Data Warehouse and the Web

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Chapter 10
Data Warehouse Fundamentals

• Data Warehouse and the Web

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Chapter - Objectives

• Understand what Web-enabled data warehouse means
  and examine the reasons for doing so
• Appreciate the importance of the convergence of
  Web technology and those of the Data Warehouse
• Probe into all the facets of Web-based
  information delivery
• Study how OLAP and the Web connect and learn the
  different approaches to connecting them
• Examine the steps for building a Web-enabled data
  warehouse

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Three Information Delivery Mechanisms

• Internet Low cost, however security concerns
  must be addressed.
• Intranet - A Web site or group of sites belonging
  to an organization, accessible only by the
  members of an organization.
• Extranet - An intranet that is partially
  accessible to authorized outsiders.
• Whereas intranet resides behind firewall and
  is accessible only to people who are members of
  same organization, extranet provides various
  levels of accessibility to outsiders.

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Why the Web?

• World-Wide Web (Web, WWW, or W3) possibly most
  popular and powerful networked information system
  to date.
• As architecture of Web was designed to be
  platform-independent, can significantly lower
  deployment and training costs.
• Organizations using Web as strategic platform for
  innovative business solutions, in effect becoming
  Web-centric.

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Why the Web?

• Universal browsers will run on any systems.
• It is easy to keep the internet/extranet updated
  so that there will be one source of information.
• Open up your data warehouse to your business
  partners over the extranet fosters and
  strengthens the partnership.

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Why the Web?

• Worldwide collection of interconnected networks.
• Began in late 60s in ARPANET, a US DOD project,
  investigating how to build networks that could
  withstand partial outages.
• Starting with a few nodes, Internet estimated to
  have over 100 million users in 1997, and over 270
  million users in over 100 countries in 1998, with
  one million new users joining each month.
• May be 199 million users of Web by year 2000.

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The Web as a Data Source

• Hypermedia-based system that provides a simple
  point and click means of browsing information
  on the Internet using hyperlinks.
• Information presented on Web pages, which can
  contain text, graphics, pictures, sound, and
  video.
• Can also contain hyperlinks to other Web pages,
  which allow users to navigate in a non-sequential
  way through information.
• Web documents written using HTML.

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The Web

• Web consists of network of computers that can act
  in two roles
• as servers, providing information
• as clients (browsers), requesting information.
• Protocol that governs exchange of information
  between Web server and browser is HTTP and
  locations within documents identified as a URL.
• Much of Webs success is due to its simplicity
  and platform-independence.

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Browser Technology for the Data Warehouse

• At present, four technologies are commonly used
  to
• build Web-based user interfaces.
• HTML
• Java
• ADO
• Plug-In

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Architecture of a Web-enabled Data Warehouse
MDDB
Firewall
Internet
Internet Client
OLAP Server
Web Server
Network Backbone
Database Server
Source Systems
DW
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Web Processing Model
Web Server
HTML
CGI
Structured Data Contents
Web Browser
SQL
HTML
Unstructured Data Contents
Query Engine
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Basic Components of Web Environment
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HyperText Transfer Protocol (HTTP)

• Protocol used to transfer Web pages through
  Internet.
• Based on request-response paradigm
• Connection - Client establishes connection with
  Web
• server.
• Request - Client sends request to Web server.
• Response - Web server sends response (HTML
• document) to client.
• Close - Connection closed by Web server.

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HyperText Transfer Protocol (HTTP)

• HTTP/1.0 is stateless protocol - each connection
  is closed once server provides response.
• This makes it difficult to support concept of a
  session that is essential to basic DBMS
  transactions.

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HyperText Markup Language (HTML)

• Document formatting language used to design most
  Web pages.
• A simple, yet powerful, platform-independent
  document language.
• HTML is an application of Standardized
  Generalized Markup Language (SGML), a system for
  defining structured document types and markup
  languages to represent instances of those
  document types.

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HyperText Markup Language (HTML)
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HyperText Markup Language (HTML)
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Uniform Resource Locators (URLs)

• String of alphanumeric characters that
  represents location or address of a resource on
  Internet and how that resource should be
  accessed.
• Defines uniquely where documents (resources) can
  be found.
• Uniform Resource Identifiers (URIs) - generic set
  of all Internet resource names/addresses.
• Uniform Resource Names (URNs) - persistent,
  location-independent name. Relies on name lookup
  services.

