Chapter 13Chapter 13

How do Web Applications How do Web Applications Work?Work?

Typical Web ApplicationsTypical Web Applications

Web BrowserWeb Browser E-mailE-mail

Web BrowsingWeb Browsing

Web SearchingWeb Searching Pop Up WindowsPop Up Windows

How does a search engine How does a search engine work?work?

It doesn’t search the Web.It doesn’t search the Web. A search engine contains a database A search engine contains a database

with information on lots of Web with information on lots of Web pages.pages.

When you do a search, it looks When you do a search, it looks through it’s database to find pages through it’s database to find pages which might be useful and returns a which might be useful and returns a list of themlist of them

Details for Search EnginesDetails for Search Engines

You submit a query.You submit a query. The search engine looks through its The search engine looks through its

database.database. The search engine orders the likely The search engine orders the likely

pages by relevance.pages by relevance. The search engine returns the list of The search engine returns the list of

pages.pages.

Web Page InformationWeb Page Information

URLURL TitleTitle KeywordsKeywords DescriptionDescription

Search Engine DatabaseSearch Engine Database

Search Engines typically use programs Search Engines typically use programs called spiders which crawl the Web.called spiders which crawl the Web.

These spiders examine the These spiders examine the Information on Web pages that they Information on Web pages that they find and save this information to the find and save this information to the database for the Search Engine.database for the Search Engine.

The spiders work 24/7/365 and they The spiders work 24/7/365 and they revisit pages to see if they have revisit pages to see if they have changed.changed.

Database (continued)Database (continued)

So the database at a typical search So the database at a typical search engine contains information on engine contains information on millions of pages that they can millions of pages that they can search when you do a query.search when you do a query.

The search engine companies have The search engine companies have algorithms to determine how algorithms to determine how relevant a page is to your query.relevant a page is to your query.

RelevanceRelevance

Different search engines use different Different search engines use different ways of determining relevance.ways of determining relevance.

For example, suppose you did a search on For example, suppose you did a search on “cat food”.“cat food”. The search engine would look for pages whose The search engine would look for pages whose

titles or descriptions or keywords were had cat titles or descriptions or keywords were had cat food or cat or food and arrange them in some food or cat or food and arrange them in some reasonable order.reasonable order.

Probably they would list the pages with cat and Probably they would list the pages with cat and food ahead of the pages with car or food.food ahead of the pages with car or food.

Relevance ContinuedRelevance Continued

Cat food example (cont)Cat food example (cont) Some search engines Some search engines mightmight determine the determine the

importance of a particular site based on how importance of a particular site based on how many OTHER sites have a link to it.many OTHER sites have a link to it.

Some search engines might determine the Some search engines might determine the importance of a particular site based on how importance of a particular site based on how often other users who typed the query “cat often other users who typed the query “cat food” chose a particular site.food” chose a particular site.

Some search engines might determine the Some search engines might determine the importance of a particular site based on money importance of a particular site based on money paid to the search engine by the web site.paid to the search engine by the web site.

Why don’t Search Engines just Why don’t Search Engines just search the Web?search the Web?

SPEED.SPEED. A typical search on Google, for A typical search on Google, for

example, takes a few secondsexample, takes a few seconds If they searched the Web it would If they searched the Web it would

probably take 5 or 10 seconds EACH probably take 5 or 10 seconds EACH for the Web pages examined. Thus a for the Web pages examined. Thus a search for “cat food” would take search for “cat food” would take several hours rather than 2 seconds.several hours rather than 2 seconds.

How can Google search billions How can Google search billions of pages in its database in only of pages in its database in only

two seconds?two seconds? The pages are indexed.The pages are indexed. So instead of having to look at each So instead of having to look at each

of the pages, the search engine only of the pages, the search engine only has to look through the index to find has to look through the index to find a page, much like you’d use the a page, much like you’d use the index or the Table of Contents to index or the Table of Contents to search a booksearch a book

Why do porn sites show up a Why do porn sites show up a lot?lot?

Pornography is big business so pornographers want Pornography is big business so pornographers want their sites to have lots of business.their sites to have lots of business.

