Application State

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Application State

• Cookies, Session Management, and Top N Queries

October 3, 2001
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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!!)

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Final Design Documents

• Due October 10
• Information online under class notes about what
  to include
• Try to be complete, but concise (you dont need
  100 pages ?).

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In the beginning
there was
5
Next came
But what can we do with it?
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The Internet
We can offer services! Sell, buy and trade
things! Post information! Chat! Surf!
You are wise
Do not forget CS433
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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

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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

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Stateless Protocol

• Advantages
• Easy to use dont 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

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Application State

• Server-side state
• Information is stored in a database, or in the
  application layers local memory
• Client-side state
• Information is stored on the clients computer in
  the form of a cookie
• Hidden state
• Information is hidden within dynamically created
  web pages

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Application State
So many kinds of state how will I choose?
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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 information
• 2. Use application layers local memory
• Can map the users IP address to some state
• BUT this information is volatile and takes up
  lots of server main memory

5 million IPs 20 MB
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Server-Side State

• Should use Server-side state maintenance for
  information that needs to persist
• Old customer orders
• Click trails of a users movement through a
  site
• Permanent choices a user makes

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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/state
ful.html
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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 users login information
• The current shopping basket
• Any non-permanent choices the user has made

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Creating A Cookie

• Cookie myCookie
• new Cookie(username", jeffd")
• response.addCookie(userCookie)
• You can create a cookie at any time

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Accessing A Cookie

• Cookie cookies request.getCookies()
• String theUser
• for(int i0 iltcookies.length i)
• Cookie cookie cookiesi
• 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()

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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

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Hidden State

• Often users will disable cookies
• You 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

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Hidden State Hidden Fields

• Declare hidden fields within a form
• ltinput typehidden nameuser valuejeffd/gt
• Users will not see this information (unless they
  view the HTML source)
• If used prolifically, its a killer for
  performance since EVERY page must be contained
  within a form.

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Hidden State Path Information

• Path information is stored in the URL request
• http//server.com/index.htm?userjeffd
• Can separate fields with an character
• index.htm?userjeffdpreferencepepsi
• There are mechanisms to parse this field in Java.
  Check out the javax.servlet.http.HttpUtils
  parserQueryString() method.

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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)

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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.

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Top N Queries
Select all the odd things Jeff wants in batches
of 2 items
Batch 1
Batch 2
Batch 3
Batch 4
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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

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Placeholder Technique

• For each page you then know
• Showing Records 3-4
• Previous Link /display.jsp?lower3
• Next Link /display.jsp?upper4
• If the user clicks on the Next Link do
• Iterate to upper value 1 (here that is 5)
• Display the next 2 results
• Prepare new lower/upper values (5,6)

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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)

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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 gt
  lowest_value)
• OR (P.Product gt item)
• Order By P.Product, P.Id

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Query Constraint Technique

• SELECT P.Product
• FROM ProductTable P
• WHERE (P.Product item AND P.Id gt
  lowest_value)
• OR (P.Product gt 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.

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Query Constraint Technique
ID
Product
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Baseball Hat
B1
27
Basketball Tickets
37
Basketball Tickets
B2
14
Laptop Computer
To move from batch B1 to batch B2 you would issue
SELECT P.Product FROM ProductTable P WHERE
(P.Product Basketball Tickets AND P.ID gt
27) OR (P.Product gt Basketball
Tickets) ORDER BY P.Product, P.ID
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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

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Query Constraint Technique

• Advantages
• No need to iterate through results so you get a
  large speedup
• Disadvantages
• More difficult to implement

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Any questions?