UML Activity Diagrams as a Workflow Specification Language

1
UML Activity Diagrams as a Workflow
Specification Language

• Marlon Dumas, Arthur H.M. ter Hofstede
• UML2001
• 2003.4.29
• presented by Hyun-Min Ko

2
Contents

• Introduction
• Background
• Activity Diagrams
• Capturing Workflow Patterns
• Conclusion

3
Introduction

• Motivation
• UML activity diagrams are intended to model
  workflows
• They need to compare to the languages currently
  used for this purpose
• Those supported by existing Workflow Management
  Systems
• Objectives
• To examine the expressiveness and adequacy of the
  activity diagrams notation for workflow
  specification by systematically evaluating their
  ability to capture a collection of workflow
  patterns

4
Background

• Workflow
• The automation of a business process, in whole
  part, during which documents, information or
  tasks are passed from on participant to another
  for action, according to a set of procedural
  rules
• Workflow Specification Perspectives
• Control-flow
• Expresses activities and their execution ordering
  through different constructors, which permit flow
  of execution control
• e.g.) sequence, choice, parallelism and join
  synchronization
• Data
• Expresses the flow of information between
  activities
• Resource
• Defines human and device roles responsible for
  handling activities
• Operational
• Describes the elementary actions executed by
  activities

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Activity Diagrams (1/2)

• States and Transition
• States
• Wait state no action or activity is performed
• Action state a single action is attached to a
  state
• Activity-in-state an activity is attached to
  the state
• Transition shows what activity state follows
  after another

concurrent region
action state
transition
wait state
AND-state
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Activity Diagrams (2/2)

• Forks and joins
• Express concurrency
• Fork a transition with one source state and
  several target states
• Join a transition with several source states
  and one target state
• Must fulfill well-formedness criteria
• to replace all forks and joins with AND-states
• Forks and joins permit deadlocking situation
  which would not occur if forks and joins were
  replaced with AND-states
• Synch states
• A synchronization point between two threads
• May hold several tokens simultaneously
• the number of tokens a given bound
• e.g.) no bound
• Dynamic invocations
• A feature to specify that multiple invocations of
  an action or subactivity execute concurrently

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Capturing Workflow Patterns

• Control-flow Workflow Patterns
• Abstracted forms of recurring situations related
  to the ordering of activities in a workflow, and
  the flow of execution between them
• Synchronization patterns
• One or several concurrent activities need to be
  completed before another activity is initiated
• State-based patterns
• Human and material resources are not always
  available, activities are more often in a waiting
  state than in a processing one
• Producer-consumer patterns
• Several instances of an activity A (the producer)
  are executed sequentially, and the termination of
  each of these instances triggers the execution of
  an instance of another B (the consumer)

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Discriminator (1/2)

• Description
• A point in a workflow that waits for one of its
  incoming branches to complete before activating
  the subsequent activity
• Example
• A solution using UML activity diagrams
• A set of concurrent regions communicating through
  signals
• The incoming branches of the discriminator, as
  well as the outgoing branch, are placed in
  separate regions of a single activity state

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Discriminator (2/2)

• Limitation
• When the discriminator is part of a loop

Undesired synchronization !!
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N-out-of-M join

• Description
• A point in a workflow where M parallel branches
  converge into one
• Example
• A solution using activity diagrams
• A counter needs to be introduced to keep track of
  the number of termination signals generated by
  the incoming branches of the N-out-of-M join
• Limitation The same as the Discriminator pattern

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Multiple instances requiring synchronization

• Description
• A point in a workflow where an activity A is
  enabled multiple times
• Example
• A solution using activity diagrams
• Embed activity A within a subactivity state, and
  attach an unbounded dynamic multiplicity to it
• Introduce a transition between this subactivity
  state and an action state labeled by B

12
Deferred choice

• Description
• A point in a workflow where one among several
  branches is chosen based on some external
  information
• Example
• A solution using activity diagrams
• A normal state which waits for an event from the
  environment, and chooses one of its outgoing
  branches accordingly

13
Interleaved parallel routing (1/2)

• Description
• A set of activities A1, A2, , An need to be
  executed in an arbitrary order
• Each activity in the set is executed exactly once
• Example

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Interleaved parallel routing (2/2)

• A solution using activity diagrams
• To enforce the interleaving of activities by
  placing each activity in a separate concurrent
  region, and blocking their execution through
  synch states emanating from a single blocking
  region

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With termination activity (1/2)

• Description
• The process starts with the execution of an
  instance of A
• When this execution completes, an instance of B
  is enabled
• Concurrently, a second instance of A can be
  started
• When this second instance of A completes, a
  second instance of B is enabled
• Example

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With Termination Activity (2/2)

• A solution using activity diagrams
• Instances of activity A and B run in two
  concurrent regions of a compound state

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With Bounded Queue

• Description
• It is similar to that of the previous one, except
  that at any time, the difference between the
  number of times that activity A has been
  executed, and the number of times that activity B
  has been executed, is bounded by an integer
  called the size of queue
• Example
• A solution using activity diagrams
• Two separate counters are kept one counting the
  executions of A (na), and the other counting the
  executions of B (nb)
• Every time that activity A is complete if the
  boolean variable more is true, a waiting state
  is entered, which is only exited when the
  condition na-nb

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Conclusion

• The strong points of activity diagrams
• They support signal sending and receiving at the
  conceptual level
• They support both waiting states and processing
  states
• They provide a seamless mechanism for decomposing
  an activity specification into subactivities
• The weak points of activity diagrams
• Some of their constructs lack a precise syntax
  and semantics
• The well-formedness rules linking forks with
  joins
• The concepts of dynamic invocation
• They do not fully capture important kinds of
  synchronization
• Discriminator and the N-out-of-M join

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Future work

• To assess the suitability of such constructs as
  object flows and swimlanes for the data and
  resource perspective respectively
• To study how the concepts and constructs of UML
  activity diagrams compare to those of the
  Workflow Management Coalitions Reference Model

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Back TP
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Background (2)

• Workflow Patterns
• Initiative started at the Fourth IFCIS
  International Conference on Cooperative
  Information Systems (CoopIS)  in Edinburgh in
  1999
• Can address business requirements in an
  imperative workflow style expression
• Control-flow Workflow Patterns
• Abstracted forms of recurring situations related
  to the ordering of activities in a workflow, and
  the flow of execution between them
• Categories
• Basic Control Patterns
• Advanced Branching and Synchronization Patterns
• Structural Patterns
• Patterns Involving Multiple Instances
• State-based Patterns
• Cancellation Patterns

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Workflow Management Systems (1/2)

• Staffware
• 25 in the world
• Developed by Staffware PLC
• Features
• Elements the Start, Step, Wait, Condition and
  Stop
• Arbitrary loops / implicit termination
• COSA
• In Europe
• Developed by Ley GmbH
• Features
• Petri-net based
• (activity, conditions)
• Explicit representation of states
• Deffered Choice / Interleaved Parallel Routing

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Workflow Management Systems (2/2)

• Verve
• A newcomer debuted in 1998
• Acquired by Versata (VIS)
• Features
• Model activities, transitions
• Multiple instances and dynamic modification of
  running instances