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Rogério de Lemos ICSE WADS 2004 – May 2004 – 1

An Architectural Approach for Improving Availability in Web Services

Evangelos Parchas and Rogério de Lemos

Computing Laboratory - University of Kent at Canterbury, UK

Motivation Architectural pattern Implementation and results Concluding and future work

Rogério de Lemos ICSE WADS 2004 – May 2004 – 2

Motivation

Dependable systems that are built from existing

components/systems/services;

existing systems cannot be trusted, they are not under the

control of the architect;

the system has to be protected against faults;

A good architecture improves the handling of faults:

error confinement, and reduction of system complexity;

Application of traditional fault tolerant techniques:

Components – self-checking and comparison for error

detection and confinement ;

System – system dynamic reconfiguration for fault handling;

Rogério de Lemos ICSE WADS 2004 – May 2004 – 3

Motivation

Not quite ‘Web services’:

HTML and text, but we are getting there;

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Architectural Solution

For improving the reliability and availability of systems by using multiple sources of information:

Components implement crash-failure semantics;

• plus timing self-checks;

System dynamic reconfiguration;

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Architectural Solution

i I n p u t B r i d g e _ 1 B r i d g e _ 2 C

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p a r a t

• r

M a n a g e r i O u t p u t i I n p u t

Failure assumptions:

Bridge: arbitrary failures; Comparator: crash-failures;

• n failures for n+1 Bridges;

Manager: does not fail;

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Architectural Solution

i I n p u t B r i d g e _ 1 B r i d g e _ 2 C

• m

p a r a t

• r

M a n a g e r i O u t p u t i I n p u t

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Architectural Solution

i I n p u t B r i d g e _ 1 B r i d g e _ 2 C

• m

p a r a t

• r

M a n a g e r i O u t p u t i I n p u t

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Architectural Solution

i I n p u t B r i d g e _ 1 B r i d g e _ 2 B r i d g e _ 3 B r i d g e _ 4 C

• m

p a r a t

• r

M a n a g e r C

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i O u t p u t i I n p u t i I n p u t i I n p u t

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N Self-Checking Programming

NSCP is a design diverse technique that uses redundancies to check its own behaviour during execution.

iInput Variant1 Variant2 Variant3 Variant4 Comparator 12 Comparator Final Comparator 34

NSCP

iOutput

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Case Study: Stock Quotes

iWeb Bridge Yahoo Bridge Lycos Bridge Quicken Bridge MSN Comparator Manager Comparator iWeb iWeb iWeb Client StockQuotes Yahoo StockQuotes Lycos StockQuotes Quicken StockQuotes MSN Lookup Services iManager

Implemented in Jini: set of APIs and network protocols that help in building and deploying distributed systems;

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Implementation Analysis

System statistics: total restoration time (ms) Reliability and availability of the services and final system

205 Total restoration 123 Stream restoration 2 Reconfiguration 80 Failure detection Average time (ms) 0.999 0.6 f/p System Service 0.998 6.9 f/p MSN 0.998 8.4 f/p Quicken 0.983 81.3 f/p Lycos 0.999 4.2 f/p Yahoo Availability Reliability

(failure rate)

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Concluding Remarks

Architectural solution for improving the reliability and

availability of Web services:

it might be naïve but it does the job;

• its feasibility was shown in the context of stock quote

information; Current and future work:

apply the architectural solution in the context of Web

services;

special attention will be given to the Bridge component for

the purpose of fault tolerance:

• addition of the non-functional interface;
• automatic generation of an implementation;