a university for the world real r © 2009, chapter 3 advanced synchronization moe wynn wil van der...

a university for the worldrealR

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© 2009, www.yawlfoundation.org YYY

Chapter 3Advanced Synchronization

Moe Wynn

Wil van der Aalst

Arthur ter Hofstede


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© 2009, www.yawlfoundation.org

Overview

• Introduction• Informal semantics• Formal semantics• Algorithm• Optimisation• Conclusions


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The Many Ways to Join

• When is task D enabled?• Three choices: XOR, AND, and OR

• AND-join– Full synchronization

• XOR-join– No synchronization

• OR-join– Active paths synchronization

A

B D

C


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Non-local semantics of a General OR-Join ("bus driver semantics")


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Advanced Synchronization: OR-join

• Synchronization is only performed for active threads

• Difficult to formally capture a desired informal semantics

• Difficult to determine when to enable an OR-join task

• Synchronizing Merge pattern in Workflow patterns


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Different interpretations

• OR Gateway in BPMN• OR-join in Event-driven Process Chains (EPCs)• OR-join in workflow systems e.g. InConcert, eProcess,

MQ workflow

• Syntactic restrictions• Vicious Circles• Local semantics• No formal semantics


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Informal semantics


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OR-join• An OR-join is enabled at a marking iff

– there is at least one token in one of its input conditions and– It is not possible for more tokens to arrive in other empty input

conditions in the reachable markings from the current marking

• An OR-join task is not enabled at a marking iff– it is possible to place tokens in the unmarked input conditions of an OR-

join task in the reachable markings from the current marking

A

B

C

D

c1

c2

c3

c4

c5

c6

E

Enabled?


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Challenges

• How to capture the non-local semantics? • Formal semantics without syntactical restrictions

– Structural restrictions - InConcert, eProcess, MQ Workflow (acyclic)

– True/False tokens• Complications for complex workflows

– Unstructured– Cancelation regions – Multiple OR-joins– Loops


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Unstructured workflows

A

B

C

D

c1

c2

c3

c4

c5

c6

E F

• E should only wait for tokens in c5 and c6 (two paths)• Not the three paths from the corresponding OR-split after A


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Cancelation regions

A

B

C

D

c1

c2

c3

c4

c5

c6

E

• Cancellation region of task D – c2, C, c5• M=c2+c3+c4 -C-> M’= c3+c4+c5, E is not enabled• M=c3+c4+c5 -D-> c4+c6 (Remove token from c5), E is enabled• How to predict that E should be enabled at c3+c4+c5 without doing

a reachability analysis?


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Multiple OR-joins

A

c1

c2 B

C

D

E

F

c3

c7c4

c5 c6

• Two OR-joins E and F• What happens at marking c1+c3?• How should we treat E when considering when F should be

enabled?


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Formal semantics


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Considerations

• How to deal with complex constructs supported in YAWL?– Unstructured workflows– Cancelation, hierarchy, multiple instances, loops– Multiple OR-joins

• Reset nets – extend Petri nets with reset arcs • OR-join semantics are defined in terms of Reset nets

– Support for cancelation regions


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Reset nets1. Transition removes all tokens and not just one when it fires

2. Reset action can be ineffective

3. A reset arc can affect any place in the entire net

•Reachability notion is undecidable for reset nets with more than two reset arcs•Coverability notion is decidable using backwards firing algorithm

Complexity of a reset arc


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Backwards firing rule

• Backwards search techniques of Well-structured Transition Systems

• Input places that are not reset places– Add one token

• Output places that are not reset places– Remove one token

• Reset places that are also input places– Add one token (coverability)


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Semantics (Reset nets)

• An OR-join in a YAWL net is enabled at a marking M iff– M has tokens in one or more input conditions of the

OR-join and– there is no marking in the future states of the

corresponding reset net that covers M.


