1June 10, 2007, IDAR’07, Beijing, China

Ensuring Consistent Termination of Composite Web Services

An Liu1,2,3 and Qing Li2,3

1 Department of Computer Science and Technology

University of Science & Technology of China (USTC), Hefei, China 

2 Joint Research Lab of Excellence

CityU-USTC Advanced Research Institute, Suzhou, China 

3 Department of Computer Science

City University of Hong Kong (CityU), Hong Kong, China

liuan@ustc.edu itqli@cityu.edu.hk

2June 10, 2007, IDAR’07, Beijing, China

Outline

Problem Statement and Background Problem Modeling and Methodology Framework Conclusion and Future Work

3June 10, 2007, IDAR’07, Beijing, China

Web Services CompositionClient App Travel Agent

Booking Request

Hotel Res. OK

Tim

eline

Web Services

Flight TaxiHotel

Flight RequestHotel RequestTaxi Request

Flight Res. OK

Taxi Res. OK

Booking Response

Hotel CancellationTaxi Cancellation

Reliable Composition Needs Transactional Support!

4June 10, 2007, IDAR’07, Beijing, China

ACID Transaction Insufficient

Composite web services Last for hours or days Work in an open environment Cross administrative boundaries

ACID transaction is insufficient Trust and timeliness no longer apply

Compensation-based transaction No common transaction semantics

5June 10, 2007, IDAR’07, Beijing, China

Some Representative Related Work Transaction semantics declaration

Mikalsen et al. (2002): declare transaction semantics at operation level and transactional property at service level

Transactional property (TP) deduction Vidyasankar and Vossen (2004): deduct TP of composite services

from TP of component services Different Atomicity Requirements

Fauvet et al. (2005): support flexible atomicity through TP-based service selection

Bhiri et al. (2005): verify Accepted-Termination-States (ATS) based atomicity by some transactional rules

Montagut and Molva (2006): support ATS-based atomicity through TP-based service selection and automatic coordination

6June 10, 2007, IDAR’07, Beijing, China

State of the Art

Current research has Defined TP of web services

Guaranteed Reliable Service Composition in the context of heterogeneous TP

Autonomy of services was overlooked Compensation has temporal and cost

constraints (Benatallah et al. 2004) e.g., deadline of hotel reservation cancellation

7June 10, 2007, IDAR’07, Beijing, China

Problem Statement

How to get reliable composition when both autonomy and heterogeneity are considered?

Given The control flow of a composite service Component services with heterogeneous TP Temporal constraints of compensation

How to get reliable composition where the composite service will terminate in a consistent state?

8June 10, 2007, IDAR’07, Beijing, China

Outline

Problem Statement and Background Problem Modeling and Methodology Framework Conclusion and Future Work

9June 10, 2007, IDAR’07, Beijing, China

Definitions

Transactional Property (TP) of web services Compensatable (c): normal operation (on) and

compensation operation (oc)

Retriable (r): Only normal operation Pivot (p): Only normal operation

Temporal property of operations Execution time (tx): e.g., 5 seconds

Available time (ta): e.g., [9am, 5pm] Deadline: Maximal Compensatable Time (MCT) For compensation op, ta=[t, t+MCT]

10June 10, 2007, IDAR’07, Beijing, China

Definitions (cont.)

Service s: 5-tuple(tp, on, oc, t, sta) tp{r, c, rc, p} on & oc : normal and compensation operation

t = (ts, te): starting and ending time sta{initial, active, completed, aborted, failed, cancelled,

compensated}: current state

Process: G(Vt, Vo, E) Vo : set of control flow op. (cfo), where cfo{AND-split, AND-

join, XOR-split, XOR-join, OR-split, OR-join, start-loop, end-loop}

G(Vs, Vo, E): an instance of process G(Vt, Vo, E)

