Transcript The Worklet Service

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Chapter 11
The Worklet Service
Michael Adams
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The Worklet Service
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• The Worklet Service has been implemented as a YAWL
Custom Service
– but it is in no way limited to the YAWL environment, and may
be ported to other environments by making the necessary links
in the service interface
• It comprises two discrete but complementary subservices:
– a Selection Service, which enables dynamic flexibility for
process instances, and
– an Exception Service, which provides facilities to handle both
expected and unexpected process exceptions at runtime
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A Worklet Is:
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•
•
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•
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a small, self-contained, complete workflow process
designed to handle one specific action (task) in a larger, composite
activity (process)
a member of an extensible
repertoire
contextually chosen at runtime to
perform a task
a dynamically substituted subprocess and/or exception
compensation process
an implicit part of the parent
process model
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The Selection Service
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• The Selection Service allows a workitem to be
substituted at runtime with a dynamically selected
worklet
– Corresponding data inputs of the original workitem are mapped
to the inputs of the worklet, and the worklet is launched as a
separate case
– When the worklet has completed, its output data is mapped
back to the original workitem, which is then checked back into
the engine, allowing the original process to continue.
• An extensible repertoire of worklets is maintained by
the service for each task in a specification.
– Each worklet may be designed and provided to the service at
any time, even while a parent process instance is executing
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Exception Handling in YAWL
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• Exception Handling is a specific area within the wider
Flexibility domain
• It therefore makes sense that an exception handling
solution would extend from ways of making workflow
more flexible
• In YAWL, an extension of the Worklet Service, known
as the Exception Service, provides dynamic exception
handling with corrective and compensatory actions
worklet
service
flexibility
exception
handling
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The Exception Service
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• The Exception Service uses the same repertoire and
Ripple-Down Rule (RDR) set framework as the wider
Worklet Service
• It can handle both expected and unexpected exceptions
at runtime
• For each anticipated exception, a set of repertoiremember exception handling processes may be
maintained, known as exlets
– an exlet is defined in the YAWLeX language using a graphical
editor and may include worklets as compensation processes
• An exlet may be dynamically incorporated into a
running workflow instance on an as-needed basis
– selected based on the context of the case and the type of
exception that has occurred
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Architecture
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Interface X Architecture
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Internal Service Architecture
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Process Definition
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Order Fulfilment: Payment Sub-Net
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Same Sub-Net, Worklet Enabled
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Issue Shipment Invoice Dialog
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Worklet Decomposition Rules
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• It is only necessary to map from the parent task to the worklet
those variables that contain values to be displayed to the user,
and/or those variables that the user will supply values for to be
passed back to the parent task when the worklet completes
• The definition of variables is not restricted to those defined in the
parent task. Any additional variables required for the operation of
the worklet may also be defined here; their values will not be
passed back to the parent task when the worklet completes
• Only those variables that have been defined with an identical name
and data type to variables in the parent task and:
– with a usage of ‘Input Only’ or ‘Input & Output’ will have data passed into them
from the corresponding variables of the parent task when the worklet is
launched
– with a Usage of ‘Output Only’ or ‘Input & Output’ will have their data values
mapped back to the corresponding variables of the parent task by the Worklet
Service when the worklet completes
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Exlets
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• Exception handling processes – or exlets – are defined graphically
using a set of 'exception handling primitives'
• A specification can have a repertoire of handlers for each exception
type - the appropriate handler will be chosen contextually at runtime
– For item-level exceptions, each item in the specification has its own
repertoire of handlers
• Each exlet may contain any number of compensation primitives,
each associated with a worklet
– Worklets can run in parallel
with the parent task, or
while the parent is
suspended
– A compensation primitive
can launch any number of
worklets simultaneously
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Exception Perspectives
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An exlet can take these actions against these targets for these
events
Targets
Events
Actions
Task
CasePreConstraint
Suspend
Case
CasePostConstraint
Continue
All Cases
ItemPreConstraint
Restart
Ancestor Cases
ItemPostConstraint
Complete
TimeOut
Fail
CaseExternal
Remove
ItemExternal
Compensate
ResourceUnavailable
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A Simple RDR
Example
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Root
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Ripple Down Rule Sets
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• A repertoire of exlets may be formed for each
exception type
• Each specification has a unique rule set that may
contain between one and nine tree sets (or sets of
rule trees)
– one for selection rules and one for each of the eight
implemented exception types
• Three relate to case-level exceptions and so each of
these will have at most one rule tree in the tree set
• The other six tree sets relate to work item-level
events, and so may have one rule tree for each task
in the specification
– the tree sets for these eight rule types may consist of a
number of rule trees
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Example Rule Tree
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• TimeOut tree for Arrange Delivery Appointment task of the
Payment subnet of the OrderFulfilment Process
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Rule Set Hierarchy
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• Rule Node: contains the details of one discrete ripple-down
rule. The conclusion of a node equates to an exlet.
