CS233: Database Systems — Lecture Slides 8
presented by
Timothy Heron∗
November 19, 2004
∗ E-mail:
theron@dcs.warwick.ac.uk
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Databases and the Web
• The Three-tier architecture
• Approaches to integrating Databases into the Web environment
• Java solutions
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What is the WWW?
• A network of information systems
• Platform independent delivery and dissemination of information
and interactive applications
• Global network
However from a database perspective it is:
• Web documents (static) are file-based which creates problems in
managing and keeping up-to-date this information
• Many useful Web sites use dynamic content (more than brochure
ware) e.g. product and pricing information.
This sort of data is bread-and-butter of databases and DBMS.
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The Hyperlink
• The WWW is a hyper-media (non-sequential) means of browsing
information.
• Simple point-and-click interface.
• Multi-media.
Web architecture is Client-Server
• Clients (browsers) request information from Servers (Web-servers
eg. Apache HTTP Server)
• Much of information stored in documents using HTML
(Hypertext Markup Language)
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Mailing List
Our Mailing List application could be easily written as a Web
application.
We can re-use our Members.java class without modification, we just
need to change the interface code.
Mailing List Web Demo
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The Protocol: HTTP
Documents are addressed by a URL (Uniform Resource Locator).
Hypertext Transfer Protocol (HTTP) defines how web-clients and
web-servers communicate:
1. Connection: the client establishes a connection with the Web
server
2. Request: The client sends a request message to the Web server.
3. Response: The Web server sends a response (eg. an HTML
document) to the client.
4. Close: The connection is closed by the Web server.
The protocol is stateless: the server retains no information between
requests.
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Statelessness
The protocol is stateless: the server retains no information between
requests.
This means that servers (and clients) can be ‘thin’ requiring little
memory or disk space.
This also means that concept of a session which is essential to
DBMS transactions is difficult to support.
(One way is to have hidden identifiers in HTML forms which get
passed back and forth between client and server, other ways include
’Cookies’ and storing a session id in the URL).
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Static v Dynamic Pages
HTML page stored in file is an example of a static page: content
does not change unless file is changed.
Dynamic Pages: generate each time it is accessed e.g. could
contain information generated from queries to a database.
Essentially, Dynamic Pages are created by programs (usually scripts)
which are run by the server (when requested by the client) that
create HTML dynamically containing results of data access e.g. to a
RDBMs.
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Web-DBMS Architecture
Two Tier client-server architecture:
• Client is user interface and ‘business logic’ within data
processing programs. Primary role is the presentation of data.
• Server: DBMS. Primary role is to supply data services.
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Three Tier architecture:
• First Tier: Client running user interface.
• Second Tier: Business logic and data processing logic handled
by an Application Server.
• Third Tier: Data validation and database access handled by
DBMS.
The Second Tier centralises the application maintainance and
application logic in an extensible framework. It reduces to burden on
Client making it ‘thin’. The client no longer has to include database
specific code.
The Three-Tier architecture maps naturally on to the Web
architecture which the Web-Server acting as an Application Server or
sending requests to a co-resident Application Server.
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What are the advantages of using Web environment to
support Three-Tier systems ?
• DBMS can manage web document hierarchies
• HTML has a simplicity in user-end markup and multi-media
GUI, it is also standardised.
• Through browsers, web access is platform independent.
• Network access is transparent and global: internet.
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What are the disadvantages ?
• Reliability: Slow, Reliability of packet delivery and bandwidth.
• HTTP is not scalable: protocol not optimised for multiple small
requests.
• HTTP is stateless: session management has to be coded.
• HTML is limited in functionality.
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Approaches to Integrating Web and DBMSs
How to get dynamic content into web pages?
• CGI: Scripting languages: C, PHP, Perl, JavaScript
• Java solutions:
– Applets - not used much now
– Servlets (API)
– JavaServer Pages (JSP)
– Enterprise Java Beans (EJB)
– Struts
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Other solutions (not discussed)
• SSI (Server Side Includes)
• Microsoft’s Web Solutions Platform: COM, DCOM, Active
Server Pages - ASP and ASP.NET, Active X Data Objects
(ADO)
• Oracle Internet Platform: Oracle Internet Application Server
(iAS)
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CGI
Common Gateway Interface (CGI) is a protocol (specification) for
communicating between a browser and a web-server.
• An HTML <form> is used to generate contents which are passed
by GET or POST methods to a named CGI script.
• GET appends the form contents to the URL as a query-stringe.g.
url/test-cgi2?stuff=Hi+there&action=dosomething
• POST first contacts the CGI script and then sends the contents to
its standard input (stdin). This hides the contents from the
user.
• Results from the CGI program are passed back to the browser.
Results must be in Multipurpose Internet Mail Extension
(MIME) format.
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Problems with CGI
• The browser-server interaction is limited: only query-string is
passed and standard output (MIME) returned.
• A new process is invoked for each HTTP connection (each user
form submit or session) which is costly.
• There is no persistence of session state between user requests.
• Badly written CGI is open to abuse.
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Servlets
Servlets are (Java) objects that extend the functionality of the
web-server.
