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Chapter 9: Application Design and Development

Application Programs and User Interfaces
Web Fundamentals
Servlets and JSP
Application Architectures
Rapid Application Development
Application Performance
Application Security
Encryption and Its Applications

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Application Programs and User Interfaces

Most database users do not use a query language like SQL
An application program acts as the intermediary between users and the database

Applications split into

front-end
middle layer
backend

Front-end: user interface

Forms
Graphical user interfaces
Many interfaces are Web-based

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Application Architecture Evolution

Three distinct era’s of application architecture

mainframe (1960’s and 70’s)
personal computer era (1980’s)
We era (1990’s onwards)

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Web Interface

Web browsers have become the de-facto standard user interface to databases

Enable large numbers of users to access databases from anywhere
Avoid the need for downloading/installing specialized code, while providing a good graphical user interface

Javascript, Flash and other scripting languages run in browser, but are downloaded transparently

Examples: banks, airline and rental car reservations, university course registration and grading, an so on.

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The World Wide Web

The Web is a distributed information system based on hypertext.
Most Web documents are hypertext documents formatted via the HyperText Markup Language (HTML)
HTML documents contain

text along with font specifications, and other formatting instructions
hypertext links to other documents, which can be associated with regions of the text.
forms, enabling users to enter data which can then be sent back to the Web server

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Uniform Resources Locators

In the Web, functionality of pointers is provided by Uniform Resource Locators (URLs).
URL example:

http://www.acm.org/sigmod

The first part indicates how the document is to be accessed

“http” indicates that the document is to be accessed using the Hyper Text Transfer Protocol.

The second part gives the unique name of a machine on the Internet.
The rest of the URL identifies the document within the machine.

The local identification can be:

The path name of a file on the machine, or
An identifier (path name) of a program, plus arguments to be passed to the program

E.g., http://www.google.com/search?q=silberschatz

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HTML and HTTP

HTML provides formatting, hypertext link, and image display features

including tables, stylesheets (to alter default formatting), etc.

HTML also provides input features

Select from a set of options

Pop-up menus, radio buttons, check lists

Enter values

Text boxes

Filled in input sent back to the server, to be acted upon by an executable at the server

HyperText Transfer Protocol (HTTP) used for communication with the Web server

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Sample HTML Source Text

<html>

<body>

<table border>�<tr> <th>ID</th> <th>Name</th> <th>Department</th> </tr>�<tr> <td>00128</td> <td>Zhang</td> <td>Comp. Sci.</td> </tr>�….

</table>

<form action="PersonQuery" method=get>�Search for: � <select name="persontype">� <option value="student" selected>Student </option>� <option value="instructor"> Instructor </option>� </select> <br>�Name: <input type=text size=20 name="name">�<input type=submit value="submit">

</form>

</body> </html>

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Display of Sample HTML Source

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Web Servers

A Web server can easily serve as a front end to a variety of information services.
The document name in a URL may identify an executable program, that, when run, generates a HTML document.

When an HTTP server receives a request for such a document, it executes the program, and sends back the HTML document that is generated.
The Web client can pass extra arguments with the name of the document.

To install a new service on the Web, one simply needs to create and install an executable that provides that service.

The Web browser provides a graphical user interface to the information service.

Common Gateway Interface (CGI): a standard interface between web and application server

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Three-Layer Web Architecture

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Two-Layer Web Architecture

Multiple levels of indirection have overheads

Alternative: two-layer architecture

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HTTP and Sessions

The HTTP protocol is connectionless

That is, once the server replies to a request, the server closes the connection with the client, and forgets all about the request
In contrast, Unix logins, and JDBC/ODBC connections stay connected until the client disconnects

retaining user authentication and other information

Motivation: reduces load on server

operating systems have tight limits on number of open connections on a machine

Information services need session information

E.g., user authentication should be done only once per session

Solution: use a cookie

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Sessions and Cookies

A cookie is a small piece of text containing identifying information

Sent by server to browser

Sent on first interaction, to identify session

Sent by browser to the server that created the cookie on further interactions

part of the HTTP protocol

Server saves information about cookies it issued, and can use it when serving a request

E.g., authentication information, and user preferences

Cookies can be stored permanently or for a limited time

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Servlets

Java Servlet specification defines an API for communication between the Web/application server and application program running in the server

E.g., methods to get parameter values from Web forms, and to send HTML text back to client

Application program (also called a servlet) is loaded into the server

Each request spawns a new thread in the server

thread is closed once the request is serviced

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Example Servlet Code

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class PersonQueryServlet extends HttpServlet {

public void doGet (HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException

{

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println("<HEAD><TITLE> Query Result</TITLE></HEAD>");

out.println("<BODY>");

….. BODY OF SERVLET (next slide) …

out.println("</BODY>");

out.close();

}

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Example Servlet Code

String persontype = request.getParameter("persontype");

String number = request.getParameter("name");

if(persontype.equals("student")) {

... code to find students with the specified name ...