• Rule Tree: consists of a number of rule nodes conceptually
linked in a binary tree structure.
• Tree Set: a set of one or more rule trees. Each tree set is
specific to a particular exception type.
– The tree set of a case-level exception type will contain exactly one
tree.
– The tree set of an item-level type may contain one rule tree for
each task of the specification that has rules defined for it.
• Rule Set: a set of one or more tree sets representing the entire
set of rules defined for a specification. Each rule set is specific
to a particular specification.
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Worklet-Exlet Hierarchy
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compensation worklet
cancel stadium
book theatre
advise fans
ChangeToMidVenue
ItemPreConstraint
exlets
C
C
C
C
‘parent’ process
book
stadium
sell tickets
do show
OrganiseConcert
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Cornerstone Cases
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• Each rule node has a set of case descriptors called
the Cornerstone Case
– describe the actual case context that was the catalyst for
the creation of the rule
• When a new rule is added to the rule set, its
conditional predicate is determined by comparing
the descriptors of the current case to those of the
cornerstone case and identifying a sub-set of
differences.
• Not all differences will be relevant—it is only
necessary to determine the factor or factors that
make it necessary to handle the current case in a
different fashion to the cornerstone case to define a
new rule.
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Contextual Data
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• The conditional expressions in each node can test
values sourced from:
–
–
–
–
The data parameters of the current workitem
The data parameters of the current case
Process state information
A discrete RDRConditionFunction class, which allows the
definition of functions that can use data from any external
source, such as:
• Archival data from process logs
• External databases
• User-defined values
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The Rules Editor
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• The Worklet Rules
Editor is a purpose
built tool that
enables:
– the browsing of rule
sets;
– the addition of new
rules to existing rule
trees;
– the creation of new
rule trees for existing
rule sets; and
– the creation of new
rule sets.
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Adding a New Rule Tree/Set
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• The New Rule
Set Screen
supports the
creation of:
– New rule trees
for non-defined
exception
types
– New tree sets
for new
specifications
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Adding a New Rule
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• Each node incorporates a set of case descriptors, called the
‘cornerstone case’, which describe the actual case that was the
catalyst for the creation of the rule.
•
A new rule is determined by
comparing the descriptors of the
current case to those of the
cornerstone case of the last
satisfied rule and identifying a subset of differences.
– it is only necessary to determine
the factors that make it necessary
to handle the current case
differently
•
The current case descriptors
become the cornerstone case for
the newly formulated rule.
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Replacing a Running Worklet
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• After a new rule is added, the Rules Editor provides an administrator with
the choice to replace the previously started (inappropriate) worklet instance
with an instance of the worklet defined in the new rule.
• If the administrator chooses to replace the worklet, the Rules Editor
contacts the worklet service to request the change. The service responds
with a dialog similar to that below.
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Static Process Model
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A Worklet Solution
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A Worklet Solution
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Meeting the Criteria for Flexibility and
Exception Handling
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• Flexibility and Re-use:
–
–
–
–
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Extensible worklets repertoire
Contextual choice
Constraint checking rules
Modular approach: loosely defined
Worklets may be reused
complete
• Different repertoires
• Selection/Compensation
• Adaptation via Reflection:
– Plans not prescriptive but fluid and extensible
– Each instance a 'learning experience'
– Dynamically extensible rule set
• Encapsulates 'organisational memory'
• Refines understanding of domain
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Meeting the Criteria for Flexibility and
Exception Handling
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• Dynamic Evolution of Tasks:
– No more straight-jacketed, regimented, demotivating work
– Different methods and efficiencies
larger repertoires
– Natural work methods
motivation, ownership,
responsibility
– Process evolves incrementally, not monolithically
• Locality of Change:
– Change emanates from local level
• Workers (domain experts) nominate change
– Worklet/Exlet addition does not change parent or repertoire
• Ease of verification
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Meeting the Criteria for Flexibility and
Exception Handling
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• Comprehensibility of Process Models:
– Allows models to concentrate on business logic
• Removes conditional control flow constructs
– Parent process is a guide
– Worklets/Exlets better understood by individual participants
– Different levels of granularity
• Exceptions as 'First-Class Citizens':
– Fully featured exception handling service
• Context-aware Exlets
– Detects, reacts, handles, maintains and incorporates
• Learning experience
• Organisation memory retained
– Supports expected and unexpected exceptions in real-time
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