A User -Perceived Availability Evaluation of a Web-based Travel Agency

Transcription

1 A User -Perceived Availability Evaluation of a Web-based Travel Agency Mohamed Kaâniche, Karama Kanoun, Magnos Martinello Partially supported by the European Community, DSoS - Project IST DSN-2003, IPDS, June 2003, San Francisco, CA, USA

2 Context and motivations Assess the impact of failures on the quality of service provided to the users User 2 Supplier 1 Supplier 2 External Suppliers Support e-business system designers in making objective architectural decisions User 1 Internet Supplier n Hierarchical modeling framework e-business service provider Case study: Travel agency e-business key players 2

3 Context and motivations Assess the impact of failures on the quality of service provided to the users Support e-business system designers in making objective architectural decisions User 1 Flight Flight Flight reservation reservation system system system AF, KLM, BA, User 2 External Suppliers Hotel Hotel Hotel reservation reservation system system system Sheraton, Sofitel Car Hotel Hotel reservation reservation system system Hertz, Visa Internet Hierarchical modeling framework Case study: Travel agency Web-based Travel Agency 3

4 Hierarchical modeling framework User User operational profile level User 1 User 2 User k A(user 1 ) A(user N ) Availability modeling at the user level Function level Service level Resource level functions invoked by the users F1 F2 Fn Internal & external services implementing functions and their interactions e-business provider Si 1 External suppliers Si 2 Si m Se 1 Se 2 Architecture on which the services are implemented e-business provider Ri 1 Se p External suppliers Ri 2 Ri k Re 1 Re 2 E-business system description Re p A(F 1 ) A(Si 1 ) A(Ri 1 ) A(F n ) Availability modeling at the function level A(Si k ) Availability modeling at the service level A(Ri m ) Availability modeling at the resource level A(Se 1 ) A(Se p ) A(Re p ) A(Re 1 ) E-business availability modeling 4

5 Function and User levels Home p 12 p 32 p 27 p 47 p 24 Start Exit Book p 54 Search p 57 p 56 p 45 p 34 p 44 p 23 p 37 p 67 Pay p 13 Browse User 1 User 2 User k User level p 33 F1 F2 Fn Function level p ij Probability of invoking Fj after Fi Functions invoked from Start to Exit User scenario Service level internal external Si 1 Si 2 Si m Se 1 Se 2 Se p Resource level internal external Ri 1 5 Ri 2 Ri k Re 1 Re 2 Re p

6 User execution scenarios User scenario 1: Start-Home-Exit 2: Start-Browse-Exit 3: Start-{Home; Browse}*-Exit 4: Start-Home-Search-Exit 5: Start-Browse-Search-Exit 6: Start-{Home; Browse}*-Search-Exit 7: Start-Home- {Search-Book}*-Exit 8: Start-Browse- {Search-Book}*-Exit 9: Start-{Home; Browse}*- {Search-Book}*-Exit 10: Start-Home- {Search-Book}*-Pay-Exit 11: Start-Browse- {Search-Book}*- Pay-Exit 12: Start-{Home - Browse}*- {Search-Book}*- Pay-Exit Class A (%) Class B (%)

7 User execution scenarios User scenario Class A (%) Class B (%) 1: Start-Home-Exit : Start-Browse-Exit SC : Start-{Home; Browse}*-Exit : Start-Home-Search-Exit : Start-Browse-Search-Exit SC : Start-{Home; Browse}*-Search-Exit : Start-Home- {Search-Book}*-Exit : Start-Browse- {Search-Book}*-Exit } } } SC : Start-{Home; Browse}*- {Search-Book}*-Exit } 10: Start-Home- {Search-Book}*-Pay-Exit 11: Start-Browse- {Search-Book}*- Pay-Exit SC4 12: Start-{Home - Browse}*- {Search-Book}*- Pay-Exit % 20.3% 7

8 Service and function levels Internal services External services Web Application Database Flight Hotel Car Payment Home Browse Search Book Pay Browse End WS q 5,6 End User 1 User 2 User k User level q 1 2 2,3 q 4,5 4 7 Begin Function level F1 F2 Fn Service level WS AS DS AS WS Si 2 Si m Se 1 Se 2 End q 1,2 q 2,4 q 4,7 q 7,8 q 8,9 q 9,10 Si 1 internal Ri 1 internal external 8 Se p Resource level external Ri 2 Ri k Re 1 Re 2 Re p

9 Service and Resource levels External service: modeled as a black box Internal services: detailed analysis of e-business provider architecture solutions o 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 Si 1 User 1 F1 internes User 2 F2 User k User level Function level Fn Service level externes Si 2 Si m Se 1 Se 2 Se p Ri 1 Resource level internes externes 9 Ri 2 Ri k Re 1 Re 2 Re p

10 Resource level: TA example E-business Provider site Disk Database server Application server Web server Flight reservation Flight component reservation system #N F component system #1 Internet Flight reservation Car component reservation system #N C component system #1 Hotel reservation component system #1 #N H Basic architecture Payment server Redundant architecture D1 D2 Database server 1 Database server 2 Application server 1 Application server 2 Web server 1 Web server 2 Web server N W 10

11 TA availability modeling User level Two operational profiles: A, B Function level Home, Browse, Search, Book, Pay Internal WS, AS, DS Internal Basic architecture Redundant architecture Perfect coverage Imperfect coverage Service level External Flight, Hotel, Car, Pay Resource level External Flight res. system: A Fi i = 1.. N F Hotel res. system: A Hi i = 1.. N H Car res. system: A Ci i = 1.. N C Payment: APS, Network: Anet 11

12 Web service: Modeling approach Composite Availability-Performance model System states resulting from hardware and software failures and recoveries System behaviour resulting from request arrival/service processes conditioned on system states A(Web service) = 1 - Prob. {WS in a down state or overloaded} 12

13 Web service: Modeling example Redundant architecture: imperfect coverage o Availability model N w cλ (N w -2) cλ 2cλ λ N w N w -1 N w µ µ µ µ µ β β β N w (1-c)λ 2 (1-c)λ y Nw -1 y Nw -2 y 2 0 o Performance model: M/M/i/K p K (i): probability a request rejected in state i due to WS buffer full A(Web service) = 1" % Nw Nw"2 ' $ p K (i) + $ # y + # & i o ( # i i=1 i=1 13

14 Evaluation results: example A(WS) α = 50/sec α = 100/sec λ = 10-2 /h 10-5 λ = 10-3 /h 10-6 λ = 10-4 /h Number of web servers (Nw) Redundant architecture: imperfect coverage 14

15 User level availability Composition o Service and Function level availabilities o Operational profile N F = N H = N C A (A users) A (B users) User perceived unavailability 173 hours/year 190 hours/year 15

16 User level availability User perceived Unavailability UA(SC2) UA(SC4) 10-3 UA(SC3) NF=NH=NC 16 hours downtime per year UA(A users) UA(SC1) SC4: Scenarios that end up with a payment UA(B users) UA(SC2) UA(SC3) UA(SC4) UA(SC1) NF=NH=NC 43 hours downtime per year 570 Million Loss of revenue 1.55 Billion Transaction rate: 100per sec Revenue per transaction:

17 Conclusion Contribution o Hierarchical modeling framework user perceived availability o Example: travel agency Illustration of the main concepts of the proposed framework Examples of dependability analysis results to support design solutions State of the art o Measurement-based evaluation o Modelling:performance oriented Extensions to handle more complex assumptions 17