the stages in between processing.State-based patterns such as deferred choice, interleaved paralled routing,and milestone can therefore be specified straightfor-wardly.A description of these patterns can be found in[9].Petri nets though also have some deficiencies when it comes to the specification of certain controlflow dependencies(see[7].This observation has led to the development of YAWL[8] (Yet Another Workflow Languageof which the formal semantics is specified as a transition system.It is interesting to observe that a concept such as the deferred choice,while easily captured in terms of Petri nets,is not often supported in languages of “classical”workflow management systems(see[9].Two recently proposed stan-dards for web service composition,BPEL4WS and BPML,however,provide direct support for thisconstruct(see[27]and[5]resp..In web services compo-sition it is important to capture the interactions between the various services and a formalism such as theπ-calculus seems to be a natural candidate to provide a formal foundation for such interactions.While it is some times claimed that BPML is based on theπ-calculus,there does not seem to be a precise definition of this relation(note that in[12],it is stated that“there is currently no evidence that BPEL4WS is based on a formal semantics”.We believe that it is important that such relations are fully formalized.Formally defined Business Process Modeling Languages can be compared in terms of their expressive power.For some classes of workflow modeling languages, abstractions of some existing approaches,comparative expressiveness has been studiedin[21,20].These results are in the context of a specific notion of equiv-alence,addressing the issue of when two workflow models can be considered expressing the sameworkflow.Expressiveness results give insight into what can and cannot be expressed in some approaches and more research is needed in this area as it could provide more guidance for language development.4Available Technology and Emerging StandardsBased on the definition of Business Process Management proposed in Section2, a characterization of its main concepts is provided,and the technology currently available or on the horizon is discussed.Some of the key aspects of business process management already mentioned in Sections1and2are re-visited,and the current state of available technology and emerging standards are discussed.One of the main aspects and certainly an activity typically carried out in early phases of business process management projects is the design of business pro-cesses.There is a close relationship between business process design and business process modeling,where the former refers to the overall design process involving multiple steps and the latter refers to the actualrepresentation of the business process in terms of a business process model using a process language.To this end,the term business process modeling is used to characterize the identifica-tion and(typically rather informalspecification of the business processes at hand.This phase includes modeling of activities and their causal and temporal rela tionships as well as specific business rules that process executions have to comply with.Business process modeling has a decade long tradition,and a variety of prod-ucts are commercially available to support this phase,based on different process languages.Given this situation,it is not surprising that the selection of a par-ticular product is an important step in many BPM projects,and,consequently, appropriate selection criteria have been studied extensively.Besides organiza-tional,economical,and aspects related to the overall IT infrastructure of the enterprise at hand,the expressive power of the process language as well as inter-faces to related software systems are important criteria,most prominently inter-faces to process enactment systems(such as workflow management systemsand to software responsible for modeling personnel and organizational structures of the enterprise.Not only the expressive power but also a well-defined semantics of the process language deserves a central role during product selection.How-ever,this aspect is considered only in a small number of recent business process management projects.Business process analysis aims at investigating properties ofbusiness pro-cesses that are neither obvious nor trivial.To this end,the term analysis is used with a rather broad meaning,including for example simulation and diagnosis, verification and performance analysis.Process simulation facilitates process di-agnosis in the sense that by simulating real-world cases,domain experts can acknowledge correct modeling or proposemodifications of the original process model.If business process models are expressed in process languages with a clear semantics,their structural properties can be analyzed.If,for example,certain parts of processes can never