3rd Dec, 2008 ICSOC, 2008 1

Determining QoS of WS-BPEL Compositions

Debdoot Mukherjee, IBM ResearchPankaj Jalote, IIT Delhi

Mangala Gowri Nanda, IBM Research

3rd Dec, 2008 ICSOC, 2008 2

Agenda Background and Motivation

Importance of WS-BPEL & QoS therein QoS in Structured Workflows WS-BPEL vs Structured Workflows

QoS Model for WS-BPEL Inputs Overall Approach Model Elements Algorithm Activity Specific QoS Rules

QoS Tool Conclusions

3rd Dec, 2008 ICSOC, 2008 3

Background With SOA gaining prominence

More composed services than written from scratch A number of services providing same functionality

in a marketplace for services Quality of Service (QoS) is the key differentiator

during orchestration BPEL is the de facto standard for composing web

services A Comprehensive QoS computation model for

WS-BPEL processes is required.

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QoS in Structured Workflows

Cardoso1 applies reduction rules on a structured workflow to compute its reliability, time and cost. Sequential Parallel Conditional Loop Fault Tolerance

1 Jorge A. Cardoso, Quality of Service and Semantic Composition of Workflows, Ph.D. Thesis, University of Georgia, 2002

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Workflow Reduction

R2 = 0.9T2 = 5

C2 = 10

R1 = 0.8T1 = 15 C1 = 5

R3 = 0.7T3 = 20 C3 = 15

R4 = 0.9T4 = 10 C4 = 10

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Workflow Reduction

R′ = R1 * R2 = 0.7T ′ = T1 + T2 = 20 C′ = C1 + C2 = 15

R′′ = R3 * R4 = 0.63T′′ = T3 + T4 = 30

C′′ = C3 + C4 = 25

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Workflow Reduction

R = R′ * R′′ = 0.43T = Max(T′, T′′) = 30

C = C′ + C′′ = 40

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Workflow Reduction

A D

C E

B

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Why is BPEL different ??

BPEL has features of both structured languages as well as graph based flow languages Greater expressive power arising from

synchronization links between activities possibly nested inside different structured constructs

Fault handling Event driven programming

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Lack of Structure…But more power !!

receive loanAmount

invoke LoanApprover invoke LoanAssessor

invoke LoanProcess

loanAmount >= $100K loanAmount < $100K

reply

risk = “low”

risk = “high”

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Passport Service Exampleflow

invoke ifinvoke invoke

invoke

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Inputs to the QoS Model

QoS parameters for all web services in the composition

Average Waiting Times for incoming messages: <receive> & <onMessage>

Control Flow parameters Branch Probabilities in <if> & <pick> Average number of unrolling for Loops Probability of success for <transitionCondition> Fraction of faults captured by <catch> blocks

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QoS Model Overview

Build an activity graph where activities / scopes / handlers are the nodes.

Each node X is annotated by its: Child Nodes – Activities directly contained within X Control Dependencies

Source of an incoming synchronization link to X An activity that precedes X in <sequence>

Catch blocks and all handlers Recursive QoS algorithm calculates QoS at

each node.

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QoS Elements

P(SX): Probability that X successfully executes in any run of the process

ETX: Expected time of completion of X measured relative to the start of the process

CostX: Sum of expected costs of all child nodes of X

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Auxiliary Elements

P(startX): Probability that X may start execution in a state that is semantically in accordance with one which is expected at that point. P(SX) = F(P(startX), f (P(Schild1

), P(Schild2), ..))

