Lecture 6
Petri Net Techniques for Modeling
Workflows and Their Support of
Reuse
2008/10
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Abstract
 Several authors propose their own technique based
on Petri Nets to model workflow processes. But
suggest totally different solutions.
 Because the proposed techniques and
fundamentally different, eleven of these
techniques are briefly discussed and compared
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What is workflow?
 The automation of a business process, in whole or part,
during which documents, information or tasks are passed
from one participant to another for action, according to a
Selection
set of procedural rules.
Sequence
Activity A
Activity B
Activity B
Activity C
Activity A
And-fork
And-join
Activity D
Activity C
Parallel
Iteration
Activity B
Or-fork
Activity A
Or-fork
Or-join
Activity C
Activity D
Activity A
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Activity B
Activity D
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Definition of Basic Workflow
 Workflow Management Coalition (WEMC):
provides a common “Reference Model” of
workflow management systems to identify
workflow management system’s characteristics,
terminology and components.
 Workflow management system is a software
system that defines, coordinates, manages, and
executes complex business activities.
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



Oberweis et al (1997)
-Integration of data and behaviour
aspects.
-Support for concurrent,
cooperative
processes.
-Different degrees of formality
-Availability of analysis
techniques
-Flexibility
Van der Aalst (1996)
-formal semantics despite their
graphical nature
-can explicitly model states
-have abundance of available and
theoretically proven analysis
techniques
Why Petri Nets
to Model
Workflow?
Merz. Et al (1995)
-mathematical foundation
-graphical representation
-possibility
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Why Petri Nets to Model Workflow?
Event
.
t3
P3
P1
P={P1、P2、P3、P4、P5}
T={t1、t2、t3}
t1
•P1=Ø
•t1={P1}
•P2={t1}
•t2={P2}
t2
P2
P4
•P3={t3}
•t3={P2}
t1•={P2}
•P4={t2}
•p5={t2}
t2•={P4、P5}
P1•={t1}
t3•={P3}
P2•={t2、t3}
P5
P3•=Ø
P4•=Ø
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P5•=Ø
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Petri Nets evolution
Ellis C.A., Nutt G.J.
Information Control Nets (ICN)
1993
Agostini A., et al.
Subclass of Elementary Net Systems
Generalized Stochastic Petri Nets (GSPN)
Ferscha A.
1994
Schömig A.K., Rau H.
Coloured Petri Nets (CPN)
Merz M., et al.
1995
Wikarski D., Han Y., Löwe M.
Van der Aalst W.M.P
1996
Coloured Generalized Stochastic Petri Nets
(CGSPN)
Wikarski D.
Higher Order Object Nets (HOON)
Workflow-nets (WF-nets)
Modular Process Nets
Oberweis A., et al.
INCOME/WF
1997
Adam N.R., et al.
Badouel E., Oliver J.
Temporal Constraint Petri Net (TCPN)
Reconfigurable Nets
1998
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High Level Petri Nets
 Information Control Nets (ICN)
adding a complementary data flow model, generalising
control flow primitives and simplifying semantics
 Coloured Petri Nets
in order to enhance the distributed systems architecture
Common Open Service Market (COSM), with concurrent
workflow modeling.
 Reconfigurable Nets
consists in fact of several Petri Nets which constitute the
different possible configurations of the system.
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Introduction
 First introduced by Carl Adam Petri in
1962.
 A diagrammatic tool to model concurrency
and synchronization in distributed systems.
 Very similar to State Transition Diagrams.
 Used as a visual communication aid to
model the system behaviour.
 Based on strong mathematical foundation.
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A Petri Net Specification ...
 consists of three types of components:
places (circles), transitions (rectangles) and
arcs (arrows):
– Places represent possible states of the system;
– Transitions are events or actions which cause
the change of state; And
– Every arc simply connects a place with a
transition or a transition with a place.
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A Change of State …
 is denoted by a movement of token(s) (black
dots) from place(s) to place(s); and is
caused by the firing of a transition.
 The firing represents an occurrence of the
event or an action taken.
 The firing is subject to the input conditions,
denoted by token availability.
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A Change of State
 A transition is firable or enabled when there
are sufficient tokens in its input places.