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Uniform Resource Locators (URLs)

• URL consists of three basic parts
• protocol used for the connection,
• host name,
• path name on host where resource stored.
• Can optionally specify
• port through which connection to host should be
  made,
• query string.
• http//www.w3.org/WWW/MarkUp.html

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Static and Dynamic Web Pages

• HTML document stored in file is static Web page.
• Content of dynamic Web page is generated each
  time it is accessed.
• Thus, dynamic Web page can
• respond to user input from browser.
• be customized by and for each user.
• Requires hypertext to be generated by servers.
• Need scripts that perform conversions from
  different data formats into HTML on-the-fly.

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Requirements for Web-DBMS Integration

• Ability to access valuable corporate data in a
  secure manner.
• Data and vendor independent connectivity to allow
  freedom of choice in DBMS selection.
• Ability to interface to database independent of
  any proprietary browser or Web server.
• Connectivity solution that takes advantage of all
  the features of an organizations DBMS.

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Requirements for Web-DBMS Integration

• Open-architecture to allow interoperability with
  a variety of systems and technologies. For
  example
• different Web servers
• Microsoft's (Distributed) Common Object Model
  (DCOM/COM)
• CORBA/IIOP (Internet Inter-ORB protocol)
• Java/Remote Method Invocation.

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Requirements for Web-DBMS Integration

• Cost-effective solution that allows for
  scalability, growth, and changes in strategic
  directions, and helps reduce applications
  development costs.
• Support for transactions that span multiple HTTP
  requests.
• Support for session- and application-based
  authentication.
• Acceptable performance.

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Requirements for Web-DBMS Integration

• Minimal administration overhead.
• Set of high-level productivity tools to allow
  applications to be developed, maintained, and
  deployed with relative ease and speed.

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Two-Tier Client-Server Architecture
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Three-Tier Client-Server Architecture

• Client side presented two problems preventing
  true scalability
• Fat client, requiring considerable resources on
  clients computer to run effectively.
• Significant client side administration overhead.
• By 1995, three layers proposed, each potentially
  running on a different platform.

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Three-Tier Client-Server Architecture

• Advantages
• Thin client, requiring less expensive hardware.
• Application maintenance centralized.
• Easier to modify or replace one tier without
  affecting others.
• Separating business logic from database functions
  makes it easier to implement load balancing.
• Maps quite naturally to Web environment.

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Three-Tier Client-Server Architecture
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Advantages of Web-DBMS Approach

• DBMS advantages
• Simplicity
• Platform independence
• Graphical User Interface
• Standardization
• Cross-platform support
• Transparent network access
• Scalable deployment
• Innovation

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Disadvantages of Web-DBMS Approach

• Reliability
• Security
• Cost
• Scalability
• Limited functionality of HTML
• Statelessness
• Bandwidth
• Performance
• Immaturity of development tools

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Approaches to Integrating Web and DBMSs

• Common Gateway Interface (CGI).
• Server-Side Includes.
• HTTP Cookies.
• Extending the Web Server.
• Java and JDBC, JSQL and JRB.
• Scripting Languages (JavaScript and VBScript).
• Microsoft Active Platform ASP and ADO.
• Oracle Network Computing Architecture.

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Common Gateway Interface (CGI)

• Specification for transferring information
  between a Web server and a CGI program.
• Server only intelligent enough to send documents
  and to tell browser what kind of document it is.
• But server also knows how to launch other
  programs.
• When server sees that URL points to a program
  (script), it executes script and sends back
  scripts output to browser as if it were a file.

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CGI - Environment
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Common Gateway Interface (CGI)

• CGI defines how scripts communicate with Web
  servers.
• A CGI script is any script designed to accept and
  return data that conforms to the CGI
  specification.
• Before server launches script, prepares number of
  environment variables representing current state
  of the server, who is requesting the information,
  and so on.
• Script picks this up and reads STDIN.

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Common Gateway Interface (CGI)

• Then performs necessary processing and writes its
  output to STDOUT.
• Script responsible for sending MIME header, which
  allows browser to differentiate between
  components.
• CGI scripts can be written in almost any
  language, provided it supports reading and
  writing of an operating systems environment
  variables.

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Common Gateway Interface (CGI)

• Four primary methods for passing information from
  browser to a CGI script
• Passing parameters on the command line.
• Passing environment variables to CGI programs.
• Passing data to CGI programs via standard input.
• Using extra path information.

