Basi di dati e World Wide Web Paolo Atzeni Paolo Merialdo 19/02/2002.

Basi di dati e World Wide Web

Paolo Atzeni

Paolo Merialdo

19/02/2002

19/02/2002 Basi di dati, capitolo 14 2

Richiami: Internet e WWW

• Internet:– Una federazione di reti che usano TCP/IP

• World Wide Web– ipertesto multimediale distribuito– con la possibilità di richiamare programmi e costruire pagine

dinamicamente


Il World Wide Web

• ipertesto• multimediale (testo, immagini, suoni, video, …)• distribuito• con componenti autonome• “attivo” (permette di richiamare programmi)


Ipertesto

• documento con struttura non sequenziale o• insieme di documenti correlati

• esempi: – un libro (struttura gerarchica, più rimandi, note, bibliografia)– una rivista (con rimandi anche a fascicoli diversi)– un insieme di libri (che si possono citare a vicenda)– …


Richiami: HTML

• linguaggio per la specifica di ipermedia• formattazione “logica” dei documenti• riferimenti basati su URL (Uniform Resource Locator):

[Protocollo://] [Server/] Risorsa

– Protocollo: http, ftp, telnet, mailto– Server: nome simbolico o indirizzo IP– Risorsa: specifica di directory e file, anche con parametri, o

anche indirizzo di posta elettronica• costrutto “form” per l’invio di informazioni (parametri, quando

l’URL specifica un programma da eseguire)


Richiami: HTTP

• protocollo per la comunicazione fra browser (client) e server Web (“server HTTP”)

• fasi:

1. Apertura della connessione: il browser contatta il server HTTP (indirizzo e protocollo come nell’URL) e richiede una connessione TCP

2. Accettazione della connessione: il server, se disponibile, accetta e invia conferma

3. Richiesta: il client invia la richiesta specifica, con i dettagli della risorsa e gli eventuali parametri

4. Risposta: il server comunica l’esito e il risultato, chiude la connessione


HTTP, protocollo senza memoria

• Chiusa una connessione, il server la “dimentica” completamente (a parte il log, che però non viene utilizzato nelle connessioni successive)

• Motivazioni:– semplicità realizzativa (ogni richiesta può essere gestita

indipendentemente dalle altre)• Conseguenze:

– la gestione di attività che richiedono interazioni multiple non è immediata


HTTP e programmi: il protocollo CGI (Common Gateway

Interface)• Gateway (o programma CGI): un qualunque progranna

richiamato da un server HTTP• I gateway “aprono” il mondo Web permettendo l’integrazione di

altre funzionalità• Meccanismo di comunicazione:

BrowserServerHTTP

Gateway … 1 2 3

45


The Web: a great opportunity

• the diffusion of the Web is ...• the Web (with its browsers) is becoming a standard interface for

the final user– the protocol is very simple and public– the interface is uniform– the content is very rich (in breadth and depth)

• it is becoming a standard interface for accessing many services, with information systems and databases of every type


Integration of information systems

• Various motivations:– interaction of components independently developed– cooperation of previously separated business processes– cooperation (or merge) of companies

• Typical requirements:– distribution– heterogeneity– autonomy


A topical issue

• a request from our “users”:– “computing facilities should become similar to standard

utilities (gas, phone, power, etc.)”• our usual reply:

– “computing services have application specific features for which standard services would only be a limited solutions (as it is the case for the other utilities)”

• however– what would a standard offer of services be?


The great opportunity

• Internet (and Intranets and Extranets) and the World-Wide-Web offer a great opportunity

• a simplified stack of layers:– cooperation (of applications)– interoperability (ftp, telnet, mail, http, ...)– connectivity

• standardization climbs stacks (functionalities get standardized and go down: think to database systems!)


Sistemi informativi su Web (WIS)

(Nota: ci riferiamo al Web come strumento “produttivo”; il surfing è marginale)

• Pubblicazione (permanente?) e consultazione su Web: siti “catalogo”– richiedono un approccio sistematico, sempre più importante

al crescere delle dimensioni:• i dati possono cambiare, come si evolve il sito?• i siti presentano ridondanza, per vari motivi

– l’uso di basi di dati può risultare conveniente (spesso poi la base di dati esiste già!)