They are servers dedicated to specific CGI requests. Once started,
they wait for CGI requests, execute them and wait for more.
Servlets are created by compiling Java code which users the packages
:
javax.servlet
javax.servlet.http
Web-servers support and manage these programs through built-in
extensions (plugins), e.g. Apache Tomcat.
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The life cycle of a servlet
1. The servlet execution engine (e.g. Tomcat) creates a servlet and
runs it in its own thread (light-weight process).
2. The servlet object is initialised by the init() method and waits
for service requests.
3. It responds to GET and POST requests by calling the user defined
doGet() or doPost() methods.
4. User content is unpackaged using the getParameterValues()
method.
5. It creates MIME response output to stdout, perhaps using a
HTML helper object like HtmlWriter.
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Why use servlets?
Servlets are preferred over CGI scripts because:
• Servlets can keep user sessions open for multiple requests.
• They can manage client state within the object (or with
supporting objects).
• They can access a DBMS through the Java database API: JDBC.
• They are easier to write securely, Java does not allow direct
access to memory and so attacks such as ’buffer overflows’ are
easier to protect against.
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Mailing List as Servlets
We can break our Mailing List application into 4 key parts :
• A HTML page to select whether we want to ’Add a member’ or
’List members’.
• A HTML page containing a form to enter a member’s details.
• An AddMemberServlet to add the form’s contents to the
database.
• A ListMembersServlet to retrieve data from the database and
present it as an HTML page.
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ListMembersServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ListMembersServlet extends HttpServlet
{
public void doGet(HttpServletRequest request,HttpServletRespon
throws IOException, ServletException
{
// Get the list of members
Members members = new Members();
String[] memberList = members.listMembers();
// Prepare the beginning of our response
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PrintWriter writer = response.getWriter();
response.setContentType("text/html");
writer.println("<html>");
writer.println("<head>");
writer.println("<title>List of Members</title>");
writer.println("</head>");
writer.println("<body bgcolor=\"white\">");
writer.println("<h1>Members</h1><hr>");
for (int i = 0; i < memberList.length; i++)
writer.println("<br>"+memberList[i]);
// Prepare the ending of our response
writer.println("<hr><br><br>");
writer.println(
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"<a href=\"../mailinglist/MailingList.html\">Return</a><br
writer.println("</body>");
writer.println("</html>");
}
}
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The Add Member form
We have to construct a web page to accept input from the user.
This requires us to use the <FORM> tag and the <INPUT> tag.
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AddMember.html
<html>
<head>
<title>Mailing List - Add Member</title>
</head>
<body>
<h1>Add a new member</h1>
<hr>
<form action="../servlet/AddMemberServlet" method="post">
<table width="100%" border="0">
<tr><td>Forename :</td>
<td><input type="TEXT" name="forename" /></td></tr>
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<tr><td>Surname :</td>
<td><input type="TEXT" name="surname" /></td></tr>
<tr><td>Address 1 :</td>
<td><input type="TEXT" name="address1" /></td></tr>
<tr><td>Address 2 :</td>
<td><input type="TEXT" name="address2" /></td></tr>
<tr><td>City :</td>
<td><input type="TEXT" name="city" /></td></tr>
<tr><td>Postcode :</td>
<td><input type="TEXT" name="postcode" /></td></tr>
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</table>
<input type="SUBMIT">
</form>
<hr>
<p><a href="MailingList.html">Return</a></p>
</body>
</html>
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AddMemberServlet
We have to create a servlet that the web browser will submit this
form to.
Since the user is submitting a form (rather than following a link) we
must override the doPost method instead of the doGet method we
overrode in the ListMembersServlet.
The fields on the form are passed to a servlet through the
HttpServletRequest request object.
We call the String getParameter(String parameterName) method
of the request object to get to the user’s submission.
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AddMemberServlet.java
public class AddMemberServlet extends HttpServlet
{
public void doPost(HttpServletRequest request,HttpServletRespo
response) throws IOException, ServletException
{
// Acquire the parameters
String forename = request.getParameter("forename");
String surname = request.getParameter("surname");
String address1 = request.getParameter("address1");
String address2 = request.getParameter("address2");
String city = request.getParameter("city");
String postcode = request.getParameter("postcode");
// Try add the member to our list
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Members members = new Members();
boolean success = members.addMember(forename,surname,
address1,address2,city,postcode);
// Prepare the beginning of our response
PrintWriter writer = response.getWriter();
response.setContentType("text/html");
writer.println("<html>");
writer.println("<head>");
writer.println("<title>Add Member Result</title>");
writer.println("</head>");
writer.println("<body bgcolor=\"white\">");
writer.println("<h3>Add Member Result</h3>");
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if (success)
writer.println("<p>Member "+forename+" added successfully.
</p>"
else
writer.println("<p>Could not add member to the list.</p>")
// Prepare the ending of our response
writer.println("<hr>");
writer.println("<a href=\"../mailinglist/AddMember.html\">
Add another member</a><br>");
writer.println("<a href=\"../mailinglist/MailingList.html\">
Return</a><br>");
writer.println("</body>");
writer.println("</html>");
}
}