Chapter 3
Web Software
Objectives
• Name the four primary components of a webbased application
• Describe the responsibilities of a web browser
• Explain cache control and cookie management
details
• Describe the responsibilities of a web server
• Explain the different web development
application environments
• Describe the purpose of an RDBMS
• Name the protocols supported by a modern
RDBMS
Web Application Components
• Four important components of a web
application:
Business
Application
Web Browser
Web Server
Database
Server
Client-Side
Server-Side
Web Application Components
• Web Browser: presents the user interface
• Web Server: processes HTTP requests
• Business Application: processes requests at the
application level by providing a service
• Database Server: maintains the database by
processing query and update requests from the
application
Web Browsers
Time-Line of Widely Used Browsers
Web Browser Responsibilities
•
•
•
•
•
•
User Interface Presentation
Client-Server Communication (HTTP)
Cache Control
Cookie Management
Handling Embedded Objects
Script Interpretation
User Interface Presentation
• Parse HTML and CSS code
– handle errors
• Format and present a graphical display
• Handle user interactions
– scroll, mouse movement, click, etc.
Client-Server Communication (HTTP)
• Format HTTP request
• Handle HTTP response
– including redirects, errors, etc.
• Request subordinate items
– images, style sheets, etc.
Cache Control
• Cache is a local memory for recently
accessed web pages, images, etc.
• Cache enables pages to be reloaded
quickly, without another HTTP transaction
reload
Browser
Cache
initial request
Server
Cache Control
HTTP Headers
• Cache-Control: <option>
(response header)
– public – client or proxy server may cache
– private – only the client may cache
– no-cache – no caching anywhere
• If-Modified-Since: <date> (request header)
– tells server to send resource only if modified
since the specified date
– otherwise the client will use a cached version
Cache Control
HTTP Headers
• HTTP response with Cache-Control header
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Last-Modified: Thu, 22 Sep 2005 12:57:30 GMT
Cache-Control: private
Expires: Wed, 26 Sep 2007 14:15:51 GMT
Content-Type: text/html
Content-Length: 327
Date: Tue, 25 Sep 2007 14:15:51 GMT
• HTTP transaction with If-Modified-Since header
GET /index.html HTTP/1.1
Host: grove.cs.jmu.edu
If-Modified-Since: Tue, 25 Sep 2007 14:15:51 GMT
Date: Tue, 25 Sep 2007 14:15:51 GMT
request
HTTP/1.1 304 Not Modified
Date: Tue, 25 Sep 2007 14:35:36 GMT
Server: Apache/2.2.3 (Red Hat)
response
Cookie Management
• An HTTP cookie is a small file that is
– provided by the server as an HTTP response header
– stored by the client
– returned to the server as an HTTP request header
Client
Cookie
Storage
Cookie contents
Server
Cookies and Sessions
• HTTP Session: a series of HTTP transactions
between a client and a server
• Cookies allow servers to identify sessions
• Typical usage
– initial transaction: server creates session (server-side)
and stores session-id in a cookie (client-side)
– subsequent transactions: client sends session-id with
each request, allowing server to locate session data
Cookies and Sessions
Server
Client
Make 1st
request
Cookie
(session-id)
Handle
response
Make 2nd
request
Handle
response
Create
session
*
*
Session
data
Continue
session
* includes session-id
Cookie Parameters
• Cookie parameters are set in a Set-Cookie
response header
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Set-Cookie: domain=widget.com; path=/; secure; Account=766324
Content-Type: text/html
Content-Length: 327
Date: Tue, 25 Sep 2007 14:15:51 GMT
Cookie Parameters
• expires=<date>: determines when cookie will be
deleted
• domain=<domain name> : cookie will be
returned to each domain that ends with this
value
• path=<path name> : cookie will be returned only
for requests that start with this path
• secure : if present, cookie will be returned only
with HTTPS (secure HTTP) requests
• <name>=<value> : allow arbitrary data to be
stored in a cookie
Handling Embedded Objects
• Objects referenced by an HTML document (images,
scripts, style sheets, etc.) must be loaded separately by
the browser
– A plug-in or helper application may be used
• Helper application: a program that handles content in a
separate window
– e.g., audio player, movie player
• Plug-in: a program that handles content within the
browser
– e.g., animation viewer or virtual machine for applets
Script Interpretation
• Most browsers interpret JavaScript and its
variants (ECMAScript, JScript, etc.)
• Scripting languages are powerful, so interpreters
are necessarily complex
• Script interpreters are not entirely standardized
across browsers, so script programmers must
test scripts on many browser versions
– The “write once, test many” principle in action
Web Server Responsibilities
• Connection Management
• HTTP request handling
Connection Management
• With HTTP/1.1, servers are responsible for
setting up and tearing down TCP
connections over a series of requests
– improves efficiency
HTTP Request Handling
• Servers must correctly interpret HTTP
request headers and respond
appropriately
• Static content includes fixed files that are
returned without any further computation
• Dynamic content is generated on request
by an application component that is
invoked by the server
Application Development Environments
(1/2)
• Common Gateway Interface (CGI)
– allows programs to be invoked at the
operating system level
• Servlet Container
– allows programs written in Java to be invoked
in a Java Virtual Machine linked to the server
Application Development Environments
(2/2)
• Template Language
– allows executable code to be inserted into an
HTML document
• Scripting Language
– a language that can be used to generate
HTML content on request
– typically have relaxed syntax, requiring less
coding than regular programming languages
Database Server Responsibilities
• Relational Database Management System (RDBMS)
• Maintains data storage for an application
– processes queries and updates
• Provides a standard interface for application programs
– Open DataBase Connectivity (ODBC)
– Java DataBase Connectivity (JDBC)
• Supports standard query language for data query and
manipulation
– Structured Query Language (SQL)
Review
•
•
•
•
•
Web application components
Web browsers
Web servers
Application development environments
Database servers
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