... using JDBC to communicate with the database ..

out.println("<table BORDER COLS=3>");

out.println(" <tr> <td>ID</td> <td>Name: </td>" + " <td>Department</td> </tr>");

for(... each result ...){

... retrieve ID, name and dept name

... into variables ID, name and deptname

out.println("<tr> <td>" + ID + "</td>" + "<td>" + name + "</td>" + "<td>" + deptname � + "</td></tr>");

};

out.println("</table>");

}

else {

... as above, but for instructors ...

}

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Servlet Sessions

Servlet API supports handling of sessions

Sets a cookie on first interaction with browser, and uses it to identify session on further interactions

To check if session is already active:

if (request.getSession(false) == true)

.. then existing session
else .. redirect to authentication page

authentication page

check login/password
request.getSession(true): creates new session

Store/retrieve attribute value pairs for a particular session

session.setAttribute(“userid”, userid)
session.getAttribute(“userid”)

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Servlet Support

Servlets run inside application servers such as

Apache Tomcat, Glassfish, JBoss
BEA Weblogic, IBM WebSphere and Oracle Application Servers

Application servers support

deployment and monitoring of servlets
Java 2 Enterprise Edition (J2EE) platform supporting objects, parallel processing across multiple application servers, etc

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Server-Side Scripting

Server-side scripting simplifies the task of connecting a database to the Web

Define an HTML document with embedded executable code/SQL queries.
Input values from HTML forms can be used directly in the embedded code/SQL queries.
When the document is requested, the Web server executes the embedded code/SQL queries to generate the actual HTML document.

Numerous server-side scripting languages

JSP, PHP
General purpose scripting languages: VBScript, Perl, Python

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Java Server Pages (JSP)

A JSP page with embedded Java code

<html>

<head> <title> Hello </title> </head>

<body>

<% if (request.getParameter(“name”) == null)

{ out.println(“Hello World”); }

else { out.println(“Hello, ” + request.getParameter(“name”)); }

</body>

</html>

JSP is compiled into Java + Servlets
JSP allows new tags to be defined, in tag libraries

such tags are like library functions, can are used for example to build rich user interfaces such as paginated display of large datasets

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PHP

PHP is widely used for Web server scripting
Extensive libaries including for database access using ODBC

<html>

<head> <title> Hello </title> </head>

<body>

<?php if (!isset($_REQUEST[‘name’]))

{ echo “Hello World”; }

else { echo “Hello, ” + $_REQUEST[‘name’]; }

</body>

</html>

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Client Side Scripting

Browsers can fetch certain scripts (client-side scripts) or programs along with documents, and execute them in “safe mode” at the client site

Javascript
Macromedia Flash and Shockwave for animation/games
VRML
Applets

Client-side scripts/programs allow documents to be active

E.g., animation by executing programs at the local site
E.g., ensure that values entered by users satisfy some correctness checks
Permit flexible interaction with the user.

Executing programs at the client site speeds up interaction by avoiding many round trips to server

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Client Side Scripting and Security

Security mechanisms needed to ensure that malicious scripts do not cause damage to the client machine

Easy for limited capability scripting languages, harder for general purpose programming languages like Java

E.g., Java’s security system ensures that the Java applet code does not make any system calls directly

Disallows dangerous actions such as file writes
Notifies the user about potentially dangerous actions, and allows the option to abort the program or to continue execution.

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Javascript

Javascript very widely used

forms basis of new generation of Web applications (called Web 2.0 applications) offering rich user interfaces

Javascript functions can

check input for validity
modify the displayed Web page, by altering the underling document object model (DOM) tree representation of the displayed HTML text
communicate with a Web server to fetch data and modify the current page using fetched data, without needing to reload/refresh the page

forms basis of AJAX technology used widely in Web 2.0 applications
E.g. on selecting a country in a drop-down menu, the list of states in that country is automatically populated in a linked drop-down menu

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Javascript

Example of Javascript used to validate form input

<html> <head>�<script type="text/javascript">� function validate() {� var credits=document.getElementById("credits").value;� if (isNaN(credits)|| credits<=0 || credits>=16) {� alert("Credits must be a number greater than 0 and less than 16");� return false� }� }�</script>