• Transazioni su Web: perché solo interrogazioni?– il sito può essere aggiornato attraverso il Web stesso– il browser diventa un’interfaccia standard per sistemi

informativi


Sistemi informativi su Web, nuove applicazioni

• Commercio elettronico:

– se i cataloghi sono la principale applicazione dei siti di consultazione, è naturale pensare alla “vendita per corrispondenza” (ma con funzionalità aggiuntive: commenti, discussioni, verifiche); sicurezza e affidabilità diventano essenziali

• Gruppi di discussione: ad esempio, recensioni di libri

• Sistemi di supporto al lavoro cooperativo: workflow

• “Data farming”

• Sistemi “embedded”: telefonini, sistemi di navigazione, Internet TV, …

• Nota: alcune applicazioni potrebbero essere su “Intranet” (con possibilità di uniformazione degli strumenti) o “Extranet” (attività “Business-to-business”)


Sistemi informativi su Web

Complessitàdei dati

Complessitàdei servizi

Siti data-intensive

Siti di presenza

Siti orientatiai servizi

WIS


Portali

• Siti che supportano l'utente nell'utilizzo del Web:– servizi generali:

• posta elettronica, • gruppi di discussione• motori di ricerca e/o ricerca guidata, con classificazione • hosting di siti

– servizi specializzati (portali "verticali"), dipendenti dallo specifico dominio, in particolare con:

• basi di dati "locali" specializzate• integrazione a grana fine di informazioni e servizi locali

e/o offerti (più o meno spontaneamente) da terzi


Two worlds?

• Databases can be queried in a flexible way; hypertexts are easy to access, but cannot be “queried”

• Web sites are often difficult to explore, use and monitor

• Database design methods are well known• Web sites are difficult to design and maintain


Web and DBs: a contradiction?

• databases are well structured and organized• how much structure and organization is there in the Web?

• it depends, both on the source and on the user• there are different degrees of granularity and structure for our

data• we need to be able to make conversions (from DB to HT and

viceversa)


Database approaches

• bottom-up: accessing information from Web sources, and integrating them

• top-down: designing and maintaining Web sites• global: integrating existing sites and offering the information

through new ones


Problems with many Web-sites

• design– information is often poorly organized and difficult to access– it is not even clear which pieces of information are available– the access structure is casual and many dangling references

occur– the style of presentation is heterogeneous

• maintenance– difficulties in updating the content (type and values)– difficulties in changing the initially defined structure– difficulties in changing the presentation details


Esistono pagine con la stessa struttura


Indipendenza dei dati negli ipertesti:stessi dati, diversa struttura ipertestuale


Indipendenza dei dati negli ipertesti:stessi dati e stessa struttura ipertestuale,

diversa presentazione


Hypertext data-independence

• Data “what information is offered through the site and what are the conceptual details and the logical organization”

• Hypertext “how data is arranged in pages and what navigation links correlate them”

• Presentation “the appearance of each piece of information in pages”


Design Issues

• Data

choosing the content

• Hypertext

choosing page organization and navigation paths

• Presentation

defining layout and graphics

Maintenance Issues

• Data

changing the (type of) content

• Hypertext

changing page organization and navigation paths

• Presentation

changing layout and graphics


Components and Models

data ER and Relational

hypertext

presentation HTML

What is missing is a model for hypertexts!


Models for hypertexts

• in data-intensive Web sites (and often in general) there are (many) pages with a similar (or even the same) structure

• thirty or forty years ago people realized that in an application it is often the case that there are records with the same structure; files with a rather fixed structure were invented with this purpose

• the notion of scheme of the database was later introduced as an overall description of the content of a database


Pagine con la stessa struttura


Pagine con un'altra struttura


A Web page


A page-scheme: ProfessorPage

ProfessorPage

NamePositionAddress

EMailResearchList

AreaToResP


ADM (Araneus Data Model): a logical model for Web hypertexts

• page-schemes• “unique” pages• simple attributes

– text, images, ...– link (anchor, URL)