STX: Denotes the time instant when X is ready to start all its dependencies have ended. ETX = STX + Time taken by children of X

PCX: Conditional probability that X will start execution given that the parent of X starts. CostX = Σ (PCX * Costchildi

)

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Algorithm : setQoS(X)

for all Z such that X is dependent on Z dosetQoS(Z)

end for

Compute P(startX), STX and PCX // Aux.for all Z such that Z is a child of X do

setQoS(Z)end for

Compute P(SX), ETX and CostX according to rules specific to Type of X

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Determine P(startX)

For each incoming link Ai , find the probability that it evaluates to true:

Represent the Join Condition in Sum of Products form and apply standard laws of union & intersection to evaluate P(joinCondition = true)

To compute P(startX) we take a product of the following probabilities, if they are applicable: P(startparentX

), if X starts along with parent(X)

P(S ) of predecessor in sequence P(joinConditionX = true)

)().()( )( trueConditiontransitionPPtrueAP ii AAsourcei S

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P(startX) Calculation Example1.0

0.8

R = 0.9 R = 0.9R = 0.8

0.7 0.9

0.8

1.0 1.01.0

1.0

P(X) = 0.64 P(Y) = 0.63 P(Z) = 0.81

P((X U Y) ∩ Z) = 0.7

0.7R = 0.9

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Compute STX and PCX

STX is taken to be the maximum of: ET of predecessor in <sequence> ET of source activities of all incoming links STparent(X)

PCX is calculated as:)|( )(XparentXX startstartPPC

1))(|( Since ,)(

)()(

)(

XXparentXparent

X startPstartPstartP

startP

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QoS for <invoke>

Rws : Conditional probability that an invocation to an external web service fails even if the call is made with proper arguments

P(S invoke) = Rws x P(start invoke) ETinvoke = STinvoke + Tws

Costinvoke = Cws

Rws and Tws incorporate failures and latencies respectively that arise both at the service site or from the network

3rd Dec, 2008 ICSOC, 2008 21

QoS in Structured Activities

Activity P(S ) ET Cost

Sequence

Flow

If

Pick

Loop

)( lastChildP S lastChildET

i

childchild iiCostPC

ii

P )( sinkS )( sink iETMax i

i

bri iPselP )()( S

i

evti iPselP )()( S

i

bri iETselP )(

i

evti iETselP )( ii evt

ievti CostPCselP )(

ii bri

bri CostPCselP )(

nchildi PstartP )()( S loopChildloop ETnST loopChildCostn

i

childchild iiCostPC

3rd Dec, 2008 ICSOC, 2008 22

Reliability of Passport Service

1.0

0.8

R = 0.9 R = 0.9R = 0.8

0.7 0.9

0.8

1.0 1.01.0

1.0

P(X) = 0.64 P(Y) = 0.63 P(Z) = 0.81

P((X U Y) ∩ Z) = 0.7

0.7R = 0.9

, 0.63

, 0.704

, 0.704

, 0.8 , 0.9 , 0.9

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Tool Screenshot

3rd Dec, 2008 ICSOC, 2008 24

Conclusions

QoS computation of WS-BPEL processes requires special attention because of the unique features supported by it

Fault handler, event handlers need to dealt with in the same light as other activities

Mechanism to estimate QoS improvement derived from Fault Tolerance constructs is important.

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Thank You!!

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Backup

3rd Dec, 2008 ICSOC, 2008 27

Fault Handlers

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Motivation – Fault Tolerance

High levels of reliability, strict adherence to performance limits required in mission critical applications

Create dependable apps out of undependable services

High risk is involved because software is not owned

One does not build redundant services, only pays a certain fee

3rd Dec, 2008 ICSOC, 2008 29

QoS in Structured WorkflowsCardoso gives reduction rules for structured constructs

Construct Reliability Response Time Cost

Sequential

Parallel

Conditional

Loop

i

iR i

iT i

iC

i

iR

ii

i Cp

i

iC}Max{ iT

i

i

p

T

1 i

i

p

C

1ii

ii

Rp

Rp

1

)1(

* In addition, he specifies QoS rule for a Fault Tolerant construct.

ii

i Tp ii

i Rp

3rd Dec, 2008 ICSOC, 2008 30

Quality vs Quality of Service

Software Quality Quality of Service

Subjective Objective

Contains both external and internal

characteristics

Contains only external attributes that are

unambiguously measurable

Reliability, Execution Time, Understandability

Portability, Maintainability

Reliability, Availability, Response Time, Cost.

3rd Dec, 2008 ICSOC, 2008 31

Passport Service Inputs