Application State

Cookies, Session Management, and Top N Queries

October 3, 2001

Administration

Preliminary Design Documents We were very impressed! If we found glaring errors, you would have received an

email from us. Egret

If you believe you cannot log into egret, reread the directions carefully, and if you still have trouble, email TAs with your group number.

Common problems: omitting “cucs/” in your user name, not using “map network drive”, and not using the correct username/password we gave you (it is NOT your CSUGLab netid/password!!)

Final Design Documents

Due October 10Information online under “class notes”

about what to includeTry to be complete, but concise (you don’t

need 100 pages ).

In the beginning…

…there was…

…nothing.

TheInternet

Next came…

But what can we do

with it?

The Internet…

Internet

We can offer services! Sell, buy and trade things! Post information! Chat! Surf! You are

wise…Do not forget CS433

The Internet…

The onset of the Web has brought about new client-server issues Reliability – servers must be online 24-7 Distributed networking – servers access many

systems Scalability – servers must be able to handle

millions of requests Stateless and State Maintained Interaction

Stateless and State Maintenance

Stateless No information is retained from one request to

another Pure web servers are stateless

State Some memory is stored between requests This memory can be saved on either the client

or the server

Stateless Protocol

Advantages Easy to use: don’t need anything Great for static-information applications Requires no extra memory space

Disadvantages No record of previous requests means

No shopping baskets No user logins No custom or dynamic content Security is more difficult to implement

Application State

Server-side state Information is stored in a database, or in the

application layer’s local memory

Client-side state Information is stored on the client’s computer in

the form of a cookie

Hidden state Information is hidden within dynamically created

web pages

Application State

So many kinds of state…

…how will I choose?

Server-Side State

Many types of Server side state:1. Store information in a database

Data will be safe in the database BUT: requires a database access to query or

update the information2. Use application layer’s local memory

Can map the user’s IP address to some state BUT: this information is volatile and takes up

lots of server main memory

5 million IPs = 20 MB

Server-Side State

Should use Server-side state maintenance for information that needs to persist Old customer orders “Click trails” of a user’s movement through a

site Permanent choices a user makes

Client-side State: Cookies

Storing text on the client which will be passed to the application with every HTTP request. Can be disabled by the client. Are wrongfully perceived as "dangerous", and

therefore will scare away potential site visitors if asked to enable cookies1

Are a collection of (Name, Value) pairs

1http://www.webdevelopersjournal.com/columns/stateful.html

Client State: Cookies

Advantages Easy to use in Java Servlets / JSP Provide a simple way to persist non-essential data on the

client even when the browser has closed Disadvantages

Limit of 4 kilobytes of information Users can (and often will) disable them

Should use cookies to store interactive state The current user’s login information The current shopping basket Any non-permanent choices the user has made

Creating A Cookie

Cookie myCookie = new Cookie(“username", “jeffd");response.addCookie(userCookie);

You can create a cookie at any time

Accessing A Cookie

Cookie[] cookies = request.getCookies(); String theUser;for(int i=0; i<cookies.length; i++) { Cookie cookie = cookies[i]; if(cookie.getName().equals(“username”))

theUser = cookie.getValue();} // at this point theUser == “jeffd”

Cookies need to be accessed BEFORE you set your response header:response.setContentType("text/html");PrintWriter out = response.getWriter();

Cookie Features

Cookies can have A duration (expire right away or persist even

after the browser has closed) Filters for which domains/directory paths the

cookie is sent to

See the Java Servlet API and Servlet Tutorials for more information

Hidden State

Often users will disable cookiesYou can “hide” data in two places:

Hidden fields within a form Using the path information

Requires no “storage” of information because the state information is passed inside of each web page

Hidden State: Hidden Fields

Declare hidden fields within a form: <input type=‘hidden’ name=‘user’ value=‘jeffd’/>

Users will not see this information (unless they view the HTML source)

If used prolifically, it’s a killer for performance since EVERY page must be contained within a form.

Hidden State: Path Information

Path information is stored in the URL request:http://server.com/index.htm?user=jeffd

Can separate ‘fields’ with an & character:index.htm?user=jeffd&preference=pepsi

There are mechanisms to parse this field in Java. Check out the javax.servlet.http.HttpUtils parserQueryString() method.

Multiple state methods

Typically all methods of state maintenance are used: User logs in and this information is stored in a cookie User issues a query which is stored in the path

information User places an item in a shopping basket cookie User purchases items and credit-card information is

stored/retrieved from a database User leaves a click-stream which is kept in a log on the

web server (which can later be analyzed)

Case Study: Top N Queries

A Top N Query requests the first n records in some table. Used when there are many search results Want to break the search result into managable

batches, say to display 20 search result records at a time (display NEXT, PREVIOUS 20)

You need to know the starting/ending point in your table.

Top N Queries

ID Product

13 Baseball Hat

27 Duke Basketball Tickets

7 DVD Player

14 Laptop Computer

2 Pocket PC

25 Sunglasses

15 Vitamins

Batch 1

Batch 2

Batch 3

Batch 4

Select all the odd things Jeff wants in batches of 2 items…

Placeholder Technique

Store the first and last result tuples and perform the same select query

Iterate through the results to return the next or previous batch of results

Tuples Shown

Lower Place

Prev Set Upper Place

Next Set

1-2 1 None 2 3-4

3-4 3 1-2 4 5-6

5-6 5 3-4 6 7

7 7 5-6 7 None

Placeholder Technique

For each page you then know: Showing Records: 3-4 Previous Link: /display.jsp?lower=3 Next Link: /display.jsp?upper=4

If the user clicks on the Next Link do:1. Iterate to: upper value + 1 (here that is 5)

2. Display the next 2 results

3. Prepare new lower/upper values (5,6)

Placeholder Technique

You perform the SAME select query each time you need a new batch of results

Advantages: Easy implementation

Disadvantages: You must fetch many unneeded records (i.e.

the ones you iterate through)

Query Constraint Technique

Instead of issuing the SAME query, push the constraint into the query so the result only contains records that have not yet been displayed

You can use sorting to do this effectively:Select P.Product

From ProductTable P

Where (P.Product = ‘item’ AND P.Id > lowest_value)

OR (P.Product > ‘item’)

Order By P.Product, P.Id

Query Constraint Technique

SELECT P.ProductFROM ProductTable PWHERE (P.Product = ‘item’ AND P.Id > lowest_value)

OR (P.Product > ‘item’)ORDER BY P.Product, P.Id

What it means:The first part of the WHERE clause is for when there

are many products with the same name, and we only want to see the new ones.

The second part of the WHERE clause is for items that are “larger” alphabetically.

Query Constraint Technique

ID Product

13 Baseball Hat

27 Basketball Tickets

37 Basketball Tickets

14 Laptop Computer

B1

B2

SELECT P.ProductFROM ProductTable PWHERE (P.Product = ‘Basketball Tickets’ AND

P.ID > 27) OR (P.Product > ‘Basketball Tickets’)

ORDER BY P.Product, P.ID

To move from batch B1 to batch B2 you would issue:

Query Constraint Technique

What information needs to be stored? Previous:

Title of FIRST record in the previous set Primary_Key of FIRST record in the previous set

Next: Title of FIRST record in the next set Primary_Key of FIRST record in next set

This information can be encoded in the path information

Query Constraint Technique

Advantages No need to iterate through results so you get a

large speedup

Disadvantages More difficult to implement

Any questions?