A LOOSELY COUPLED ARCHITECTURE FOR DIGITAL

LIBRARIES

The Phronesis Case

Juan C Lavariega, Andan Salinas, David Garza, Lorena Gomez, Martha Sordia

Centro de Investigacion ene Informatica, ITESM-Campus Monterrey, Mexico

Keywords: Digital libraries, Software arc

hitecture, Information systems

Abstract: Phronesis is a software tool for the creation and administration of digital libraries that can be geographically

distributed and that are accessible over the WWW. Phronesis was developed by using open source software

with the intention to make the project accessible to other developers, who can improve its functionality.

However, one of the major drawbacks in Phronesis was its data centric architecture and the highly coupled

subsystems which made hard to maintain or to add new functionality. This paper addresses the problems

with the old data centric Phronesis architecture. Throughout the paper we discuss the functionality provided

by the subsystems, and present a loosely coupled architecture for digital libraries. The approach presented

here follows the style of services oriented architectures (SOA). The SOA for Phronesis is a framework that

provides services for the submission, indexing and compression of documents. Phronesis SOA is organized

into layers of functionality that favour maintenance, reuse, and testing of the entire project. Also the SOA

increases Phronesis performance and availability..

1 INTRODUCTION

A Digital Library (DL) (Fox,2002) is an organized

collection of documents stored in digital format (i.e.,

text, image, video or any combination of these

formats). Digital libraries must also face challenges

for integration of diverse information about the

documents (i.e. metadata), and interoperation with

other digital libraries. A digital library must provide

services for submission, indexing, classification,

storage, searching, retrieval, and administration of

the digital documents within its collection or

collections.

Phronesis project (Garza, 2003) started with the

goal

of contributing to the research and development

of digital libraries technology. The main result of the

Phronesis project is a freely-available single-system

software tool that can be used for the creation of

distributed digital libraries on the Internet. The

Phronesis system supports storage and retrieval of

any digital documents (video, software, text, etc),

search and retrieval for text-based documents,

support for indexing , searching and retrieving

documents written in English or Spanish, use and

administration of the library via WWW over the

Internet, interoperability with digital libraries that

use the Z39.50 protocol and stemmer functionality.

Phronesis follows a client-server model with

cen

tralized data, in which all of the digital library

functionality is located in one server. The document

repository or digital collection resides in one site

that is accessed remotely by clients through a web

interface. The interface allows users to perform

different tasks such as: creation of digital

documents; classification and indexing of the

documents within the same collection; search and

retrieval based on keywords or metadata,

administration; and access control. This architecture

is in essence monolithic and all the components are

highly coupled with each other. Adding new

functionality to Phronesis became a major issue to

consider when new requirements arise.

For this reason, a new Phronesis' architecture

th services as components organized into several

abstraction layers was developed. This idea comes

from two architectural styles: Service Oriented

Architecture (SOA) and Layered Architecture.

These architectural styles provide the capability to

move tasks to specialized software servers and the

scalability that allows the incremental and structured

growth on the functionality that a digital library

requires.

495

C Lavariega J., Salinas A., Garza D., Gomez L. and Sordia M. (2005).

A LOOSELY COUPLED ARCHITECTURE FOR DIGITAL LIBRARIES - The Phronesis Case.

In Proceedings of the Seventh International Conference on Enterprise Information Systems, pages 495-498

DOI: 10.5220/0002533204950498

 SciTePress

2 PHRONESIS DATA CENTRIC

ARCHITECTURE

Phronesis follows the client server architecture, with

the server as the main component composed of

several internal subcomponents for search, retrieval,

indexing and managing of documents in the digital

library. Clients are World Wide Web interfaces

using HTTP, HTML and PERL Common Gateway

Interfaces (CGI’s) technology.

Phronesis clients are users accessing the system

for performing actions such document search and

retrieval as well as document submission. Three

types of users are presented in Phronesis:

Collection Contributors. They are users that

have the proper permissions to submit documents to

the collection.

Administrators. They are users that maintain a

Phronesis server.

Patrons. They are users who access the server to

search and retrieve full documents.

The server, key component of the system,

performs the following tasks: administration, access

control, physical storage of documents, indexing,

local and distributed search and retrieval.

Functionality for document storage and retrieval is

based on MG (Witten 1999), a powerful research

tool for the compression, indexing and retrieval of

textual documents. We have extended the MG

system in order to provide all the desired

functionality in Phronesis.