(Lots of people search for porn at work or at school)(Lots of people search for porn at work or at school) But you will often get porn sites even when you search But you will often get porn sites even when you search

for something else.for something else. Porn sites can manipulate information about web Porn sites can manipulate information about web

pages.pages. The official web site of the white house is The official web site of the white house is www.whitehouse.govwww.whitehouse.gov.. www.whitehouse.comwww.whitehouse.com used to be a porn site. , and may still be. used to be a porn site. , and may still be. One porn site added key words “One porn site added key words “windows, windows 95, windows, windows 95,

windows 98 …” windows 98 …” and several others to it’s keyword metatag and several others to it’s keyword metatag and to its title page.and to its title page.

How do Web sites increase How do Web sites increase their visibility?their visibility?

Use metatags to make their sites more Use metatags to make their sites more visible to search engines.visible to search engines.

Put relevant words in the page titlePut relevant words in the page title Put relevant words at the beginning of the Put relevant words at the beginning of the

text in the page.text in the page. Put relevant words in several times.Put relevant words in several times. Use relevant words as the name of web Use relevant words as the name of web

pages i.e. “cat-food.html”pages i.e. “cat-food.html” Error 404 tricksError 404 tricks

Pop-up’sPop-up’s

Two kinds of pop-ups:Two kinds of pop-ups: One that comes up when you visit a Web One that comes up when you visit a Web

site.site. One that comes up from another cause One that comes up from another cause

(which we will not discuss in this chapter)(which we will not discuss in this chapter) Web Pages consist of HTML tags which Web Pages consist of HTML tags which

describe how the information on the describe how the information on the page looks and the information itself.page looks and the information itself.

None of this can cause a pop-up.None of this can cause a pop-up.

Pop-Ups ContinuedPop-Ups Continued

Pop-ups are generated by scripts Pop-ups are generated by scripts which are part of Web Pages.which are part of Web Pages.

If you load my Web page and look at If you load my Web page and look at the source you will see a <script> tag.the source you will see a <script> tag.

Script tags come in several types, the Script tags come in several types, the most common of which is JavaScript.most common of which is JavaScript.

JavaScript can be used to make Web JavaScript can be used to make Web pages dynamicpages dynamic

Controlling Pop-UpsControlling Pop-Ups

Turn JavaScript off. Unfortunately Turn JavaScript off. Unfortunately this will keep many Web sites from this will keep many Web sites from operating properly.operating properly.

Pop-up blockers built into Web Pop-up blockers built into Web browsers can also be used but they browsers can also be used but they also tend to have problems.also tend to have problems.

E-MailE-Mail

MIME (multipurpose internet mail MIME (multipurpose internet mail extensions) is a standard that is used to extensions) is a standard that is used to send attachments to e-mail messages.send attachments to e-mail messages.

MIME determines how certain files are MIME determines how certain files are interpreted.interpreted.

In general, today it’s probably better not In general, today it’s probably better not to take advantage of MIME’s capabilities to take advantage of MIME’s capabilities since these techniques can be used to since these techniques can be used to send viruses.send viruses.

What does an E-mail Message What does an E-mail Message Contain?Contain?

The message itself.The message itself. Header InformationHeader Information AttachmentsAttachments

AttachmentsAttachments

Could be anything including sounds, Could be anything including sounds, pictures, other multimedia, pictures, other multimedia, programs, viruses, etc.programs, viruses, etc.

HeaderHeader

Original To: / Deliver to:Original To: / Deliver to: From: / Reply to:From: / Reply to: Subject:Subject: Return path:Return path: Message ID:Message ID: Other stuffOther stuff

SpammingSpamming

Where do the Spammers get Where do the Spammers get addresses:addresses: Web sitesWeb sites NewsgroupsNewsgroups From youFrom you Purchase listsPurchase lists Random addressesRandom addresses

How do you control Spam?How do you control Spam?

Don’t give out your e-mail address.Don’t give out your e-mail address. Keep several addresses including Keep several addresses including

several that you don’t use.several that you don’t use. FirewallsFirewalls Spam filtersSpam filters

SpamSpam

Legal system is largely ineffective Legal system is largely ineffective because:because: Spam may originate from outside the Spam may originate from outside the

countrycountry Spam providers can be forgedSpam providers can be forged Laws must be technology based.Laws must be technology based.

TerminologyTerminology

EventEvent Event-driven Event-driven

programmingprogramming IndexingIndexing Infinite LoopInfinite Loop

MIME (Multipurpose MIME (Multipurpose Internet Mail Internet Mail Extension)Extension)

SpamSpam Web CrawlerWeb Crawler