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Transformations

t

p1

AND-joint

pN

p1

XOR-join t

tp

pN

p1

AND-splitt

pN

p1

XOR-splitt

OR-splitt

pN

pN

YAWL Reset net

ts tept

p1

ts

pN

pt

p1 tsp1

pN

pt

tspN

ts teptp

YAWL Reset net

pN

p1

tept

pN

p1tep1

pt

tepN

pN

p1tep1

pt

tepN

tep1N

p1


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Constructs affecting the semantics

• Cancelation regions (reset nets)• Structured, unstructured and infinite loops (coverability)• Composite tasks, Multiple instances (atomic tasks)• Multiple OR-joins

• Ignore other OR-joins

• Treat them as AND-joins (Pessimistic)

• Treat them as XOR-joins (Optimistic)


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Characteristics

• General – no syntactical restriction– closely matches the informal semantics– behave as expected even for unsound nets

• Formal– defined using reset nets formalism – decided using backwards coverability algorithm

• Decidable– algorithm is applicable for workflows with

cancelation, multiple OR-joins and loops


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Algorithm


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Algorithm

1. Transform a YAWL net into a Reset-net

2. Generate a set of possible enabling markings X

3. For each marking x in X, generate a set of backwards coverable markings X’

4. Check if the current Marking M or a smaller marking is found in X’

5. If so, returns False – the OR-join is not enabled at M

6. Otherwise, returns True - the OR-join is enabled at M


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At marking c1+c7 task G is enabled if and only if ....

Example


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At marking c1+c7 task G is enabled if and only if ....

... it is possible to cover c6+c7!

Example (Reset net)


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G is not enabled !


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Carrier Appointment Example

the freight is physically picked up

[no delivery information]

[Truck-Load (TL)] [Single Package (SP)]

[Less than Truck-Load (LTL)]

[else: both]

[no delivery information]

[no pickup information]

4-eye pattern

Prepare Route GuideEstimate Trailer

Usage

Prepare Transportation Quote

CarrierTimeout

Arrange Delivery Appointment

Arrange Pickup Appointment

Create Bill of Lading

c1

o: Client Liaison c: Warehouse Officer


[else]





Create Shipment Infomation Document

sArrange Pickup

AppointmentArrange Delivery

Appointment

Modify Pickup Appointment

Modify Pickup Appointment

Modify Delivery Appointment

Modify Delivery Appointment

ProduceShipment Notice

s: Supply Officer

o: Client Liaison

o: Client Liaison

o: Client Liaisono: Client Liaison

c: Warehouse Officer

s: Supply Officer

c: Warehouse Officer s: Supply Officer

s: Supply Officer c: Warehouse Officer

c: Warehouse Admin Officer



s: Senior Supply Officer: Masters in SC & L Mgt

Create Carrier Manifest

s: Senior Supply Officer: Masters in SC & L Mgt

c4

c2

c3

Complete Pick up / Delivery

[Truck-Load (TL)]

[Single Package (SP)]



Usage


CarrierTimeout




c2

c3 c4

c1

OR-join Analysis: Create Bill of Ladingc1+c2 -> False (can cover c1+c4)c2+c3 -> False (can cover c3+c4)c1+c4 -> Truec3+c4 -> True


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Optimization


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Optimization

– Structural restriction• Remove parts of the net not associated with an OR-join• Restrict the search space using structural info• Apply once for each OR-join

– Active projection• Ignore parts of the net that cannot be marked from a given

marking• Restrict the search space using marking-dependent info• Apply once for every marking change


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Applying Optimisation to Carrier Appt example

[Truck-Load (TL)]

[Single Package (SP)]



Usage


CarrierTimeout




Structural Restriction Region for Create Bill of Lading

Active Projection Region for marking c1+c2

c3 c4


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Implementation in YAWL

• OR-join enabling algorithm– Computationally very expensive

• Engine – Algorithm with optimization techniques

• Editor– Reduction rules for OR-joins– Soundness property– OR-joins in a cycle– Unnecessary OR-joins check


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Conclusions

• Introduction– Advanced synchronization: the OR-join construct in YAWL

• Informal semantics– Wait to synchronize and proceed when it is not possible

• Formal semantics– Reset nets, Backwards coverability

• Algorithm– Expensive, repetitive

• Optimization– Structural restriction, Active projection

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