Web application architecture

Web application architectureAnd protocols of the www

SE-2840 Dr. Mark L. Hornick

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Q: What does a Browser do?

Web Browsers


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The www message model consists of Requests and Responses

Web Browser(the client) Web Server

Client Request: “I need a resource (html page, picture, pdf doc, mp3 file…)”

Server Response: “Here you go!”

Resource access

Q: How does a user initiate a request?

How a browser initiates a request: anatomy of a url

The most commonly used form:www.msoe.edu

Represents the DNS name of the server which listens for HTTP requests on port 80

The complete url is really: http://www.msoe.edu:80/index.html, where the http:// prefix explicitly instructs the browser to communicate using the http protocol, and the :80 suffix instructs the browser which port to use, and /index.html is the path of the resource to be retrieved. When the protocol and port are omitted, the browser assumes

http and 80 index.html is one of several default files the server looks for

when none is specified by the browser (another is welcome.html) 4

Other protocols and ports can also be specified

https://sapphire.msoe.edu:8443/OnTrack/login.jsp https:// specifies that a secure version of HTTP

(using SSL) should be used (recall from CS2910) :8443 is a commonly-selected port that servers

use for https. /OnTrack/login.jsp is the path to a web page being

requested by the browser

CS-4220 Dr. Mark L. Hornick 5

Browsers can use protocols other than http

ftp://myfiles.msoe.edu/public/pictures/dog.jpg ftp:// specifies that the browser should use the ftp

protocol to retrieve the specified resource The default port for ftp is used unless specified

explicitly

file:///D:/MyDocs/SampleHTML/BasicHTML.htm file:// specifies that the browser should retrieve

the specified resource directly from the filesystem The filepath syntax is specific to the operating system

running the browser (Windows in this case)CS-4220 Dr. Mark L. Hornick 6

A three-tier model (Client, Server, Database) is a common web application architecture


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Databases are often on a separate server,but not always.

ClientWeb Server Database Server

Q: What’s in the lightning bolts ??


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How are messages sent back and forth?

Generic Client

Generic Server

Request: Internet message from Client to Server

Response: Internet message from Server to Client

router

router

router

router router


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Packaging a message for delivery

Generic ClientGeneric Server

Internet message from Client to Server

router

router

router

router router

Ethernet Frame Header

IP Header(address of

Server)

TCP Header(Part 1 of N)

Application-specific data

of the message

Ethernet Frame Footer

Application-specific data of the message

Internet message packet

Peeling the onion of an internet message


Ethernet Frame Header

IP Header(address of

Server)

TCP Header(Part 1 of N)

Application-specific data

of the message

Ethernet Frame Footer

The sequence for retrieving a static web page (no db needed)


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Note: This diagram canbe found in your textbook

Browser formats therequest and sends itto the server

HTML (and other)files

…or types aurl into theaddress bar

Server formats theresponse and sends itto the client (browser)


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HTTP: The message protocol of the www

Web BrowserWeb Server

HTTP request

HTTP response

Simplepagefetch…

Q: Does a response always carry a payload?


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The HTTP GET Request is the most commonly issued message

GET requests the retrieval of a resource specified via a URL

Example: http://faculty-web.msoe.edu//hornick/index.html

URLs you type into the address bar of a web browser are always formatted into GET Requests:

GET /hornick/index.html HTTP/1.1Host: faculty-web.msoe.eduAccept: */*\r\n

http://faculty-web.msoe.edu/hornick/index.html



Request / Response illustrated


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Note: This diagram canbe found in your textbook


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Of all other HTTP requests, POST is the only one commonly used POST

Submits data to be processed (commonly from a HTML form) to the identified resource. The data is included in the body of the request. The data thus provided typically changes the state of the web application. More on this later.

PUT Uploads a representation of the specified resource.

DELETE Deletes the specified resource.

TRACE Echoes back the received request, so that a client can see what intermediate servers

are adding or changing in the request. OPTIONS

Returns the HTTP methods that the server supports. This can be used to check the functionality of a web server.

CONNECT For use with a proxy that can change to being an SSL tunnel.

HEAD Asks for the response identical to the one that would correspond to a GET request, but

without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content. .

Wfetch is free HTTP protocol viewer

Live link on course webpage under Tools.

http://download.microsoft.com/download/iis50/Utility/5.0/W9XNT4/EN-US/wfetch.exe


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Use Wfetch forhttp://www.msoe.edu/welcome.html

Documents

Web application architecture