• complex attributes: lists (possibly nested)• heterogeneous union• form (as virtual list over form fields and link to the result)


A Web page (containing a list of links)


A “unique” page-scheme: ProfessorListPage

ProfessorListPage

ProfessorListNameToProfP


An ADM Scheme

ProfessorListPage

ProfessorListNameToProfP

ProfessorPage

NamePositionAddress

EMailResearchList

AreaToResP


Heterogeneous Union and Forms


Heterogeneous Union and Forms in ADM

ProfessorListPage

ProfessorListName

ToProfP

ProfessorPageNamePosition

Address

EMailResearchList

Area

ToResP

Name

Submit

U

SearchProfPage


Data Models

ER

ADM

Database ConceptualScheme

(entities - relationships)

Hypertext LogicalScheme

(page-schemes, links)

There is a lot of ‘distance’ between the two!


A simple ER scheme


An ADM scheme


Data Models

ER

ADM







Data Models

ER

ADM






NCMHypertext Conceptual

Scheme(macroentities,

directed relationships,aggregations)


Data Models



ER

NCM Hypertext ConceptualScheme

(macroentities,directed relationships,

aggregations)ADM

NCM fills the gap between the two




Navigation Conceptual Model (NCM)

Hypertext Conceptual Features

• Which concepts should be the hypertext nodes

• Which should be the navigation paths between nodes

• How nodes should be aggregated to build the hierarchical access structure

NCM Constructs

• Macroentity

• Directed Relationship

• Aggregation


NCM: Macroentities and directed relationships

Professor

Course

Student

Teacher

Tutorship

Name

Room

Email

Name

Description

Day

Room

HourLessonN

1:1

1:N

1:1Name

...


NCM: aggregation nodes

Professor CourseStudent

TeacherTutorship1:11:N1:1

Seminar

People

Department

Activities


An NCM scheme


The Araneus Methodology

Database conceptual design

Hypertext logical design

Presentation design

Page GenerationSite generation

Presentation design

Requirements analysis

Database logical design


Hypertext conceptual design


design from scratch



Presentation design


Presentation design






design from an existing database (with an ER scheme)



Presentation design


Presentation design






design from an existing database (without an ER scheme)

Database conceptual design(reverse engineering)


Presentation design


Presentation design






Hypertext conceptual design:from ER to NCM



Presentation design


Presentation design






Hypertext Conceptual DesignER scheme NCM Scheme

• step 1 choose and describe macroentities: design “views” over the input ER scheme

• step 2choose navigation paths

• step 3shape the hypertext access structure on the basis of (“bottom-up”) conceptual aggregation



step 1 choose and describe macroentities: design “views” over the input ER scheme

usually it corresponds to “de-normalize” the input ER scheme

Course

Lesson

Course

Name

DescriptionName

Description

Day

HourLesson

Day

Hour

1:1

1:N

1:N

ER NCM



step 2 choose navigation paths

it may introduce redundancies

Professor

Research-Group

1:1

1:N

ER NCM

Paper

Professor

Research-Group

1:1

1:NPaper

1:N

1:N

1:N

1:N

1:N

1:N



step 3 shape the hypertext access structure it is based on “bottom-up” conceptual

aggregations

Professor

Research-Group

1:1

1:N

NCM NCM

Seminar

Professor

Research-Group

1:1

1:N

Seminar

...

...

ResearchActivities


The Input ER scheme


The resulting NCM scheme


Hypertext logical design:from NCM to ADM



Presentation design


Presentation design






Hypertext Logical DesignNCM scheme ADM Scheme

step 1 map each macroentity into either

a page-scheme or a list inside a page-scheme

step 2map each directed relationship into a (list of) link attribute(s)

step 3map each aggregation into a unique page-scheme with link attributes to the target page-schemes


Hypertext Logical DesignStep 1 (example)


R e s u l t i n g A D M S c h e m e


Maintenance

• The Schemes help designers to maintain the hypertext structure• Maintenance activities correspond to apply scheme

transformations:– introduce multilevel lists– introduce forms– split pages– ...


Maintenance: example

Basi di dati e World Wide Web Paolo Atzeni Paolo Merialdo 19/02/2002.

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