Process Execution and Semantic

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Process Execution andSemantics


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CS752, November 2012 www.hilcoe.com.et 2

Process Mapping and Execution in BPMN 1.X


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Process Mapping and Execution in BPMN 2.0


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Important features of BPMN 2.0 BPMN 2.0 has a valid Metamodel

A metamodel is a precise definition of theconstructs and rules needed for creatingspecific model

Several benefits associated to metamodel: It formalizes the definition of models and elements

It formalizes the relationship between elements

It enables interoperability A valid metamodel is an ontology


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Important features of BPMN 2.0

BPMN metamodel defines all BPMN

entities with all other attributes andrelations

Where entities could be any visual shapeslike, events, activities or gateways or invisiblestructures like web service operations or data

structures

The metamodel enables interchange,interoperability and execution of models


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BPMN 2.0 conformance levels

Software compliance or conformance with BPMN 2.0 isachieved, iff, the software fully matches the applicableconformance points specified

Process modeling conformance BPMN process types, BPMN process elements, visual appearance,

structural, attributes, extended and optional elements, visualinterchange

Process execution conformance Execution semantics, import of process diagrams

BPEL Process Execution Conformance

Choreography Modeling conformance BPMN choreography types, BPMN choreography elements, visual

appearance, choreography semantics, visual interchange


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BPMN 2.0 Execution Semantics

The purpose of the execution semantics is to describe aclear and precise understanding of the operation of theelements

BPMN execution semantics include: Process instantiation and termination semantics Semantics of different types of activities

Normal, subprocess, loop, multiple instances Semantics of different kinds of gateways

Parallel, event based, exclusive,

Semantics of different kinds of events Start, intermediate, end, event sub-process,

Some elements are non-operational Manual task, abstract task, ad-hoc subprocess,


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BPMN 2.0 Execution Semantics

BPMN 2.0 presents an execution semantics that

defines precisely how models in the BPMNnotation should behave when executed in a tool

The execution semantics is defined informally

using natural language


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Execution semantics of elements are presented

in the following structure:A description of the operational semantics of theelement

Exception issues for the element where relevant

List of workflow patterns supported by the elementwhere relevant

A process can start as a result of any of thefollowing:

Its start event occurs With event based gateways With receive task


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In these circumstances new process instances will becreated and the instantiate flag of the process is set totrue. [a runtime b/r]

For a Process instance to become completed, all tokensin that instance MUST reach an end node, => (1) There is no token remaining within the Process instance,

(2) No Activity of the Process is still active.

* Note also that if a token reaches a Terminate End Event,the entire Process is abnormally terminated


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Description of Some BPMN Elements:

FlowElement

Flow nodes: Event Activity

ChoreographyActvity Gateway

SequenceFlow

Some attributes of a FlowElement

Is an abstract super class for all elements that can appear in a Process flow


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Class diagram for FlowElement


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Activity

Task

SubProcess

CallActivity (Reusable Activity /Global Tasks)

It inherits the attributes and model associations of

FlowElement Some more attributes of Activity element:

Is an abstract super class for all Concrete Activity types


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The default value is 1. The value MUST NOT be less than 1. This attribute

defines the number oftokens that MUST arrive before the Activity can

begin. Note that any value for the attribute that is greater than 1 is an

advanced type of modeling and should be used with caution.

startQuantity: integer

= 1

This references the Intermediate Events that are attached to the boundary

of the Activity

boundaryEventRefs:

BoundaryEvent [0..*]

The Sequence Flow that will receive a token when none of the

conditionExpressions on other outgoing Sequence Flows evaluate to true.

The defaultSequence Flow should not have a conditionExpression; any

such Expression SHALL be ignored.

default: SequenceFlow

[0..1]

An Activity MAY be performed once or MAY be repeated. If repeated, theActivity MUST have loopCharacteristics that define the repe-tition criteria

(if the isExecutable attribute of the Process is set to true).

loopCharacteristics:LoopCharac-teristics

[0..1]

A flag that identifies whether this Activity is intended for the purposes ofcompensation. Iffalse, then this Activity executes as a result of normal

execution flow. Iftrue, this Activity is only activated when a

Compensation Event is detected and initiated under Compensation

Event visibility scope (see page 281 for more information on scopes).

isForCompensation:boolean = false

DescriptionAttribute Name


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Whathapp

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BPMNA

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diagram


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The Start Quantityattribute of an activity defines thenumber of token required for the activity to be Readyfor execution

The first InputSet where all data inputs are available(by means of data associations) is used to start theexecution of the Activity. If no InputSet is available,then the execution will wait until this condition is met

this InputSet is based on data associations

For tasks that are attached after Event-BasedExclusive Gateways, the first element (Task or Event)that completes causes all other Tasks to be withdrawn

The diagram shows complete conditions faced by anactivity from start to end


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If the Activity has no outgoing Sequence Flows,

the Activity will terminate without producing anytokens and termination semantics for thecontainer is then applied

Automated activities are executed by a system,while manual activities are executed by humans(or a system of that capability)

All Activities share common attributes andbehavior such as states and state transitions.