 After firing, tokens will be transferred from
the input places (old state) to the output
places, denoting the new state.
 Note that the EFTPOS example is a Petri
net representation of a finite state machine
(FSM).
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Net Structures
 A sequence of events/actions:
e1
e2
e3
 Concurrent executions:
e2
e3
e4
e5
e1
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Net Structures
 Non-deterministic events - conflict, choice or
decision: A choice of either e1, e2 … or e3, e4 ...
e1
e2
e3
e4
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Net Structures
 Synchronization
e1
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Net Structures
 Synchronization and Concurrency
e1
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Online EC transaction example:
 A sequence of as-is events/actions (i.e.,
INCOME behavior model):
Login
Choose items
(place items in
shopping cart)
Proceed
check-out
(confirm
purchased
items)
 A sequence of to-be events/actions:
Choose items
(place items in
shopping cart)
Proceed
check-out
Login
Confirm
Purchased
items
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Online EC transaction example:
 To-be behavior model and linkages to object
model (SQL data tables!) and organization
model (Customers!).
Choose items
(place items in
shopping cart)
Proceed
check-out
Login
Product
List
Custom
er List
Confirm
Purchased
items
Shopping
Cart List
Your web exercise (due on 11/19)
– Sample web program: as-is model
– Challenge 1 web program: to-be model
– Challenge 2 “your own” to-be model
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Stochastic Petri Nets
GSPN (Generalized Stochastic Petri Nets)
1.Quantitative analysis
2.Qualitative analysis
CGSPN (the Coloured Generalized Stochastic
Petri Nets) – which are based on Coloured
Petri Nets as pure Petri Net formalism
instead of Place/Transition Petri Nets.
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Behavioural Properties
Reachability
• “Can we reach one particular state from
another?”
Boundedness
• “Will a storage place overflow?”
Liveness
• “Will the system die in a particular state?”
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Other Types of Petri Nets
• High-level Petri nets
• Tokens have “colours”, holding complex
information.
• Timed Petri nets
• Time delays associated with transitions and/or
places.
• Fixed delays or interval delays.
• Stochastic Petri nets: exponentially distributed
random variables as delays.
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Other Types of Petri Nets
• Object-Oriented Petri nets
• Tokens are instances of classes, moving from
one place to another, calling methods and
changing attributes.
• Net structure models the inner behaviour of
objects.
• The purpose is to use object-oriented constructs
to structure and build the system.
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An O-O Petri Net
Producer
Consumer
accepted
ready
produce
send
Storage
accept
consume
ready
Producer
Consumer
data: ITEM
data: ITEM
ITEM produce( )
ITEM accept( )
void send(ITEM)
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void consume(ITEM)
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Application Generators and Very
High-Level Languages
 Allow users to specify the requirements at a very
high level of abstraction.
 In the long term, it would have the highest
potential profit. However, it remains very difficult
to build generators that scale up to industrial
production
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Evaluation
 Most common approach black-box reuse
 The notion of compositionality
 Questions(which advantages exactly can be
achieved or which type of reuse leads to
these advantages)remains unanswered
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A Critical Remark Concerning
Reuse in the IS-Literature
 It has remained extremely difficult to realize
a systematic approach to reuse.
 The potential reasons for the lack of
systematic reuse：some technical but
many are managerial
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Hidden Assumptions
 A fundamental problem of software
 Intended environment
(1)implicit
(2)don’t match the actual environment or
(3)conflict with those of other parts of the
system
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Final Remarks
 Realizing a systematic form of reuse has proven to
be a very ambitious goal ---To reach this goal
,good modeling constructs alone have been
insufficient
 Making quick progress towards deciding which
modeling constructs are most appropriate
 Empirical and experimental studies are
required(have yet find in the literature)
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Conclusion
 Identify the existing Patri Net formalisms proposed by




various authors
Is that even a very good Petri Net formalism for modeling
workflows is not worth much if there are no Workflow
Management Systems or other computer based on it?
V.S database models：far from being mature
Much progress is being made towards developing an
adequate modeling construct for modeling workflows
using Patri Nets
Construct alone isn’t sufficient in IS-field, and how’s in
workflow field?
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