1. How do Web Applications Work?
Chapter 13
How do Web Applications Work?

2. Typical Web Applications
Web Browser

3. Web Browsing
Web Searching
Pop Up Windows

4. How does a search engine work?
It doesn’t search the Web.
A search engine contains a database with information on lots of Web pages.
When you do a search, it looks through it’s database to find pages which might be useful and returns a list of them

5. Details for Search Engines
You submit a query.
The search engine looks through its database.
The search engine orders the likely pages by relevance.
The search engine returns the list of pages.

6. Web Page Information
URL
Title
Keywords
Description

7. Search Engine Database
Search Engines typically use programs called spiders which crawl the Web.
These spiders examine the Information on Web pages that they find and save this information to the database for the Search Engine.
The spiders work 24/7/365 and they revisit pages to see if they have changed.

8. Database (continued)
So the database at a typical search engine contains information on millions of pages that they can search when you do a query.
The search engine companies have algorithms to determine how relevant a page is to your query.

9. Relevance
Different search engines use different ways of determining relevance.
For example, suppose you did a search on “cat food”.
The search engine would look for pages whose titles or descriptions or keywords were had cat food or cat or food and arrange them in some reasonable order.
Probably they would list the pages with cat and food ahead of the pages with car or food.

10. Relevance Continued Cat food example (cont)
Some search engines might determine the importance of a particular site based on how many OTHER sites have a link to it.
Some search engines might determine the importance of a particular site based on how often other users who typed the query “cat food” chose a particular site.
Some search engines might determine the importance of a particular site based on money paid to the search engine by the web site.

11. Why don’t Search Engines just search the Web?
SPEED.
A typical search on Google, for example, takes a few seconds
If they searched the Web it would probably take 5 or 10 seconds EACH for the Web pages examined. Thus a search for “cat food” would take several hours rather than 2 seconds.

12. How can Google search billions of pages in its database in only two seconds?
The pages are indexed.
So instead of having to look at each of the pages, the search engine only has to look through the index to find a page, much like you’d use the index or the Table of Contents to search a book

13. Why do porn sites show up a lot?
Pornography is big business so pornographers want their sites to have lots of business.
(Lots of people search for porn at work or at school)
But you will often get porn sites even when you search for something else.
Porn sites can manipulate information about web pages.
The official web site of the white house is
used to be a porn site. , and may still be.
One porn site added key words “windows, windows 95, windows 98 …” and several others to it’s keyword metatag and to its title page.

14. How do Web sites increase their visibility?
Use metatags to make their sites more visible to search engines.
Put relevant words in the page title
Put relevant words at the beginning of the text in the page.
Put relevant words in several times.
Use relevant words as the name of web pages i.e. “cat-food.html”
Error 404 tricks

15. Pop-up’s Two kinds of pop-ups:
One that comes up when you visit a Web site.
One that comes up from another cause (which we will not discuss in this chapter)
Web Pages consist of HTML tags which describe how the information on the page looks and the information itself.
None of this can cause a pop-up.

16. Pop-Ups Continued
Pop-ups are generated by scripts which are part of Web Pages.
If you load my Web page and look at the source you will see a <script> tag.
Script tags come in several types, the most common of which is JavaScript.
JavaScript can be used to make Web pages dynamic

17. Controlling Pop-Ups
Turn JavaScript off. Unfortunately this will keep many Web sites from operating properly.
Pop-up blockers built into Web browsers can also be used but they also tend to have problems.

18.
MIME (multipurpose internet mail extensions) is a standard that is used to send attachments to messages.
MIME determines how certain files are interpreted.
In general, today it’s probably better not to take advantage of MIME’s capabilities since these techniques can be used to send viruses.

19. What does an E-mail Message Contain?
The message itself.
Header Information
Attachments

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

21. Header Original To: / Deliver to: From: / Reply to: Subject:
Return path:
Message ID:
Other stuff

22. Spamming Where do the Spammers get addresses: Web sites Newsgroups
From you
Purchase lists
Random addresses

23. How do you control Spam? Don’t give out your e-mail address.
Keep several addresses including several that you don’t use.
Firewalls
Spam filters

24. Spam Legal system is largely ineffective because:
Spam may originate from outside the country
Spam providers can be forged
Laws must be technology based.

25. Terminology Event Event-driven programming Indexing Infinite Loop
MIME (Multipurpose Internet Mail Extension)
Spam
Web Crawler