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Class diagram for Task

A Task is a Concrete Activity, and has multiple variations

A task has no more attributes than it inherits from the Activity super class; while thevariations have


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All about Tasks

Service Task: Upon activation, the data in the inMessageof the Operationis assigned from the data in the Data Inputof the Service Task theOperation is invoked. On completion of the service, the data in the DataOutputof the Service Task is assigned from the data in the outMessageofthe Operation, and the Service Task completes. If the invoked servicereturns a fault, that fault is treated as interrupting error, and the Activity (theService Task) fails

Send Task: Upon activation, the data in the associated Message isassigned from the data in the Data Input of the Send Task. The Message issent and the Send Task completes

Receive Task: Upon activation, the Receive Task begins waiting for theassociated Message. When the Message arrives, the data in the DataOutput of the Receive Task is assigned from the data in the Message, andReceive Task completes. For key-based correlation, only a single receive

for a given CorrelationKey can be active, and thus the Message matches atmost one Process instance. For predicate-based correlation, the Messagecan be passed to multiple Receive Tasks. If the Receive Tasks instantiateattribute is set to true, the Receive Task itself can start a new Processinstance


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All about Tasks

User Task: Upon activation, the User Task is distributed to the assignedperson or group of people. When the work has been done, the User Taskcompletes

Manual Task: Upon activation, the manual task is distributed to theassigned person or group of people. When the work has been done, theManual Task completes. This is a conceptual model only; a Manual Task isnever actually executed by an IT system

Business Rule Task: Upon activation, the associated business rule is

called. On completion of the business rule, the Business Rule Taskcompletes

Script Task: Upon activation, the associated script is invoked. Oncompletion of the script, the Script Task completes

Abstract Task: Upon activation, the Abstract Task completes. This is aconceptual model only; an Abstract Task is never actually executed by an ITsystem.


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Tasks


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Read on the execution semantics of SubProcess, Call Activity, Ad-Hoc Sub-Process, Loop Activity and MultipleInstance Activity


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Send task

A task specifically designed to send a Message to an external Participant


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Send Task Model Associations


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Execution Semantics for Gateway

Example: Parallel Gateway

The Parallel Gateway cannot throw any exceptionException Issues

The Parallel Gateway is activated if there is at least

one token on each incoming Sequence Flow

The Parallel Gateway consumes exactly one token

from each incoming Sequence Flow and producesexactly one token at each outgoing Sequence Flow

If there are excess tokens at an incoming Sequence

Flow, these tokens remain at this Sequence Flow

after execution of the Gateway

Operational

Semantics


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Execution Semantics for Events

Start Events For single Start Events, concerned with starting a new Process

instanceeach time the Event occurs. Sequence Flows leaving

the Event are then followed as usual

Intermediate Events The handling consists of waiting for the Event to occur. Waiting

starts when the Intermediate Event is reached Once the Event occurs, it is consumed

End Event

The behavior associated with the Event type is performed

Study all the different variation of events


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BPMN to WS-BPEL

Here a mapping of a BPMN model to WS-BPELthat is derived by analyzing the BPMN objectsand the relationships between the objects is

considered

Not all BPMN orchestrationProcesses can bemapped to WS-BPEL in a straight-forward way. One reason being BPMN allows the modeler to draw

almost arbitrary graphs to model control flow

Only sound and well synchronized BPMNOrchestration processes can be directly mappedto WS-BPEL


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Two possible, but undesirable states:

Deadlock: when token on some sequenceflow couldnt be removed in known futuretime

Lack of synchronization: when there is morethan one tokenon some Sequence Flow

Q: Please give examples to each of thesescenarios


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Directed graphs

The Gateways and the Sequence Flows of the BPMNorchestration Process form a directed graph

A blockof a diagram is a connected sub-graph that isconnected to the rest of the graph only through exactlytwo Sequence Flows:

exactly one Sequence Flow entering the block and exactly oneSequence Flow leaving the block

A block hierarchyfor a Process model is a set of blocks

of the Process model in which each pair of blocks iseither nested or disjoint and which contains the wholeprocess model


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Example Block Hierarchy


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The mapping process [BPMN Construct] = WS-BPEL Construct

E.g.

[c1]

[T1]

[c2]

[T2]

=


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Process Execution and Semantic