CS101 Lecture 28: Dynamic Web Pages Aaron Stevens 6 April 2009 1 Overview/Questions – Review: how the WWW works – What does HTTP actually do? – How do web pages change to provide dynamic content? – A python web page example 2 1
Review: Displaying a WWW Page How do you “visit a website”? 3 Displaying a WWW Page – Browser decodes URL to parse out host name and document location on that host. – DNS used to obtain IP address of host. – Establish TCP/IP connection. – Client requests resource, and waits for the server to respond (using the hypertext transfer protocol). – Browser parses the response, requests any embedded data, and formats/displays output. 4 2
Hyper Text Transfer Protocol HTTP is a protocol which specifies request and responses between clients and servers. – Presumes a reliable transport, so TCP/IP is typically used (but not required). – The client (called a browser) connects to a web server, usually on port 80. HTTP is not limited to webpages. It can be used to transfer any kind of data. 5 HTTP Request Messages An HTTP request has two main parts: a method (action) and a URI (uniform resource identifier) upon which to perform the action. HTTP methods are OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, CONNECT. The URI must specify the path to a resource (e.g. a file or directory). 6 3
HTTP Request Example Suppose the user types the URL http://www.bu.edu. The browser will find the IP address for the host www.bu.edu, and create a TCP/IP connection to it on port 80. 7 HTTP Request Example The HTTP request message is like the following: GET / HTTP/1.1 Host: www.bu.edu:80 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en- US; rv:1.8.1.5) Gecko/20070713 Firefox/2.0.0.5 Accept: text/xml,application/xml,application/xhtml+xml,text/html;q =0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive 8 4
HTTP Response Messages An HTTP response has three main parts: a status, some headers, and the message body. HTTP status codes include OK, FORBIDDEN, NOT FOUND, INTERNAL SERVER ERROR, … For successful requests, the status of OK is followed by headers which explain how to decode the message body. 9 HTTP Response Example The HTTP response message is like the following: HTTP/1.1 200 OK Date: Wed, 01 Aug 2007 17:33:41 GMT Server: Apache/1.3.37 (Unix) mod_ssl/2.8.28 OpenSSL/0.9.7l Keep-Alive: timeout=5, max=100 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html 77b <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" ... 10 5
Content Type Notice the header field called Content-Type HTTP/1.1 200 OK Date: Wed, 01 Aug 2007 17:33:41 GMT Server: Apache/1.3.37 (Unix) mod_ssl/2.8.28 OpenSSL/0.9.7l Keep-Alive: timeout=5, max=100 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html ... This indicates the that the message can be interpreted as encoded in plain text and/or HTML. 11 Dynamic Web Pages The earliest web pages were static – fixed content which did not change unless edited by a human editor. A dynamic web page is generated by a computer program, based on some transaction between client and server. – Example: news feeds on facebook.com 12 6
Dynamic Web Pages There are several approaches to developing dynamic web pages. Java Servlet A server-side Java application which processes HTTP requests and generates HTTP responses. Java Server Pages A scripting language which mixes HTML tags, plain text, and Java code scriptlets. 13 Dynamic Web Pages Continued… PHP: Hypertext Preprocessor A scripting (programming) language which is embedded in HTML documents. Active Server Pages Microsoft’s server side scripting language, based on Visual Basic Script. Python Scripting A python script can read HTTP headers, and use print statements to generate HTML output. 14 7
LAMP Model The most common structure for web applications uses this configuration: Linux operating system for a server Apache web server software MySQL database software PHP/Perl/Python scripting language to create dynamic HTML 15 A Python Example 16 8
A Python Example -- Output 17 Webserver Stuff The wesberver we will use in class is an Apache server running on azs.bu.edu . The URL for all class work will be http://azs.bu.edu/cs101/current/<USERNAME>/<PROGRAM> for example: http://azs.bu.edu/cs101/current/azs/hello.py 18 9
Putting your files on the Web To get your files to display on the web, you need to do 2 things: 1. Put your files under your CS UNIX’s account’s ~/public_html/apps/ directory (or Z:/public_html/apps/ ) 2. Set the permissions of your files to be readable and executable by yourself and your group. Permissions: rwxrwx--- (770) Step 2 is being done automatically by a – script, updated every minute. 19 How to Transfer Files to csa2.bu.edu Use a client-program like: – Fetch (Mac) – WinSCP (Windows) http://www.cs.bu.edu/courses/cs101/faq.html has some information about WinSCP and Fetch. 20 10
WinSCP to csa2.bu.edu After you connect: – Change directory (double click) to public_html/apps – Transfer files by drag’n’drop. – Right-click and open the Properties menu: 21 Setting Permissions by WinSCP On the properties dialog, set the permissions to rwxrwx--- (code 770): 22 11
Transferring File by Fetch After you connect: – Change directory (double click) to public_html/apps – Transfer files by drag’n’drop. – Use GetInfo button to open the Properties menu: 23 Setting Permissions by Fetch Set permissions to rwxrwx--- (code 770): 24 12
What to do when… If you see this… Some possible causes: Python syntax error -- run “check module” in IDLE Incorrect “shebang” line, double check WinSCP file transfer encoding… see next slide. 25 WinSCP File Transfer Encoding Check that WinSCP is transferring your Python files as text: In the drop down list, add “ ; *.py ” 26 13
Other Parts of Web Applications Data driven content: use database to store content – Example: facebook.com’s news friends, news User input: HTML forms – Example: Google’s search box Maintaining a session: cookies – Example: Amazon.com shopping cart 27 Take-Away Points – HTTP request and response messages – HTTP headers – Overview of Web Applications: LAMP – Generating HTML in Python – Transferring Python scripts to the web server 28 14
Student To Dos – HW11 is due TUESDAY 4/7 – QUIZ 5 is on WEDNESDAY 4/8 Lectures 23-27 Python: functions, decisions, repetition; Data Structures. – Aaron’s Office Hours changed this week only: Monday: 4:15-5pm (regular time) Wednesday: 8:30-9:30 am (changed from 2-3) Friday: Cancelled – NO LECTURE on FRIDAY 4/10 29 15
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