11June 10, 2007, IDAR’07, Beijing, China

Consistent Termination Condition A composite service sc will always terminate

in a consistent state if the consistent termination condition (CTC) holds:

vi.tp{c, rc}, sc.t.te vi.oc.ta for vi Vs in G Example:

sc, s1s2

if s1.on.tx=2 & s1.oc.MCT=5 & s2.on.tx=4

then s1.oc.ta=[2,7], sc.t.te=6

12June 10, 2007, IDAR’07, Beijing, China

CTC Verification

Precondition: global view of a composite service: control flow, transactional and temporal property

Method: simple calculation Problem: NOT easy to get a global view

Dynamic selection Multi-level composition

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Methodology

Assumptions A1: only elementary component services A2: static service selection A3: only compensatable component services A4: only AND-split and AND-join operators

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Outline

Problem Statement and Background Problem Modeling and Methodology Framework Conclusion and Future Work

15June 10, 2007, IDAR’07, Beijing, China

Framework Overview

selection

CEP

YESNO

re-selection

YES

NO

redesign

available service?

G

CTC verification

design

Engine

TEA

Consistency Monitor

Process pool

Selector

G

G

G

16June 10, 2007, IDAR’07, Beijing, China

CTC Verification ExamplesTable1 A B C D

tx 2 8 4 6

MCT 15 20 8 12A

B

C

D

T2 A B C D

ts 0 2 2 10

te 2 10 6 16

ta [2,17] [10,30] [6,14] [16,28]

T3 A B C D

ts 0 2 4

te 2 8

ta [2,17]

T4 A B C D

ts 0 2 6

te 2 10

ta [10,30]

T5 A B C D

ts 0 2 5

te 2

ta [2,17]

[10,30]

10

[11,31]

11

10 10

[16,28]

10

16

10

[8,16] [11,31]

17

11

[8,16]

[10,18]

12

[12,20]

12

16

[16,28]

18

[18,30][2,17][2,17]

16

[16,28]

9

[9,17][10,30]

1011

[11,31] [11,19]

11

11

17

[17,29]

17June 10, 2007, IDAR’07, Beijing, China

Scheduling Algorithms

Motivation: different scheduling algorithms lead to different results of CTC verification, which decide whether to abort the service or not

Objective: design a scheduling algorithm which can decrease the possibility of service abortion

Some algorithms Immediate scheduling Minimal delay scheduling Maximal delay scheduling Average delay scheduling …, optimal scheduling algorithm?

18June 10, 2007, IDAR’07, Beijing, China

Dynamic Monitoring

TEA CEP execution engine

1. instance of process graph

2. verify CTC3. YESstart time

4. service completes

4. service completes

5. verify CTC6. YES

new start time

7. add record

8. exception

CM

6. NOabort

9. compensation

9. compensation

selection

CEP

YESNO

re-selection

YES

NO

redesign

available service?

G

CTC verification

design

Engine

TEA

Consistency Monitor

Process pool

Selector

G

G

G

19June 10, 2007, IDAR’07, Beijing, China

Inaccurate Global View

Problems Uncertainty of Execution Time (UET) Information Missing (IM)

IM1: execution time of normal operation is unknown IM2: available time of compensation operation is

unknown Solution

UET Scheduling algorithms Dynamic monitoring mechanism

IM1: UET variation (increased execution time) IM2: Dynamic monitoring mechanism

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Removing Assumptions

A4 (only AND-split and AND-join operators) Loop: UET variation XOR/OR: pseudo AND operator

A3 (only compensatable component services) Retriable: safely ignored Pivot: process graph division

A2 (static service selection) IM1 & IM2

A1 (only elementary component services) IM1 & IM2

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Time Complexity

CTC verification: O(n+e) Dynamic monitoring: O(n2+ne) Total time complexity: O(n2+ne)

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Outline

Problem Statement and Background Problem Modeling and Methodology Framework Conclusion and Future Work

23June 10, 2007, IDAR’07, Beijing, China

Conclusion and Future Work

A framework Assists selection of services based on

transactional and temporal property Ensures consistent termination under constrained

compensation Some next steps

Minimal-Abort-Possibility scheduling algorithms Cost constraints Integration with web services execution engine

24June 10, 2007, IDAR’07, Beijing, China