Transcript Web Application Resources - gozips.uakron.edu

2440: 141
Web Site Administration
Web Application Resources
Instructor: Enoch E. Damson
Client/Server Basics
 The client/server model generally has clients and servers, and
business/application logic (rules)
 Client – hardware or software used to communicate with a data
provider (server)
 Normally, one user uses a specific client at a time
 Server – a computer capable of providing data to many clients at the
same time

The term server can mean:


the physical computer or piece of hardware
the actual server software or daemon running on that machine
 Daemon – a program that offers a service to other programs, usually over a
network
 Application/business logic – rules written in a programming language
to help the communication between clients and the server
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Network Connections and Ports
 Computers connected to the Internet typically
communicate using TCP/IP (Transmission Control
Protocol and the Internet Protocol)
 TCP/IP – allows computers to communicate at a network-
level using IP (Internet Protocol) addresses


TCP (Transmission Control Protocol) – invented by Robert Kahn
IP (Internet Protocol) – created by Dr. Vinton Cert together with
Robert Kahn
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Network Connections and Ports…
 IP Address – a unique identifier assigned to each
computer on the Internet
 Port number – a unique identifier that indicates each
service running on a computer
 Most services have standard port numbers
 Some standard ports numbers are:

ftp (20,21), telnet (23), smtp (25), http (80)
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Web Servers
 Service client requests for documents
 The European Lab for Particle Physics (CERN) produced one of
the first Web servers
 The World Wide Web Consortium (W3C) took over the
development of the CERN HTTPD (W3C HTTPD)
 The National Center for Supercomputing Applications (NCSA)
also created an HTTP server which quickly became popular
between 1993 and 1995
 The Apache server (based on the NCSA implementation) has
become very popular
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Web Browsers
 Used as a client to retrieve and display information
from Web servers
 Earlier Web browsers were text-based
 E.g. gopher, WAIS, telnet, FTP
 NCSA Mosaic was the first real HTML browser that
was developed in 1993
 There are several browsers on the market now
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Electronic Publishing
 The two main types of files in Web publishing are:
 ASCII Text Files – uses numeric values (0 – 127) to represent
letters, numbers, and other characters

Each byte of the file represents a specific character
 Binary Files – contain images, sounds, compressed ASCII
text, etc
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Images
 There are several file formats available for storing graphics and
images
 Web browsers support just a few image format
 The most common type of formats are:
 GIF (Graphic Interchange Format) – mostly used for graphics (like clip
arts logos, icons, etc) requiring fewer colors because it displays only 256
colors
 JPEG/JPG (Joint Photographic Expert Group) – mostly used for
photographs and images covering a wide spectrum of color because it
uses the full 16.7 million colors
 PNG (Portable Network Graphic) – offer millions of colors, lossless
compression, and other features that make them a good alternative to
GIF images in most cases
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Audio
 Embedded in Web applications to play automatically,
or used as links to be played when a user clicks on a
link to the sound file
 Some of the common sound files include:
 WAV – originated on Windows-based machines
 AIFF – originated on Macintosh
 AU – originated on Sun Microsystems workstations (UNIX)
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Multipurpose Internet Mail Extensions
(MIME) Types
 Set of rules that allow multimedia documents to be
exchanged among many different computer systems
 Originally designed for sending email attachments
and incorporated into http
 Both the Web server and the Web browser must have
a MIME types database
 Mime type database – used to identify the file extension
and determine how the file must be sent
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The Hypertext Transfer Protocol (HTTP)
 Provides a standard way of communication and information
exchange between the Web browsers and Web servers
 Expects the client to initiate a request and the server to
respond
 Each request and response has three parts:
 Request/Response line – contains a request method, the document
location, and the protocol version
 Header section – series of lines with http headers that are used to pass
other information about the request and the client to the server
 Entity body – contains other data to be passed to the server
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HTTP Request/Response Example
 Client Request
 Server Response
GET /hello.html HTTP/1.1
HTTP/1.1 200 OK
Host: test.com
User-Agent: Morzilla/4.5
Date: Wed, 06 Sept 2006 13:56:05 GMT
Last-Modified: Wed, 06 Sept 2006 13:50:23 GMT
Server: Apache/2.0.52
Connection: close
Content-Length: 25
Content-Type: text/html
<html>
Hello world!
</html>
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HTTP Request/Response Example…
 Client Request
 Server Response
 The hello.html file was
 The request was successful
requested from the server
test.com using HTTP version
1.1
 the Web browser used was
Mozilla version 4.5
(status code 200) and returned
25 bytes of text
 The server indicates the date
and time it thinks the resource
was requested and last
modified and the software it is
running (Apache version
2.0.52)
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HTTP Request Methods
 The HTTP command of the Web browser’s request
line
 The Web server uses the request method to
determine what to do with the browser’s request
 There are several methods defined by the HTTP 1.1
standard but only a few are widely supported by the
HTTP servers
 The most widely used methods are:
 GET – for retrieving information from a Web server



