This document provides an overview of building dynamic web applications. It discusses clients and servers, with web browsers serving as clients that make requests to web servers. It also covers how to write a server using ports, how the request-response cycle works for websites, and why web frameworks are useful for building web applications. Key aspects of web frameworks covered include routing URLs, templating, databases, sessions and cookies, and security considerations.

1. Building Dynamic Web Applications
Howie Mao
May 2, 2013

2. Clients and Servers
The web runs on a client-server model
A server is a program that listens on a network for requests
and services the requests. A webserver is a server that sends
webpages as its service.
A client is a program that requests services from the server.
Your web browser is an example of a client.

3. How to write a server?
Server calls attaches itself to a port and starts listening for
connections
Each computer has many ports (literally numbers from 1 to
65535) allowing internet traffic to be delivered to the correct
server even if there are multiple servers on the same computer
A client connects to a server by specifying its address and port

4. What happens when I go to a website
Web browser (client) connects to webserver (usually on port
80)
Web browser sends an HTTP request, asking for a certain
resource
Server sends an HTTP response, consisting of a header, which
contains metadata about the resource being sent, followed by
the data for the resource itself (a web page) in the body.

5. Web Frameworks
For making web applications, you probably want more than
just a socket library
You use a web framework, which provides you with the utilities
to deal with HTTP requests and other parts of a website
In this talk, we will use Flask, a very simple framework for
Python

6. Routing URLs
The key function of any web framework
Takes a URL and decides what code to run
Can also use parts of the URL as data

7. Templating
Makes it easier to generate HTML (or anything else) using
changing data.
Key concepts are inheritance and substitution.

8. Databases
Allows for efficient storage and retrieval of user data.
Many different kinds of databases.
Relational, document-oriented, key-value, graph-based, etc.
Choice of database depends on type of data stored

9. Sessions and Cookies
Allows users to stay "logged in".
Server sends browser a cookie, a piece of data which the
browser stores locally.
When the site is revisited, the cookie is sent back to the server.
This tells server the user is logged in already.
Cookies can be given an expiration time, or can be told to
expire once browser is closed.

10. Security
Encrypt cookies used to store login info
Login credentials and session cookies should be sent in
HTTPS (encrypted HTTP).
Passwords should be hashed before storing in database.
Always escape user input before storing in a database or
putting in a template.