Petri net-based
modelling of workflow
system：An overview
Professor：Dr. Ou-Yang
Reporter：陳星豪 李高樺
洪淨池 魏廷璋
Outline
1.Introduction
2. Workflow and workflow management system
3. Petri nets
4. Petri net-based workflow modeling
5. Petri net-based workflow software
Introduction
Introduction
• The core of workflow system is the
notion of business process, which is a
set of activities with a common goal
• Workflow符合企業先前定義的需求，支
援執行一連串需要使用人力以及各種資
源的活動
Introduction
• Petri nets are applied in different discreteevent dynamic systems, which are
characterised by parallelism and
synchronization
• Petri nets-based modelling and analysis of
workflow and workflow system is an active
research area in academic
• A small number of vendor have developed
Petri nets-based workflow management
system （WfMS）
Workflow and workflow
management system
Workflow and workflow
management system
• 1970年代，workflow的概念從工廠自動化的觀
念中延伸而出，第一個workflow的雛形被發展
出來
• 1970～1980年間，Backtalk、Office Talk-P、
Office Information System是最早發行的
workflow系統雛形
• 1980年代晚期開始，WfMS和ㄧ些computerbased system進行結合，例如：DBMS、
document imaging或e-mail
Workflow系統特徵
• 一個workflow本身就是一個流程，使文件、資訊或工
作在一個與多個參與者之間傳遞，共同協調執行，涵
蓋到組織之間與內部的資訊流和控制流
• Workflow Management Coalition
– A Workflow as a computerised-based facilitation or
automation of business process, in whole or part
• A specific software system, WfMS, manages and
executes workflow through the execution of software
whose order of execution is driven by a computer
representation of the workflow logic
Workflow系統特徵
• WfMS由兩元件所組成
– The workflow model（process definition）
– The workflow execution module（workflow
enactment service ）
Workflow system component
and phases
Workflow型態
• 以流程定義與工作派工來看，有三種型態
workflow
– Production workflow
• 固定的工作定義與執行次序
– Administrative workflow
• Cases 大致遵循原本已定義完成的程序，但是有另一條途
徑供選擇
– Ad hoc workflow
• 以事先定義的流程樣版來驅動cases進行
• 流程樣版能被修正以符合某些特定cases的需求
Petri nets
Petri nets
• 一種modelling工具，由Petri所建構
• 圖形化概念具有自我文件化(self-documenting)
的能力，成為從事設計流程的有力工具
• 建立數學模式來描述系統的行為
• 驗證模式
• 適用於離散事件動態系統（discrete event
dynamic systems）的modelling與分析，具有
平行與同步作業的特性
Petri nets
• Three structural components
– Places
– Transitions
– Arcs
• Places以圓圈表示，代表系統的狀態與條件
• Transitions以長柱形或長盒形表示，描述可能改變系
統狀態的事件
• Places與Transitions之間的關係以單一方向之Arcs連
結
• 兩個相同型態的節點之間不能相互連結
• 系統的動態行為可以用tokens來呈現
Petri nets
• Petri nets的正式定義為
– A Petri net is a 4-tuple N = {P, T, F, M0}
M which represents a state of the modelled system is a
distribution of tokens over the set of places
Petri net model of workflow
Petri nets
• Two classes of properties of Petri netbased model system
– Behavioural or marking dependent properties
• Depending on the initial marking of the model
– Structural properties
• Relate to the structure of the modeled system and
independent of the initial marking
Petri net-based
workflow modeling
Workflow modelling
• It is criticized that they have
– no needed functionality
– no clear set of definition
– no general conceptual model
Petri net-based
workflow modeling
• Zismann第一次應用Petri nets在辦公室流程上，
為workflow系統的早期想法
• Ellis延伸Zismann，推出Information Control
Net (ICN)，並且應用在辦公室資訊系統的建構
• Vander Aalst已經證實workflow可以與Petri net
完全結合
• Vander Aalst提出workflow有三種不同的維度：
process、case以及resource
Petri net-based
workflow modeling
• 每一個workflow的工作由transition呈現
• Places則是描繪出一個需執行任務之precondition與post-condition，或是所需的
資源
• 弧線表達出任務之間的邏輯關係以及它
的流向
• Example
Petri net model of workflow
Resource modeling of workflow
• 前ㄧ模式只是描繪出一個workflow流程
的單一instance，沒有討論到時間或資源
的觀點
• 實際的情況下，每一個流程可能需要執
行超過一個以上的instance
• Example
Resource modeling of workflow
Petri net-based
workflow software
Petri net-based workflow
software
• Protos
– to export the processes modeled with
Protos into some WfMS products such as
COSA and FLOWer
• Structware
– a tool that allows the designer to design
and simulate business processes based on
Petri net models.
Petri net-based workflow
software
• ExSpect
– a business process modeling tool, able to
model and analyze workflow processes in
terms of Petri nets
• Woflan
– interfaces with the other commercial
workflow products such as COSA ,
Staffware and Protos
• INCOME
Modelling and qualitative analysis
of workflow
• qualitative analysis
–
–
–
–
deadlock freeness
the correct flow of the work
absence of overflow
the presence of certain mutual exclusion
wherever shared resource are used
• mainly concentrated on structural modelling
and analysis of the qualitative properties of
workflow processes
Temporal Constraint Petri Net(TCPN)
• a modified version of an ordinary Petri net
• has been used to model workflow application
at conceptual level and as a test-bed to verify
the system properties.
• Each task is decomposed into some
primitives and therefore the states between
the primitives can be represented. Task
dependencies are categorized into controlflow dependency, value dependency and
external dependency
Business Procedure net (BP-net)
• Free software
• It is proposed for representation, validation
and verification business processes.
• Then this model has been modified into a
workflow net (WF-net) as a sound model for
workflow
• In order to obtain a strong sound workflow
model, he has added 1-safe property to WFnet
Workflow Authorization Model
• This model ensures that in a workflow
system, authorized resources are
granted privileges on required tasks
only while the task is being executed
Inter-Organizational Workflow
• Inter-organizational workflow and the
availability of the Internet have affect
workflow technology
• SGML
– A Petri net structure as a base layer
– can be used in a distributed environment
• CPN-based Colored Petri-Net
Workflow Fragmentation
• Ad hoc changes
– related to rare events
• Radical changes
• Evolutionary changes
– are often the outcomes of reengineering
the system
Object Oriented Modeling
• Modular Process Nets
• Higher-Order Object Net (HOON)
• Business Process Petri nets (BPP-nets)
GSPN
• The one that mostly has been used
• Continuous Time Markov Chain
– Time
– Conversion
GSPN Modeling
Standard Petri-net
GSPN
Petri nets
•
•
•
•
OCPN(Object Composition Petri nets)
XOCPN (eXtended Object
Composition Petri Net)
DOCPN (Distributed Object
Composition Petri Net)
TSPN (Time Stream Petri Net)