Workflow patterns
Encyclopedia
A workflow pattern is a specialized form of a design pattern
as defined in the area of software engineering
or business process engineering respectively. Workflow patterns refer specifically to recurrent problems and proven solutions related to the development of workflow
applications in particular, and more broadly, process-oriented applications.
Workflow is performed in real time. The mechanisms of control shall support the typical pace of work. Design patterns shall not delay execution of workflow.
et al. (2003) in their seminal paper Workflow Patterns. with earlier versions published in 2000-02. This collection of patterns focuses on one specific aspect of process-oriented application development, namely the description of control flow
dependencies between activities in a workflow/process. These patterns are divided into the following categories:
The above workflow patterns have been used to evaluate the functionality of commercial products supporting the development of process-oriented applications. They have also been used to evaluate a number of proposed standards, including BPEL, BPMN, UML
Activity diagram
, XPDL
, etc. It has been noted that not all these patterns are relevant in all application domains, so care must be taken when using the above workflow patterns to select a particular language or system for a given application.
The workflow patterns have also been used as initial requirements in the design of a workflow language and open-source system called YAWL
.
Several extensions to the above set of workflow patterns have been proposed. In particular, the same research groups that developed these patterns, have also proposed a set of Workflow Data Patterns, Workflow Resource Patterns, Workflow Exception Handling Patterns, and Service Interaction Patterns.
Independent/Pooled: where each component of scheduled work is completed independent of each other component and no component has a specific dependency on any other component. An example would be where staff are serving at a counter - Raoul can serve a customer in his queue without waiting for Jamie to serve a customer in his queue.
Sequential: where each component of scheduled work is dependent on the preceding component. In this case the preceding component controls the advancement of the workflow through subsequent components. An example would be on a production line - Betty cannot affix the radiator cap to the Model T Ford until Veronica has put the radiator in place.
Interdependent/Networked: where each component of scheduled work is dependent on one or a number of other components being completed. In this case the preceding components control the workflow through subsequent components. An example would be a project team - Sarah must wait for several tasks to be completed by Kevin and George before she can execute her task.
Other (workflow) pattern collections include:
These patterns collections have been used to evaluate a variety of workflow processes, both commercial (Websphere, Oracle BPEL, Staffware, SAP workflow, Windows Workflow Foundation, etc.) and open source.
Design pattern
A design pattern in architecture and computer science is a formal way of documenting a solution to a design problem in a particular field of expertise. The idea was introduced by the architect Christopher Alexander in the field of architecture and has been adapted for various other disciplines,...
as defined in the area of software engineering
Software engineering
Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
or business process engineering respectively. Workflow patterns refer specifically to recurrent problems and proven solutions related to the development of workflow
Workflow
A workflow consists of a sequence of connected steps. It is a depiction of a sequence of operations, declared as work of a person, a group of persons, an organization of staff, or one or more simple or complex mechanisms. Workflow may be seen as any abstraction of real work...
applications in particular, and more broadly, process-oriented applications.
Concept
Workflow patterns follow concepts of economised development. The usage shall follow strategies of simplifying maintenance as well as reducing modeling work.Workflow is performed in real time. The mechanisms of control shall support the typical pace of work. Design patterns shall not delay execution of workflow.
Aggregation
Workflow patterns usually may be aggregated to chains, hence the connectors for such patterns shall suffice reasonable set standards as well as the conditions for start and terminate shall be explicitly defined.Application
Workflow patterns usually get applied in various context, hence the conditions for use shall be explicitly defined and shown in example to prevent from misinterpreting.Van der Aalst classification
A well-known collection of Workflow Patterns are those proposed by Wil van der AalstWil van der Aalst
Wil M.P. van der Aalst is a Dutch computer scientist, and professor at the Department of Mathematics & Computer Science of the Technische Universiteit Eindhoven, where he chairs the Architecture of Information Systems group, His research and teaching interests include information systems, workflow...
et al. (2003) in their seminal paper Workflow Patterns. with earlier versions published in 2000-02. This collection of patterns focuses on one specific aspect of process-oriented application development, namely the description of control flow
Control flow
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
dependencies between activities in a workflow/process. These patterns are divided into the following categories:
Basic Control Patterns
- Sequence - execute two or more activities in sequence
- Parallel Split - execute two or more activities in any order or in parallel
- Synchronize - synchronize two or more activities that may execute in any order or in parallel; do not proceed with the execution of the following activities until all these preceding activities have completed; also known as barrier synchronisation.
- Exclusive Choice - choose one execution path from many alternatives based on data that is available when the execution of the process reaches the exclusive choice
- Simple Merge - wait for one among a set of activities to complete before proceeding; it is assumed that only one of these activities will be executed; typically, these activities are on different paths stemming from an exclusive choice or a deferred choice (see below)
- Terminate - terminate execution of activities upon defined event or status change
Advanced Branching and Synchronization Patterns
- Multiple Choice - choose several execution paths from many alternatives
- Conditional Choice - choose one execution path from many alternatives according to discriminated status conditions
- Synchronizing Merge - merge many execution paths; synchronize if many paths are taken; do the same as for a simple merge if only one execution path is taken
- Multiple Merge - wait for one among a set of activities to complete before proceeding; if several of the activities being waited for are executed, the simple merge fires each time that one of them completes.