Can also be used to pass information to the Web server as part of a URL
HEAD – for verifying the existence of information on a Web server
POST – for sending (updating) information to a Web server
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HTTP Request Methods…
 Other http methods include:
 PUT
 DELETE
 TRACE
 TRACK
 OPTIONS
 CONNECT
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HTTP Server Responses
 Process client requests
 The server generates an error if the resource requested cannot
be located
 The response line of the server response has 3 parts:
 Protocol version – should always be HTTP with
 Status code – a three-digit indicator result code defined by the HTTP
specification


The first digit represents the category of the response
There are currently five categories:




Informational – the request was received and is being processed
Success – the client request was successful
Client error – the client’s request was incomplete or incorrect
Server error – the request was not fulfilled due to a server problem
 Description phrase
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Common Response Codes
 Informational 1xx
 100 Continue – the initial part of the request has been received and the
client should continue
 200 OK – the client’s request was successful and the server’s response
contains the resource requested
 204 No Content – The request was successful but the response is
empty
 Redirection 3xx
 301 Moved Permanently – the URL requested is no longer valid
 302 Found (Moved Temporarily) – the URL requested currently resides
in a different location
 304 Not Modified – the client performed a conditional GET and the
document has not been modified
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Common Response Codes…
 Client Error 4xx
 400 Bad Request – the server could not understand the
request
 403 Forbidden – the client requested data that it did not
have permission to access
 404 Not Found – the resource requested was not found on
the server
 Server Error 5xx
 500 Internal Server Error – something unexpected
happened on the server side
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HTTP Headers
 Used to transfer Information between the client and
server
 Has a name and value associated with it
 There is one header per line
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Common Request Headers
 Accept – specifies the client’s preferred media type
 Cookie – contains cookie information (name/value
pair) for the requested URL
 If-Modified-Since – used for a conditional GET
request (when a document has been modified)
 Referrer – allows the client to specify the URL of the
page from which the currently requested URL was
obtained
 User-Agent – identifies the Web browser software
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Common Response Headers
 Server – identifies the server software
 Set-Cookie – Allows the server to set a cookie on the
client browser (when permitted) for the given URL or
domain
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Common Entity Headers
 Content-Length – specifies the size (in bytes) of the data
transferred in the entity body
 Content Type – specifies the MIME Type of the data
returned in the entity body
 Expires – specifies the date/time after which the
response is considered outdated
 Last-Modified – specifies the date/time the document
was last modified
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Other Web-Related Servers
 Some of the most common servers that run along HTTP
servers are:
 Proxy servers
 Streaming Audio/Video
 File Transfer Protocol (FTP)
 Databases
 Secure Socket Layer (SSL)
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Proxy Servers
 An intermediary server between a client and destination
server
 Allows extra processing of returned data
 Have three main uses:
 Security – act as a firewall by rejecting other protocols
 Content filtering – restricts access to content based on file
type
 Caching – stores frequently accessed pages
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Streaming Audio and Video
 Allows a media player (or plug-in) to start playing multimedia
content while data is still being received
 Can broadcast live audio/video feeds or serve prerecorded
clips
 Web browsers employ a plug-in to view any type of streaming
content
 Unlike HTTP, many streaming media formats may use User
Data Protocol (UDP) instead of TCP/IP as a network protocol
 UDP is good for transmitting very small pieces of data quickly
 UDP will not retransmit data if there is an error
 UDP is faster than TCP/IP although TCP/IP is more reliable
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File Transfer Protocol (FTP)
 Used to transfer files between computers on a
network
 Relies on client and server software, unlike HTTP
 FTP daemon (FTPD) runs on the server to allow
clients to connect
 It provides a means of authentication for only authorized
users to transfer files
 UNIX servers generally install an FTPD by default
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Server-Side Programming
 Server-side programs provide dynamic content and allows
interaction with users using:
 Perl
 Active Server Pages (ASP)
 ASP.NET
 Java Server Pages (JSP) and Java Servlets
 PHP: Hypertext Preprocessor (PHP)
 Coldfusion
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Databases
 Database – a collection of data and metadata about related
entities
 A large corporate database should typically be installed on its own
dedicated server and not on a Web server machine
 Metadata – data about other data
 Database management system (DBMS) – software that is
used to manipulate a database
 Structured Query Language (SQL) – language built in relational DBMSs to
create and manipulate databases
 E.g. of relational DBMSs include Oracle, MySQL, Microsoft SQL Server,
Microsoft Access etc
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Secure Socket Layer (SSL)
 A protocol that allows secure, encrypted
communication over TCP/IP
 Often used with HTTP to allow information exchange
between a Web browser and a Web server
 Most commercial Web server software includes an
SSL server that can run alongside the HTTP daemon
(HTTPD)
 An Internet socket is defined as a combination of an
IP address, a port, and a protocol
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