be reached,an obvious modeling mistake occurred that should befixed.While basic structural properties of process models have been studied for some time,it is remarkable that few software products actually support them.However structural analysis of process models requires a clear for-mal semantics of the underlying process language,which might not be present. In some products,a pragmatic approach to process modeling is preferred to aformal one;especially if the main goal of process modeling is discussion with domain experts rather than process analysis or process enactment.However,we mention that formal semantics of process languages and intuitiveness and ease of use are no contradicting goals,and recent approaches seem to support this observation.The next aspect of BPM and traditionally a very strong one is process en-actment.However,before process enactment isdiscussed,we provide a coarseclassification of business processes that paves the way for a discussion of dif-ferent types of process enactment systems.In the early days of BPM when in the application side business process modeling and in the IT enactment side workflow management were the only options,processes with a static structure were focused.The main reason behind this obvious limitation was as follows: Modeling a process and providing infrastructure for its enactment incurs con-siderable effort.To provide satisfactory return on investment,a large number of individual cases have to benefit from this new technology.This type of straight-through-process is also ca lled production workflow[23].While there are success-ful workflow projects on this type of straight-through processes,this restriction ofworkflow technology proved fatal for applications in more dynamic environ-ments.In some cases where traditional workflow t echnology was used in these advanced settings,new workflow solutions were partly circumvented or even ne-glected.As a response to this situation,considerable work in ad-hoc,flexible and case-based workflowwas(and is beingconducted,both in academia and in industry.Recently,case handling is studied in depth as a new paradigm for supporting knowledge-intensive business processes with loose structuring.Based on the brief characterization of case handling provided above,we mention that in the case handling paradigm knowledge workers enjoy a great degree of freedom inorganizing and performing their work which they are knowledgeable about. Some of the concepts of case handling are already present in commercial case handling systems.Standardization has a long histor y in workflow management.Fueled by infor-mation system heterogeneity that also includes workflow management systems, organizations started to form interest groups aiming at standardizing interfaces between workflow management systems and components,with the goal of en-hancing interoperability and fostering the workflow market.The most prominent organization in this context is the Workflow Management Coalition(WfMCthat was formed in1993and today hasover300member organizations,including all major workflow vendors as well as workflow users and interested academia[22]. The basis of WfMC activities is the so called WfMC Reference Architecture that defines standard workflow system components interfaces.Despite the fact that all major vendors are organized in WfMC and a number of important contri-butions on practical workflow aspects have been made,many people feel that WfMC’s ambitious goals have yet to be reached.A more recent standardization effort in the BPM context is related to the cur-rent momentum of XML and Web services technology.Web services is a promis-ing technology to foster interoperability between information system based–conceptually–on the service oriented architectureparadigm[11]and–tech-nologically–on open standards and light-weight protocols and systems.While Web services technology has not yet reached maturity level,there is considerable effort under way by literally all major software vendors.The need for standard-ization is clearly acknowledged in this context,and important contributions have been made.However,as。

２ ０ １ ５年 ６月
Ｄ Ｏ Ｉ ：１ ０ ． １ ３ ３ ８ ２ ／ ｊ ． ｊ ｅ ｍｉ ． ２ ０ １ ５ ． ０ ６ ． ０ １ ９
Ｐ ｅ ｔ ｒ ｉ 网结 合 Ｄ ｉ ｊ ｋ ｓ ｔ ｒ ａ算 法 的并 行 测 试 任 务 调 度 方 法 研 究
周 强 司丰 炜 修 言彬
（ 北 京航空航 天大学 自 动化 科学与 电气 工程学 院 摘 北京 １ ０ ０ １ ９ １ ）
要： 针对测试 系统 中的并行任务调 度方法复 杂且 难 以优化 的问题 ， 提 出并 实现 了一 种利用 Ｐ ｅ ｔ ｒ ｉ 网并结合 Ｄ ｉ ｊ ｋ ｓ ｔ ｒ ａ 算法
的并行测 试任务调 度方法 ， 从而减少 了搜索算法 的复杂度 。该方法通 过对 Ｐ ｅ ｔ ｒ ｉ 网可 达数分析工具 的充分利 用 ， 省 去 了传统
方法 在得到任 务调度结果过程 中的一些 步骤 。首先 建立测试 系统 的 Ｐ ｅ ｔ ｒ ｉ 网模 型并 生成其 可达 树 ； 然后 对可达 树模 型进行
加工 和改进 ， 添加状态节 点问 的联 接关系 ； 进而在加 工 、 改进 后的可达 树模型基础 上应用 Ｄ ｉ ｊ ｋ ｓ ｔ ｒ ａ 算 法得 到并行 测试 任务最 佳 的任务调度 策略。针对一个 工程实例应 用任务 调度方法进 行 了仿真 实验 与分 析 ， 仿真 实验 结果 证 明 了该方法 的有 效性
第２ ９ 卷 第６ 期
・
电子测量 与仪器学报
Ｊ ＯＵ Ｒ Ｎ Ａ Ｌ ＯＦ Ｅ Ｌ ＥＣ Ｔ Ｒ Ｏ ＮＩ Ｃ ＭＥ Ａ Ｓ Ｕ Ｒ Ｅ ＭＥ ＮＴ Ａ ＮＤ Ｉ ＮＳ Ｔ Ｒ ＵＭＥ Ｎ Ｔ Ａ Ｔ ｔ ｏＮ
］ Ｚ ． ２ ９
， ｖ ０ ． ６

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