- Discriminator - wait for one among a set of activities to complete before proceeding; if several of the activities being waited for are executed, the discriminator only fires once.
- N-out-of-M Join - same as the discriminator but it is now possible to wait until more than one of the preceding activities completes before proceeding by setting a parameter N to some natural numberNatural numberIn mathematics, the natural numbers are the ordinary whole numbers used for counting and ordering . These purposes are related to the linguistic notions of cardinal and ordinal numbers, respectively...
greater than one.
Structural Patterns
- Arbitrary Cycle - do not impose any structural restrictions on the types of loops that can exist in the process model.
- Implicitly Terminate - terminate an instance of the process if there is nothing else to be done
Multiple Instances (MI)
- MI without synchronizing - generate many instances of one activity without synchronizing them afterwards
- MI with a prior known design time knowledge - generate many instances of one activity when the number of instances is known at the design time (with synchronization)
- MI with a prior known runtime knowledge - generate many instances of one activity when a number of instances can be determined at some point during the runtime (as in FOR loop but in parallel)
- MI without a prior runtime knowledge - generate many instances of one activity when a number of instances cannot be determined (as in WHILE loop but in parallel)
State-based patterns
- Deferred Choice - execute one of a number of alternative threads. The choice which thread is to be executed is not based on data that is available at the moment when the execution has reached the deferred choice, but is rather determined by an event (e.g. an application user selecting a task from the worklist, or a message being received by the process execution engine).
- Interleaved Parallel Routing - execute a number of activities in any order (e.g. based on availability of resources), but do not execute any of these activities at the same time/simultaneously.
- Milestone - allow a certain activity at any time before the milestone is reached, after which the activity can no longer be executed.
Cancellation Patterns
- Cancel Activity - stop the execution of an enabled activity
- Cancel Case - stop the execution of a running process
- Cancel Wait - continue execution of a running process without prior completion event
The above workflow patterns have been used to evaluate the functionality of commercial products supporting the development of process-oriented applications. They have also been used to evaluate a number of proposed standards, including BPEL, BPMN, UML
Unified Modeling Language
Unified Modeling Language is a standardized general-purpose modeling language in the field of object-oriented software engineering. The standard is managed, and was created, by the Object Management Group...
Activity diagram
Activity diagram
Activity diagrams are graphical representations of workflows of stepwise activities and actions with support for choice, iteration and concurrency. In the Unified Modeling Language, activity diagrams can be used to describe the business and operational step-by-step workflows of components in a system...
, XPDL
XPDL
The XML Process Definition Language is a format standardized by the Workflow Management Coalition to interchange business process definitions between different workflow products, i.e...
, etc. It has been noted that not all these patterns are relevant in all application domains, so care must be taken when using the above workflow patterns to select a particular language or system for a given application.
The workflow patterns have also been used as initial requirements in the design of a workflow language and open-source system called YAWL
YAWL
This article is about the workflow system. For the sailing craft, see yawl.YAWL is a workflow language based on the Workflow patterns. The language is supported by a software system that includes an execution engine, a graphical editor and a worklist handler...
.
Several extensions to the above set of workflow patterns have been proposed. In particular, the same research groups that developed these patterns, have also proposed a set of Workflow Data Patterns, Workflow Resource Patterns, Workflow Exception Handling Patterns, and Service Interaction Patterns.
Another classification
Another classification of workflow patterns is the following:Independent/Pooled: where each component of scheduled work is completed independent of each other component and no component has a specific dependency on any other component. An example would be where staff are serving at a counter - Raoul can serve a customer in his queue without waiting for Jamie to serve a customer in his queue.
Sequential: where each component of scheduled work is dependent on the preceding component. In this case the preceding component controls the advancement of the workflow through subsequent components. An example would be on a production line - Betty cannot affix the radiator cap to the Model T Ford until Veronica has put the radiator in place.
Interdependent/Networked: where each component of scheduled work is dependent on one or a number of other components being completed. In this case the preceding components control the workflow through subsequent components. An example would be a project team - Sarah must wait for several tasks to be completed by Kevin and George before she can execute her task.
Other perspectives
The workflow patterns are not limited to control-flow.Other (workflow) pattern collections include:
- resource patterns ,
- data patterns ,
- exception patterns,
- and service interaction patterns .
These patterns collections have been used to evaluate a variety of workflow processes, both commercial (Websphere, Oracle BPEL, Staffware, SAP workflow, Windows Workflow Foundation, etc.) and open source.
Further reading
- Marlon Dumas, Wil van der Aalst and Arthur H.M. ter Hofstede ed. (2005). Process-Aware Information Systems. John Wiley and Sons. ISBN 0-471-66306-9.
- Volker Kramberg (2006) [ftp://ftp.informatik.uni-stuttgart.de/pub/library/medoc.ustuttgart_fi/STUD-2052/STUD-2052.pdf Pattern-based Evaluation of IBM WebSphere BPEL]: Evaluation of IBM's WebSphere Integration Developer based on Workflow Patterns.
External links
- Workflow Patterns
- BPMN 2 examples of original 20 workflow patterns