Phronesis supports search and retrieval of

English and Spanish documents. The user interface

is also available in both languages. The server

implements five different types of full-text

document and/or metadata search. The search query

can include diacritic characters common in the

Spanish language. When searching words that

contain diacritic characters, the server tolerates

simple mistakes common in Spanish language, such

as the omission of an accent. For example, a search

for the word computing in Spanish will be

performed using the keyword “computación”. If the

keyword “computacion” (accent omitted) is used,

the query will find the same documents as the

previous one. A stemmer algorithm to support

Spanish language is also available as part of the

system. A single Phronesis system can interact not

only with other Phronesis systems but also provides

interoperability support for Z39.50 based libraries

and Open Archives Initiative based libraries.

The current components of the data-centric

architecture of a Phronesis server are a set of

subsystems working together to satisfy services of a

digital library. Since all of these services are tightly-

coupled, making improvements is a time consuming

process that requires a good amount of knowledge

about the internals of the system.

As a result, the Phronesis system has become a

complex, monolithic piece of software, hard to

maintain, with no flexibility to easily evolve. A clear

sign of the inherent problems with the current

Phronesis' architecture is the time that it takes to run

a test case compared with the time it took to run the

same test case in previous versions. Also, it takes

longer to add new functionality. The differences in

time are because Phronesis' components are highly

coupled and with a poor cohesion. This complexity

has an impact in the quality assurance process since

fixing a bug in one component may introduce

problems in another component. Therefore, a

redesign of the architecture was highly needed in

order to support future improvements of the system.

2.1 Analysis of the Data Centric

Architecture

Phronesis' subsystems are not implemented as

independent components with well-defined

boundaries. Phronesis is a highly integrated set of

programs and tools that interact by means of the

shared data in the central repository.

Based on their functionality (Garza, 2003) each

program or tool can be further classified as follows:

Document Searching Subsystem. This subsystem

allows the searching of documents in different and

distributed Phronesis' repositories.

Document Browsing Subsystem. This subsystem

includes all the programs for visualizing the

documents stored in the repository. Visualization is

based on predefined categories such as document

title, year of publication, or author's name.

Document and Library Builder Subsystem. This

subsystem groups the programs for document

processing and index creation that is required during

document searching.

Document Storage Subsystem. This subsystem

includes the programs and file structures required to

assign unique identifiers to documents. It also

includes the programs to save and to retrieve

documents, and to do the preprocessing needed prior

to the indexing of documents.

Management Subsystem. This subsystem

includes programs for the digital library

configuration.

Z39.50 Interoperability Subsystem. This

subsystem includes all the programs that allow

Phronesis to perform queries to/from libraries that

support Z39.50 protocol

According to (Bass,1998), the following criteria

allow to determine the strengths and weaknesses of

different architectural styles in software.

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Maintainability. This parameter indicates the

degree of complexity to make changes in the

software as a result of new user's requirements.

Reusability. This criterion indicates if the

software product is designed to reuse components

from other products, or if its components can be

reused in other products.

Ease of Testing. This parameter indicates the

degree of difficulty to test a software component. It

also determines the complexity to detect defects in

the software.

Performance. This parameter is related to the

system's response time, or the number of events

processed during a time period. Performance also is

related to the communication costs (in time) between

different modules of the system.

Availability. This parameter measures the time

in which the system is operational.

With these criteria, we can evaluate not only the

software behaviour and functionality (Performance

and Availability), but also characteristics about the

software lifecycle (Maintainability, Reusability and

Ease of Testing). All of these parameters were used

to evaluate the current data centric architecture and

the new service oriented architecture of Phronesis

2.2 Opportunity and Impact

Following is a summary of the opportunity areas and

the impact in the original data centric architecture

for Phronesis.

Subsystem: Searching

Area of Opportunity: There is not a clear

separation between searching requests and the

results interface

Impact Maintenance is affected because code

from interface and code from searching process are

mixed which produces program that are difficult to

modify. New errors can be introduced with each

change. Also without a clear interface it is difficult

to interoperate with other DLs

Subsystem: Browsing

Area of Opportunity: Each time the browser

is invoked all of the metadata components are

accessed, which makes the process more time

consuming.

Impact Performance application deteriorates

because waiting time for construction in the

browsing menu increases as the collection becomes

bigger. Maintenance is difficult in the interfaces

that present results.

Subsystem: Storage

Area of Opportunity: Since there are several

subsystems that perform operations over the same

data, subsystems are dependent, which makes more

complex any change on an individual subsystem

Impact Maintenance and testing are affected

due to strongly coupled data between the programs.