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CGI - Passing Parameters on Command Line
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CGI - Advantages

• CGI is the de facto standard for interfacing Web
  servers with external applications.
• Possibly most commonly used method for
  interfacing Web applications to data sources.
• Advantages
• simplicity,
• language independence,
• Web server independence,
• wide acceptance.

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CGI - Disadvantages

• Communication between client and database server
  must always go through Web server.
• Lack of efficiency and transaction support, and
  difficulty validating user input inherited from
  statelessness of HTTP protocol.
• HTTP never intended for long exchanges or
  interactivity.
• Server has to generate a new process or thread
  for each CGI script.
• Security.

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Server-Side Includes (SSI)

• Allows a program to be executed, like CGI, and to
  incorporate its output into the document.
• Generally, end result is a text document.
• SSI is not governed by an Internet RFC or other
  standard. Each server vendor is free to implement
  SSI on an ad-hoc basis, if at all.
• Most servers follow NCSAs specification.
• All SSI commands are embedded within regular HTML
  comments, making the HTML portable.
• Security risks of SSI are similar to those of CGI.

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HTTP Cookies

• Cookies can make CGI scripts more interactive.
• Cookies are small text files stored on Web
  client.
• CGI script creates cookie and has Web server send
  it to clients browser to store on hard disk.
• Later, when client revisits Web site and uses a
  CGI script that requests this cookie, clients
  browser sends information stored in the cookie.
• However, not all browsers support cookies.

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Extending the Web Server

• To overcome limitations of CGI, many servers
  provide an API that adds functionality to server.
  
• Two of main APIs are Netscapes NSAPI and
  Microsofts ISAPI.
• Scripts are loaded in as part of the server,
  giving back-end applications full access to all
  the I/O functions of server.
• One copy of application is loaded and shared
  between multiple requests to server.

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Extending the Web Server

• Approach more complex than CGI, possibly
  requiring specialized programmers.
• Can provide very flexible and powerful solution.
• API extensions can provide same functionality as
  a CGI program, but as API runs as part of the
  server, API approach can perform significantly
  better than CGI.
• Extending Web server is potentially dangerous,
  since server executable is being changed.

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Comparison of CGI and API

• CGI and API both extend capabilities of server.
• CGI scripts run in environment created by Web
  server program.
• Scripts only execute once Web server interprets
  request from browser, then returns results back
  to the server.
• API approach not nearly so limited in its ability
  to communicate.
• API-based extensions are loaded into same address
  space as Web server.

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Java

• Proprietary language developed by Sun and
  currently marketed by JavaSoft.
• Originally intended to support environment of
  networked machines and embedded systems.
• Now, Java is rapidly becoming de facto language
  for Web computing.
• Interesting because of its potential for building
  Web applications (applets) and server
  applications (servlets).

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Java

• Java is a simple, object-oriented, distributed,
  interpreted, robust, secure, architecture
  neutral, portable, high-performance,
  multi-threaded and dynamic language.
• Has a machine-independent target architecture,
  the Java Virtual Machine (JVM).
• Since almost every Web browser vendor has already
  licensed Java and implemented an embedded JVM,
  Java applications can currently be deployed on
  most end-user platforms.

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Java
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Java

• Before Java application can be executed, it must
  first be loaded into memory.
• Done by Class Loader, which takes .class
  file(s) containing bytecodes and transfers it
  into memory.
• Class file can be loaded from local hard drive or
  downloaded from network.
• Finally, bytecodes must be verified to ensure
  that they are valid do not violate Javas
  security restrictions.

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Java

• Loosely speaking Java is a safe C.
• Safety features include strong static type
  checking, automatic garbage collection, and
  absence of machine pointers at language level.
• Safety is central design goal ability to safely
  transmit Java code across Internet.
• Security is also integral part of Javas design -
  sandbox ensures untrusted application cannot gain
  access to system resources.

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JDBC

• Modeled after ODBC, JDBC API supports basic SQL
  functionality.
• With JDBC, Java can be used as host language for
  writing database applications.
• On top of JDBC, higher-level APIs can be built.
• Currently, two types of higher-level APIs
• An embedded SQL for Java.
• A direct mapping of relational database tables to
  Java classes.