</head> <body>�<form action="createCourse" onsubmit="return validate()">� Title: <input type="text" id="title" size="20"><br />� Credits: <input type="text" id="credits" size="2"><br />� <Input type="submit" value="Submit">�</form>

</body> </html>

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Application Architectures

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Application Architectures

Application layers

Presentation or user interface

model-view-controller (MVC) architecture

model: business logic
view: presentation of data, depends on display device
controller: receives events, executes actions, and returns a view to the user

business-logic layer

provides high level view of data and actions on data

often using an object data model

hides details of data storage schema

data access layer

interfaces between business logic layer and the underlying database
provides mapping from object model of business layer to relational model of database

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Application Architecture

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Business Logic Layer

Provides abstractions of entities

e.g. students, instructors, courses, etc

Enforces business rules for carrying out actions

E.g. student can enroll in a class only if she has completed prerequsites, and has paid her tuition fees

Supports workflows which define how a task involving multiple participants is to be carried out

E.g. how to process application by a student applying to a university
Sequence of steps to carry out task
Error handling

e.g. what to do if recommendation letters not received on time

Workflows discussed in Section 26.2

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Object-Relational Mapping

Allows application code to be written on top of object-oriented data model, while storing data in a traditional relational database

alternative: implement object-oriented or object-relational database to store object model

has not been commercially successful

Schema designer has to provide a mapping between object data and relational schema

e.g. Java class Student mapped to relation student, with corresponding mapping of attributes
An object can map to multiple tuples in multiple relations

Application opens a session, which connects to the database
Objects can be created and saved to the database using session.save(object)

mapping used to create appropriate tuples in the database

Query can be run to retrieve objects satisfying specified predicates

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Object-Relational Mapping and Hibernate (Cont.)

The Hibernate object-relational mapping system is widely used

public domain system, runs on a variety of database systems
supports a query language that can express complex queries involving joins

translates queries into SQL queries

allows relationships to be mapped to sets associated with objects

e.g. courses taken by a student can be a set in Student object

See book for Hibernate code example

The Entity Data Model developed by Microsoft

provides an entity-relationship model directly to application
maps data between entity data model and underlying storage, which can be relational
Entity SQL language operates directly on Entity Data Model

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Web Services

Allow data on Web to be accessed using remote procedure call mechanism
Two approaches are widely used

Representation State Transfer (REST): allows use of standard HTTP request to a URL to execute a request and return data

returned data is encoded either in XML, or in JavaScript Object Notation (JSON)

Big Web Services:

uses XML representation for sending request data, as well as for returning results
standard protocol layer built on top of HTTP
See Section 23.7.3

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Disconnected Operations

Tools for applications to use the Web when connected, but operate locally when disconnected from the Web

E.g. Google Gears browser plugin

Provide a local database, a local Web server and support for execution of JavaScript at the client
JavaScript code using Gears can function identically on any OS/browser platform

Adobe AIR software provides similar functionality outside of Web browser

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Rapid Application Development

A lot of effort is required to develop Web application interfaces

more so, to support rich interaction functionality associated with Web 2.0 applications

Several approaches to speed up application development

Function library to generate user-interface elements
Drag-and-drop features in an IDE to create user-interface elements
Automatically generate code for user interface from a declarative specification

Above features have been in used as part of rapid application development (RAD) tools even before advent of Web
Web application development frameworks

Java Server Faces (JSF) includes JSP tag library
Ruby on Rails

Allows easy creation of simple CRUD (create, read, update and delete) interfaces by code generation from database schema or object model

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ASP.NET and Visual Studio

ASP.NET provides a variety of controls that are interpreted at server, and generate HTML code
Visual Studio provides drag-and-drop development using these controls

E.g. menus and list boxes can be associated with DataSet object
Validator controls (constraints) can be added to form input fields

JavaScript to enforce constraints at client, and separately enforced at server

User actions such as selecting a value from a menu can be associated with actions at server
DataGrid provides convenient way of displaying SQL query results in tabular format

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Application Performance

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Improving Web Server Performance

Performance is an issue for popular Web sites

May be accessed by millions of users every day, thousands of requests per second at peak time

Caching techniques used to reduce cost of serving pages by exploiting commonalities between requests

At the server site:

Caching of JDBC connections between servlet requests

a.k.a. connection pooling

Caching results of database queries

Cached results must be updated if underlying database changes

Caching of generated HTML

At the client’s network

Caching of pages by Web proxy

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Application Security

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SQL Injection

Suppose query is constructed using

"select * from instructor where name = ’" + name + "’"

