A User -PerceivedAvailability Evaluation of aWeb-based Travel Agency

Mohamed Kaâniche, Karama Kanoun, Magnos Martinello

DSN-2003, IPDS, 22-25 June 2003, San Francisco, CA, USA

Partially supported by the European Community, DSoS - Project IST-1999-11585

2

Hierarchical modeling framework

Assess the impact of failures onthe quality of service provided tothe users

Support e-business systemdesigners in making objectivearchitectural decisions

e-Business key players

User 1

User 2

ExternalSuppliers

e-Businessservice provider

Supplier 1

Supplier 2

Supplier n

Internet

Context and motivations

Case study: Travel agency

3

Context and motivations

Case study: Travel agency

Flight reservationsystem

Flight reservationsystem

User 1

User 2

Web-basedTravel Agency

Flight reservationsystem

Internet

Hotel reservationsystem

Hotel reservationsystemHotel reservation

system

Hotel reservationsystem

Hotel reservationsystem

Car reservationsystem

AF, KLM, BA, …

Sheraton, Sofitel …

Hertz, Visa …

ExternalSuppliers

Hierarchical modeling framework

Assess the impact of failures onthe quality of service provided tothe users

Support e-business systemdesigners in making objectivearchitectural decisions

4

A(user1) A(userN)

Availability modelingat the user level

…

A(Fn)

A(Sep)

A(F1)

…

A(Se1)…

Availability modelingat the function level

A(Si1) A(Sik)

…

Availability modelingat the service level

A(Rep)A(Re1) …

Userlevel User 1 User k…User 2

User operational profile

Functionlevel

Servicelevel

Resourcelevel

F1 …

functions invoked by the users

F2 Fn

Si1

e-business provider External suppliers…

Internal & external services implementingfunctions and their interactions

Si2 Sim …Se2 SepSe1

Architecture on which the servicesare implemented

e-business provider External suppliers

Ri1 …Ri2 Rik …Re2 RepRe1

Hierarchical modeling framework

E-business system description E-business availability modeling

A(Ri1) A(Rim)

…

Availability modelingat the resource level

5

Function and User levels

pij Probability of invoking Fj after Fi

p12

Search

Browse

Start BookExit

Pay

p13

p23

p32

Home

p44

p33

p24

p34

p45p56

p57

p47p27

p37 p67

p54

Functions invoked from “Start” to “Exit” ⇔ User scenario

User level

User 1 User k…User 2

Function levelF1 …F2 Fn

Service level

Si1

internal external…Si2 Sim …Se2 SepSe1

Resource levelinternal external

Ri1 …Ri2 Rik …Re2 RepRe1

6

User execution scenarios

4.5

9.4

6.4

3.3

6.9

4.7

9.7

20.4

13.9

4.2

6.6

10.0

Class B (%)

1.512: Start-{Home - Browse}*- {Search-Book}*- Pay-Exit

2.411: Start-Browse- {Search-Book}*- Pay-Exit

3.610: Start-Home- {Search-Book}*-Pay-Exit

1.39: Start-{Home; Browse}*- {Search-Book}*-Exit

2.08: Start-Browse- {Search-Book}*-Exit

3.07: Start-Home- {Search-Book}*-Exit

7.66: Start-{Home; Browse}*-Search-Exit

12.25: Start-Browse-Search-Exit

18.44: Start-Home-Search-Exit

11.33: Start-{Home; Browse}*-Exit

26.72: Start-Browse-Exit

10.01: Start-Home-Exit

Class A (%)User scenario

!

7

User execution scenarios

4.5

9.4

6.4

3.3

6.9

4.7

9.7

20.4

13.9

4.2

6.6

10.0

Class B (%)

1.512: Start-{Home - Browse}*- {Search-Book}*- Pay-Exit

2.411: Start-Browse- {Search-Book}*- Pay-Exit

3.610: Start-Home- {Search-Book}*-Pay-Exit

1.39: Start-{Home; Browse}*- {Search-Book}*-Exit

2.08: Start-Browse- {Search-Book}*-Exit

3.07: Start-Home- {Search-Book}*-Exit

7.66: Start-{Home; Browse}*-Search-Exit

12.25: Start-Browse-Search-Exit

18.44: Start-Home-Search-Exit

11.33: Start-{Home; Browse}*-Exit

26.72: Start-Browse-Exit

10.01: Start-Home-Exit

Class A (%)User scenario

!

!

}

!

}

!

}

!