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JDBC

• JDBC API consists of two main interfaces an API
  for application writers, and a lower-level driver
  API for driver writers.
• Applications and applets can access databases
  using
• JDBC API with pure Java JDBC drivers,
• ODBC drivers and existing database client
  libraries.

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JDBC - Advantages/Disadvantages

• Advantage of using ODBC drivers is that they are
  a de facto standard for PC database access, and
  are available for many DBMSs, for very low price.
  
• Disadvantages with this approach
• Non-pure JDBC driver will not necessarily work
  with a Web browser.
• Currently downloaded applet can connect only to
  database located on host machine.
• Deployment costs increase.

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JDBC
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JSQL

• Another JDBC-based approach uses Java with static
  embedded SQL.
• JSQL comprises a set of clauses that extend Java
  to include SQL constructs as statements and
  expressions.
• JSQL translator transforms JSQL clauses into
  standard Java code that accesses database through
  a CLI.

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Java Relational Binding (JRB)

• Middleware product that bridges from Java to
  RDBMSs.
• Provides orthogonal persistence through
  three-stage process
• database creation,
• an import program,
• JRB API.

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Java Relational Binding (JRB)
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Java Relational Binding (JRB)

• API is set of public classes used by programmer,
  which includes methods to connect to database
  server, open database, start/end transactions,
  create/update/read objects.
• JRB relies on security and integrity of
  underlying RDBMS
• references between objects implemented as FKs
• OIDs modeled as system generated PKs.

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Java Relational Binding (JRB)

• JRB also provides notion of class extents.
• Can associate predicate with class extent and
  retrieve objects based on their contents (in a
  select-from-where style).
• The runtime system is implemented on top of a
  JDBC-compliant interface layer.

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Scripting Languages (JavaScript and VBScript)

• Scripting languages can be used to extend browser
  and Web server with database functionality.
• As script code is embedded in HTML, it is
  downloaded every time page is accessed.
• Updating browser is simply a matter of changing
  Web document on server.
• Two popular scripting languages are JavaScript
  and VBScript.
• Both are interpreted languages, not compiled,
  making it easy to create small applications.

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Server-Side JavaScript for Database Access
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Microsoft Active Platform

• Microsoft Active Platform is an open,
  standards-based software architecture for
  delivering applications over the Internet and
  intranets.
• Contains HTML, scripting languages, and
  components (Java, ActiveX).
• On client machine called an Active Desktop.
• On Web server called an Active Server.
• Active Platform is encompassing term given to
  these related technologies.

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Object Linking and Embedding for DataBases (OLE
DB)

• Microsoft has defined set of data objects,
  collectively known as OLE DB.
• Allows OLE-oriented applications to share and
  manipulate sets of data as objects.
• OLE DB is an object-oriented specification based
  on C API.
• Components can be treated as data consumers and
  data providers. Consumers take data from OLE DB
  interfaces and providers expose OLE DB
  interfaces.

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OLE DB
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Active Server Pages (ASP)

• ASP is programming model that allows dynamic,
  interactive Web pages to be created on server.
• ASP provides flexibility of CGI, without
  performance overhead discussed previously.
• ASP runs in-process with the server, and is
  optimized to handle large volume of users.
• When an .asp file is requested, Web server
  calls ASP, which reads requested file, executes
  any commands, and sends generated HTML page back
  to browser.

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Active Server Pages (ASP)
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Active Data Objects (ADO)

• Programming extension of ASP supported by
  Microsoft IIS for database connectivity.
• Supports following key features
• Independently-created objects.
• Support for stored procedures.
• Support for different cursor types.
• Batch updating.
• Support for limits on number of returned rows.
• Designed as an easy-to-use interface to OLE DB.

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Microsoft Internet Database Connector (IDC)

• Similar approach to ASP, again specific to
  Microsoft Internet Information Server.
• IDC is an ISAPI that reads an .idc file that
  contains SQL commands.
• IDC communicates with a DBMSs ODBC driver to
  retrieve necessary data from database and format
  it using information in an .htx file.

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Microsoft Internet Database Connector (IDC)
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Oracle Network Computing Architecture(NCA)

• NCA aimed at providing extensibility for
  distributed environments.
• It is three-tier architecture based on industry
  standards such as
• OMGs CORBA 2.0 technology.
• HTTP and HTML for Web enablement.
• IIOP for object interoperability.
• OMGs IDL for language neutral interfaces.