Reuse is also affected because it is not possible for a

storage subsystem to store documents from multiple

collections.

Subsystem: Management

Area of Opportunity: Administration

programs are completely duplicated for different

language versions (Spanish and English)

Impact Reuse and Maintenance are affected

because administrative processes have to be

duplicated for each language version of the user

interface.

Subsystem: Interoperability

Area of Opportunity: There is duplicity in

the metadata and indexes, which implies a big effort

to keep consistency for that information in different

versions.

Impact Maintenance is affected because it is

required to assign resources for servers that require

communication.

Subsystem: Builder

Area of Opportunity: The process that

extracts the text from documents can become a

bottleneck in the system.

Impact Availability is affected. As the number

of documents increases, the index reconstruction

process takes longer limiting the availability of the

system because during index construction time,

searches cannot be performed in the library.

3 PHRONESIS LOOSELY

COUPLED ARCHITECTURE

A Service Oriented Architecture is an architectural

style for software products whose objective is to

have a loosely-coupled interaction among

components in the system. These components are in

essence a collection of services communicating with

each other. Communication between components

may be a simple data interchange, or one or more

services coordinating one activity. Each service is a

well-defined function, self-contained, and

independent of the state of another service. After

analysing the functions each subsystem provides, six

basic services were identified:.

Searching and Indexing Service. This component

maintains communication with most of the other

services and provides basic services needed for

location of documents within a collection. This

service also builds documents' indexes and processes

searches requested by users that invoked parallel

searching or interoperability services.

Storage Service. This service provides

functionality for storing and retrieving any

A LOOSELY COUPLED ARCHITECTURE FOR DIGITAL LIBRARIES: The Phronesis Case

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document in the collection. This service is usually

accessed by the searching service in order to index

new documents; and by the user interface, so users

can get any document.

Parallel Searching Service. This service

replicates a searching request in several repositories

at once and generates a global result list.

Configuration Service. This service manages

information about the services and resources that

form the digital library.

Interoperability Service. This service provides a

wrapper for accessing Phronesis' services by using

Z39.50 protocol. It is possible to create

communication services with other digital libraries.

User Service. This service manages and

maintains user information such as access control

data and preferences. It also authenticates users

accessing Phronesis.

The new Phronesis Loosely Coupled

Architecture, is based on a layered architectural style

in which each layer groups a set of services that

together provide the functionality of the digital

library.

Presentation Logic Layer. This layer provides

the user interface. Programs at this level work with

the outputs resulting from programs at the business

layer and transform such results into a format useful

for users. This layer includes menu lists, forms for

capturing data, and programs for transforming XML

into HTML documents.

Business Logic Layer. This layer includes the

programs implementing business rules. At this level,

programs coordinate the flow of events that occur

during the operation of the digital library.

Service Access Layer. This layer is an

application program interface that allows access to

service function in the digital library. This layer

hides the existence of different specialized services

such as searching, users' administration and

configuration.

Service Layer. This layer consists of the services

for data management in the digital library. Each

service performs specific and limited functionality.

These services are used as building blocks by upper

layers.

Proxy Layer. This layer is used for interaction

with other applications. The proxy layer works as an

external application accessing the services of the

Business logic layer. The proxy layer allows sharing

the digital library functionality by using other

communication protocols.

4 CONCLUSIONS

We have presented two different architectures of

Phronesis, an open, self-contained software product

that provides digital library services.

The first architecture, called a data centric

architecture, provides the functionality required.

However, the integration of new user requirements

is a process that becomes more complex as the

system grows. Because of the complexity of the

software, the performance also is affected. To avoid

all these problems, a new architecture based on

services was developed. The new architecture

contains independent, self-contained modules that

performed a well-defined function. As a result, the

software produced should have properties such as

maintainability, reusability, testability and

availability

REFERENCES

Bass, L., et al, 1998. Software Architectures in Practice, .

Addison Wesley..

Fox, E.A,2002, Overview of Digital Libraries,

Proceedings of the second ACM/IEEE-CS Joint

Conference on Digital Libraries, ACM press.

Garza-Salazar, D et al 2003 “Information Retrieval and

Administration of Distributed Documents in Internet,”

in Knowledge-Based Information Retrieval and

Filtering from the Web Edited by W. Abramowicz,

Kluwer Academic Publishers

Witten, I.H.; et al 1999 Managing Gigabytes

Compressing and Indexing Documents and

Images, 2nd. Edition, Morgan Kauffman

Publishers,

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