}

SC1

SC2

SC3

SC4

7.5% 20.3%

8

Service and function levels

FlightExternal services

Hotel

Car

PayBookSearchBrowseHome

PaymentDatabaseApplicationWebInternal services

Browse

Begin ASDSAS

1 2 4 7

WS WS End

End

3 5

WS End

6

q1,2 q7,8 q8,9 q9,10

8 9 10

q4,7q2,4

q2,3 q4,5

q5,6

User level

User 1 User k…User 2

Function levelF1 …F2 Fn

Service level

Si1

internal external…Si2 Sim …Se2 SepSe1

Resource levelinternal external

Ri1 …Ri2 Rik …Re2 RepRe1

9

Service and Resource levels

External service: modeled as a black box

Internal services: detailed analysis of e-business provider architecture solutionso Organization of services on the hardware support

Dedicated hosts for each server ↔ Multiple servers on the same host

o Fault tolerance strategies Non-redundant servers ↔ Replicated servers

o Maintenance policies Dedicated ↔ Shared repair resources Immediate↔ Delayed maintenance

o Fault assumptions permanent, transient

User level

User 1 User k…User 2

Function levelF1 …F2 Fn

Service level

Si1 …Si2 Sim …Se2 SepSe1

Resource level

Ri1 …Ri2 Rik …Re2 RepRe1

internes externes

internes externes

10

Resource level: TA example

E-business Provider site

Databaseserver

Webserver

Applicationserver

Disk

Webserver 1Database

server 1Web

server 2

Webserver NW

Applicationserver 2

D1 Applicationserver 1

Databaseserver 2D2

Flight reservationcomponent system #NC

Internet

Payment server

Car reservationcomponent system

#1

#NH

Hotel reservationcomponent system #1

Flight reservationcomponent system #NF

Flight reservationcomponent system

#1

Basic architecture

Redundant architecture

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TA availability modeling

Function level

Service level

Resource level

User levelTwo operational profiles: A, B

Home, Browse, Search, Book, Pay

WS, AS, DS

Internal External

Flight, Hotel, Car, Pay

Basic architectureRedundant architecture Perfect coverage Imperfect coverage

Internal ExternalFlight res. system: AFi i = 1.. NFHotel res. system: AHi i = 1.. NHCar res. system: ACi i = 1.. NCPayment: APS, Network: Anet

12

Web service: Modeling approach

Composite Availability-Performance model

System behaviour resulting fromrequest arrival/service processes

conditioned on system states

System states resulting fromhardware and softwarefailures and recoveries

A(Web service) = 1 - Prob. {WS in a down state or overloaded}

13

Web service: Modeling example Redundant architecture: imperfect coverage

o Availability model

o Performance model: M/M/i/K pK(i): probability a request rejected in state i due to WS buffer full

Nwcλ

µ

(Nw-2) cλ

µ µ…….

µ

λ

Nw Nw-1 Nw-2

2cλ

µ01

yNw-1 yNw-2 y2

Nw(1-c)λβ β β

2 (1-c)λ

!

A(Web service) = 1" # ii=1

Nw

$ pK(i) + #iy

i=1

Nw"2$ + # o

%

&

'

(

14

Evaluation results: example

Redundant architecture: imperfect coverage

1 2 3 4 5 6 7 8 9 10

α = 50/sec

α = 100/sec

10-1

Number of web servers (Nw)

1- A

(WS)

1

10-2

10-3

10-4

10-5

10-6

10-7

λ = 10-4/h

λ = 10-3/h

λ = 10-2/h

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User level availability

Compositiono Service and Function level availabilitieso Operational profile

0.9782518790.9782286880.9780199880.9759350540.9552915810.768755879

A (B users)

0.980197168100.98018190950.98004459140.9786727730.96509002420.8423554821A (A users)NF = NH = NC

173 hours/year 190 hours/year User perceived unavailability ☞

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User level availability

SC4: Scenarios that end up with a payment

43 hours downtime per year16 hours downtime per year

570 Million€ Loss of revenueTransaction rate: 100per sec

Revenue per transaction: 100€

1.55 Billion€

1 2 3 4 5 6 7 8 9 10

NF=NH=NC

UA(SC2)

UA(SC4)

UA(SC1)

UA(B users)

UA(SC3)

10-1

1

10-2

10-3

User

per

ceive

d Un

avai

labi

lity

1 2 3 4 5 6 7 8 9 10NF=NH=NC

10-1

1

10-2

10-3 UA(SC3)

UA(A users)

UA(SC4)

UA(SC2)

UA(SC1)

17

Contributiono Hierarchical modeling framework ↔ user perceived availabilityo Example: travel agency

Illustration of the main concepts of the proposed framework Examples of dependability analysis results to support design solutions

State of the arto Measurement-based evaluationo Modelling:performance oriented

Extensions to handle more complex assumptions

Conclusion