Suppose the user, instead of entering a name, enters:

X’ or ’Y’ = ’Y

then the resulting statement becomes:

"select * from instructor where name = ’" + "X’ or ’Y’ = ’Y" + "’"
which is:

select * from instructor where name = ’X’ or ’Y’ = ’Y’

User could have even used

X’; update instructor set salary = salary + 10000; --

Prepared statement internally uses:�"select * from instructor where name = ’X\’ or \’Y\’ = \’Y’
Always use prepared statements, with user inputs as parameters
Is the following prepared statemen secure?

conn.prepareStatement("select * from instructor where name = ’" + name + "’“)

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Cross Site Scripting

HTML code on one page executes action on another page

E.g. <img src = http://mybank.com/transfermoney?amount=1000&toaccount=14523>
Risk: if user viewing page with above code is currently logged into mybank, the transfer may succeed
Above example simplistic, since GET method is normally not used for updates, but if the code were instead a script, it could execute POST methods

Above vulnerability called cross-site scripting (XSS) or cross-site request forgery (XSRF or CSRF)
Prevent your web site from being used to launch XSS or XSRF attacks

Disallow HTML tags in text input provided by users, using functions to detect and strip such tags

Protect your web site from XSS/XSRF attacks launched from other sites

..next slide

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Cross Site Scripting

Protect your web site from XSS/XSRF attacks launched from other sites

Use referer value (URL of page from where a link was clicked) provided by the HTTP protocol, to check that the link was followed from a valid page served from same site, not another site
Ensure IP of request is same as IP from where the user was authenticated

prevents hijacking of cookie by malicious user

Never use a GET method to perform any updates

This is actually recommended by HTTP standard

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Password Leakage

Never store passwords, such as database passwords, in clear text in scripts that may be accessible to users

E.g. in files in a directory accessible to a web server

Normally, web server will execute, but not provide source of script files such as file.jsp or file.php, but source of editor backup files such as file.jsp~, or .file.jsp.swp may be served

Restrict access to database server from IPs of machines running application servers

Most databases allow restriction of access by source IP address

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Application Authentication

Single factor authentication such as passwords too risky for critical applications

guessing of passwords, sniffing of packets if passwords are not encrypted
passwords reused by user across sites
spyware which captures password

Two-factor authentication

e.g. password plus one-time password sent by SMS
e.g. password plus one-time password devices

device generates a new pseudo-random number every minute, and displays to user
user enters the current number as password
application server generates same sequence of pseudo-random numbers to check that the number is correct.

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Application Authentication

Man-in-the-middle attack

E.g. web site that pretends to be mybank.com, and passes on requests from user to mybank.com, and passes results back to user
Even two-factor authentication cannot prevent such attacks

Solution: authenticate Web site to user, using digital certificates, along with secure http protocol
Central authentication within an organization

application redirects to central authentication service for authentication
avoids multiplicity of sites having access to user’s password
LDAP or Active Directory used for authentication

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Single Sign-On

Single sign-on allows user to be authenticated once, and applications can communicate with authentication service to verify user’s identity without repeatedly entering passwords
Security Assertion Markup Language (SAML) standard for exchanging authentication and authorization information across security domains

e.g. user from Yale signs on to external application such as acm.org using userid joe@yale.edu
application communicates with Web-based authentication service at Yale to authenticate user, and find what the user is authorized to do by Yale (e.g. access certain journals)

OpenID standard allows sharing of authentication across organizations

e.g. application allows user to choose Yahoo! as OpenID authentication provider, and redirects user to Yahoo! for authentication

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Application-Level Authorization

Current SQL standard does not allow fine-grained authorization such as “students can see their own grades, but not other’s grades”

Problem 1: Database has no idea who are application users
Problem 2: SQL authorization is at the level of tables, or columns of tables, but not to specific rows of a table

One workaround: use views such as

create view studentTakes as�select *�from takes�where takes.ID = syscontext.user_id()

where syscontext.user_id() provides end user identity

end user identity must be provided to the database by the application

Having multiple such views is cumbersome

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Application-Level Authorization (Cont.)