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Oracle NCA
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Oracle NCA
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Security

• All Internet traffic travels in the clear and
  anyone who monitors traffic can read it.
• Need to ensure with communication that
• It is inaccessible to anyone but sender and
  receiver (privacy).
• It has not been changed during transmission
  (integrity).
• Receiver can be sure it came from sender
  (authenticity).

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Security

• Sender can be sure receiver is genuine
  (non-fabrication).
• Sender cannot deny he or she sent it
  (non-repudiation).
• Must also protect information once it has reached
  Web server.

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Security

• Download may have executable content, which can
  perform following malicious actions
• Corrupt data or execution state of programs.
• Reformat complete disks.
• Perform a total system shutdown.
• Collect and download confidential data.
• Usurp identity and impersonate user.
• Lock up resources.
• Cause non-fatal but unwelcome effects.

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Security

• Look at
• Proxy Servers. - Firewalls
• Message Digest - Digital
• Algorithms Signatures.
• Digital Certificates. - Kerberos.
• SSL and S-HTTP. - SET and SST.
• Java Security. - ActiveX Security.

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Proxy Servers

• Proxy server is computer that sits between
  browser and Web server.
• It intercepts all requests to Web server to try
  to fulfil requests itself.
• Has two main purposes
• improve performance
• filter requests.

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Firewalls

• Designed to prevent unauthorized access to/from a
  private network.
• Can be implemented in both hardware and software,
  or a combination of both.
• Several types of firewall techniques
• Packet filter.
• Application gateway.
• Circuit-level gateway.
• Proxy server.

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Message Digest Algorithms

• Message digest algorithm takes an arbitrary-sized
  string (message) and generates fixed-length
  string (digest or hash).
• A digest has following characteristics
• It should be computationally infeasible to find
  another message that will generate same digest.
• Digest does not reveal anything about message.

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Digital Signatures

• Digital signature consists of two parts
• string of bits computed from data being signed
• private key of individual or organization wishing
  the signature.
• Can be used to verify data comes from this
  individual or organization.

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Digital Signatures

• Digital signature has many useful properties
• Authenticity can be verified, using public key.
• Cannot be forged (assuming private key is kept
  secret).
• Function of data signed and cannot be claimed to
  be signature for any other data.
• Signed data cannot be changed or signature will
  no longer verify data as being authentic.

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Digital Certificates

• Attachment to electronic message used for
  security purposes (e.g. verify user sending
  message), and provide receiver with means to
  encode reply.
• Sender applies for certificate from Certificate
  Authority (CA).
• CA issues encrypted certificate containing
  applicants public key and other identification
  information.

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Digital Certificates

• CA makes its own public key readily available.
• Recipient uses CAs public key to decode
  certificate attached to message, verifies it as
  issued by CA, and obtains senders public key and
  identification information held within
  certificate.
• With this information, recipient can send an
  encrypted reply.
• CAs role is critical, acting as go-between in
  relationship between two parties.

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Kerberos

• A server of secured user names and passwords.
• Provides one centralized security server for all
  data and resources on network.
• Database access, login, authorization control,
  and other security features are centralized on
  trusted Kerberos servers.
• Has similar function to that of Certificate
  server to identify and validate a user.

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Secure Sockets Layer (SSL)

• Encryption protocol for transmitting private
  documents
• Designed to prevent eavesdropping, tampering, and
  message forgery.
• Works by using private key to encrypt data that
  is transferred over SSL connection.
• Layered between application-level protocols such
  as HTTP and TCP/IP transport-level protocol.
• Thus, may be used for other application-level
  protocols such as FTP and NNTP.

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Secure-HTTP (S-HTTP)

• Protocol for securely transmitting individual
  messages over Web.
• Both SSL and S-HTTP use techniques such as
  encryption, and digital signatures, and
• Allow browsers and servers to authenticate each
  other.
• Allow controlled access to Web site.
• Ensure data exchanged between browser and server
  is secure and reliable.

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Secure Electronic Transactions

• Open, interoperable standard for processing
  credit card transactions over Internet, in simple
  and secure way.
• Transaction is split in such a way that merchant
  has access to information about
• what is being purchased,
• how much it costs,
• whether payment is approved,
• but no information on what payment method
  customer is using.

