Tracking Web Visitors Aaron Stevens Computer Science What Youll - PDF document

4/17/13 CS108: Lecture 26 Tracking Web Visitors Aaron Stevens Computer Science What You’ll Learn Today Computer Science  How do you know who’s using your Application?  Using hidden fields to keep session state  HTTP Environment Variables  Cookie Cookie Cookie! 1

4/17/13 Session State Computer Science HTTP is a stateless protocol.  Each request/response pair is a distinct conversation -- no “ memory ” How to keep track of a transaction which spans multiple pages?  confirm order and check out  e.g. shopping cart Some well-known ideas:  Use hidden form fields  HTTP provides out-of-band info about the environment  Cookies Hidden HTML Form Fields Computer Science Consider this web application: HTML code which creates the buttons: 2

4/17/13 Hidden HTML Form Fields Computer Science Here’s the Python code to generate the HTML Form: HTTP Environment Variables Computer Science Show: snoop.py Discuss environment variables 3

4/17/13 Cookie Cookie Cookie! Computer Science Session State: Cookies Computer Science A cookie is like a special bookmark shared between client and server in a “ session. ”  Server sets a cookie in HTTP response headers  Client will always include this cookie with requests  Cookie can be any key-value pairs as text.  Server typically uses the cookie to identify a client ’ s data in a database. http://tools.ietf.org/html/rfc2109.html 4

4/17/13 Session State: Cookies Computer Science How Cookies Work Computer Science Web Browser Web App (client) (server) GET / No Cookie? Send login form RESP: login form GET + login form Got login form? Send cookie + RESP: cookie + welcome welcome message GET + Cookie Yes Cookie? Send cookie + RESP: cookie + data regular response 5

4/17/13 Setting a Cookie Computer Science Upon authentication, the server-side app can set a cookie to the client ’ s browser: Retrieving a Cookie Computer Science Browser sends the cookies to the server in the HTTP request. Web app reads from os.environ : 6

4/17/13 Cookies get saved in the browser Computer Science First Things First: Look for Cookies Computer Science Cookies are sent by browser to server, received with HTTP request, stored in environment variables. 7

4/17/13 No Cookie? Ask User to Ident Computer Science Setting the Cookie Computer Science 8

4/17/13 Computer Science What You Learned Today Computer Science  Using hidden fields to keep session state  HTTP Environment Variables  Cookie Cookie Cookie! 9

4/17/13 Announcements and To Do Computer Science  Readings for this week:  HTML Tutorial: http://www.w3schools.com/HTML/  Cookies: http://webpython.codepoint.net/cgi_cookie  QUIZ 6 on FRI 4/19  Web applications, database integration  Session state, environment variables, and cookies  Lab Exam is THU 4/25  Covers material from Zelle book only (first half of semester)  Final project due THU 5/2 at 12pm.  5 points for early presentation on TUE 4/30 at 10am.  Use http://aaronstevens.youcanbook.me to make an appointment. 10