Currently, authorization is done entirely in application
Entire application code has access to entire database

large surface area, making protection harder

Alternative: fine-grained (row-level) authorization schemes

extensions to SQL authorization proposed but not currently implemented
Oracle Virtual Private Database (VPD) allows predicates to be added transparently to all SQL queries, to enforce fine-grained authorization

e.g. add ID= sys_context.user_id() to all queries on student relation if user is a student

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Audit Trails

Applications must log actions to an audit trail, to detect who carried out an update, or accessed some sensitive data
Audit trails used after-the-fact to

detect security breaches
repair damage caused by security breach
trace who carried out the breach

Audit trails needed at

Database level, and at
Application level

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Encyption

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Encryption

Data may be encrypted when database authorization provisions do not offer sufficient protection.
Properties of good encryption technique:

Relatively simple for authorized users to encrypt and decrypt data.
Encryption scheme depends not on the secrecy of the algorithm but on the secrecy of a parameter of the algorithm called the encryption key.
Extremely difficult for an intruder to determine the encryption key.

Symmetric-key encryption: same key used for encryption and for decryption
Public-key encryption (a.k.a. asymmentric-key encryption): use different keys for encryption and decryption

encryption key can be public, decryption key secret

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Encryption (Cont.)

Data Encryption Standard (DES) substitutes characters and rearranges their order on the basis of an encryption key which is provided to authorized users via a secure mechanism. Scheme is no more secure than the key transmission mechanism since the key has to be shared.
Advanced Encryption Standard (AES) is a new standard replacing DES, and is based on the Rijndael algorithm, but is also dependent on shared secret keys.
Public-key encryption is based on each user having two keys:

public key – publicly published key used to encrypt data, but cannot be used to decrypt data
private key -- key known only to individual user, and used to decrypt data. Need not be transmitted to the site doing encryption.

Encryption scheme is such that it is impossible or extremely hard to decrypt data given only the public key.

The RSA public-key encryption scheme is based on the hardness of factoring a very large number (100's of digits) into its prime components

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Encryption (Cont.)

Hybrid schemes combining public key and private key encryption for efficient encryption of large amounts of data
Encryption of small values such as identifiers or names vulnerable to dictionary attacks

especially if encryption key is publicly available
but even otherwise, statistical information such as frequency of occurrence can be used to reveal content of encrypted data
Can be deterred by adding extra random bits to the end of the value, before encryption, and removing them after decryption

same value will have different encrypted forms each time it is encrypted, preventing both above attacks
extra bits are called salt bits

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Encryption in Databases

Database widely support encryption
Different levels of encryption:

disk block

every disk block encrypted using key available in database-system software.
Even if attacker gets access to database data, decryption cannot be done without access to the key.

Entire relations, or specific attributes of relations

non-sensitive relations, or non-sensitive attributes of relations need not be encrypted
however, attributes involved in primary/foreign key constraints cannot be encrypted.

Storage of encryption or decryption keys

typically, single master key used to protect multiple encryption/decryption keys stored in database

Alternative: encryption/decryption is done in application, before sending values to the database

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Encryption and Authentication

Password based authentication is widely used, but is susceptible to sniffing on a network.
Challenge-response systems avoid transmission of passwords

DB sends a (randomly generated) challenge string to user.
User encrypts string and returns result.
DB verifies identity by decrypting result
Can use public-key encryption system by DB sending a message encrypted using user’s public key, and user decrypting and sending the message back.

Digital signatures are used to verify authenticity of data

E.g., use private key (in reverse) to encrypt data, and anyone can verify authenticity by using public key (in reverse) to decrypt data. Only holder of private key could have created the encrypted data.
Digital signatures also help ensure nonrepudiation: sender�cannot later claim to have not created the data

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End of Chapter

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Digital Certificates

Digital certificates are used to verify authenticity of public keys.
Problem: when you communicate with a web site, how do you know if you are talking with the genuine web site or an imposter?

Solution: use the public key of the web site
Problem: how to verify if the public key itself is genuine?

Solution:

Every client (e.g., browser) has public keys of a few root-level certification authorities
A site can get its name/URL and public key signed by a certification authority: signed document is called a certificate
Client can use public key of certification authority to verify certificate
Multiple levels of certification authorities can exist. Each certification authority

presents its own public-key certificate signed by a �higher level authority, and
uses its private key to sign the certificate of other web sites/authorities

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A formatted report

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Figure 9.11

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Web Interfaces to Database (Cont.)

Dynamic generation of documents

Limitations of static HTML documents

Cannot customize fixed Web documents for individual users.
Problematic to update Web documents, especially if multiple Web documents replicate data.

Solution: Generate Web documents dynamically from data stored in a database.

Can tailor the display based on user information stored in the database.

E.g., tailored ads, tailored weather and local news, …

Displayed information is up-to-date, unlike the static Web pages

E.g., stock market information, ..�

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