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SET

• Card issuer (e.g. Visa) has access to purchase
  price, but no information on type of merchandise
  involved.
• Certificates are heavily used by SET, both for
  certifying cardholder and for certifying that
  merchant has relationship with financial
  institution.

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SET
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Secure Transaction Technology (SST)

• Protocol designed to handle secure bank payments
  over Internet.
• Uses DES encryption of information, RSA
  encryption of bankcard information, and strong
  authentication of all parties involved in
  transaction.

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Java Security

• Sandbox ensures untrusted application cannot gain
  access to system resources.
• Involves three components
• class loader,
• bytecode verifier,
• security manager.
• Safety features provided by language and JVM, and
  enforced by compiler and runtime system.
• Security is a policy built on top of safety layer.

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Class Loader

• Allocates a (hierarchically structured) namespace
  for each class.
• Never allows class from less protected
  namespace to replace class from more protected
  namespace.
• Thus, I/O primitives, defined in local Java
  class, cannot be invoked or overridden by classes
  from out with local machine.
• As browsers and Java applications can provide
  their own class loader, this may be viewed as
  weakness in security.

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Bytecode Verifier

• JVM verifies bytecode instructions before
  allowing application/applet to run
• Typical checks include verifying
• Compiled code is correctly formatted.
• Internal stacks will not overflow/underflow.
• No illegal data conversions will occur.
• Bytecode instructions are appropriately typed.
• All class member accesses are valid.

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Security Manager

• Each Java application defines and implements its
  own security policy.
• A Java-enabled browser contains its own Security
  Manager, and any applets it downloads are subject
  to its policies.
• Generally, downloaded applets are prevented from
• Reading and writing files on clients file
  system.
• Making network connections to machines other than
  host.
• Starting other programs on the client.

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Security Manager

• Loading libraries.
• Defining method calls.
• These restrictions apply to applets downloaded
  over Internet/intranet.
• Also do not apply to applets on clients local
  disk and in directory on CLASSPATH.
• Local applets are loaded by file system loader
  and can read and write files, exit JVM, and are
  not passed through bytecode verifier.

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ActiveX Security

• ActiveX security model places no restrictions on
  what a control can do.
• Instead, each ActiveX control can be digitally
  signed by its author using system called
  Authenticode.
• Digital signatures are then certified by CA.
• This security model places responsibility for the
  computers security on the user.

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HTTP/1.1

• Number of new features added. Look at two
• Persistent connections become default behavior.
  While open, client can send synchronous or
  asynchronous messages, and server can respond to
  them in order.
• Digest authentication provided as replacement for
  basic authentication. Password remains secret
  between client and server. Client and server
  compute digest value using the MD5 and digest is
  sent across network.

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XML (eXtensible Markup Language)

• XML is new standard that could preserve general
  application independence that makes HTML portable
  and powerful.
• Pared-down version of SGML, designed especially
  for Web documents.
• Designers can create their own customized tags to
  provide functionality not available with HTML.

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XML (eXtensible Markup Language)

• SGML allows document to be logically separated
  into two
• Document Type Definition (DTD)
• other containing the text itself.
• Useful features include
• Database Schema Definition.
• Linking to relative objects or elements.
• Support for bi-directional links.
• Simplicity may be lost with move to XML.

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Put the Pieces Together

• The data warehouse configuration is beyond
  client/server computing. Therefore, consider a
  distributed component architecture.
• Strive for minimum administration on the client
  side. Thru thin-client technology like Java is
  likely to provide a zero-administration client
  setup.
• Ensure complete metadata integration. Both IT and
  variety of user types will benefit from unified
  metadata

100
Put the Pieces Together (contd)

• Select the right database to support the
  distributed environment. Thus, a RDBMS with the
  Java engine in the database would prove useful.
• In many Web applications, the HTTP server becomes
  a point of congestion as all data from a session
  is fed to the browser through this server.
  Implementing a CORBA model would provide
  distributed object computing and scalability.
• The Web interface consists of a browser, search
  engine, groupware, push technology, home pages,
  hypertext links, and downloaded Java and ActiveX
  applets.

101
Put the Pieces Together (contd)

• Tools supporting HTML can be universally
  deployed. However, for complex analysis, HTML is
  cumbersome. Use HTML as much as possible and
  reseve Java or plug-ins for complex ad hoc
  analysis.
• Ensure that you pay enough attention to the
  administration and maintenance of fact and
  dimension tables.