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Cookies & Web-Caches (Proxy Servers)

COMP211 Lectures

Cookies

Cookies are used to maintain user state. Many websites use four components to achieve this:

  1. Cookie header line of HTTP response message.
  2. Cookie header line in next HTTP request message.
  3. Cookie file kept on user’s host, managed by user’s browser.
  4. Back-end database at website.

A cookie is just a unique ID number stored in a file.

sequenceDiagram
client ->> server: Usual HTTP request msg
server ->> database: Create ID entry
server ->> client: Usual HTTP response & "set-cookie:1678"
client ->> server: Usual HTTP request msg & "cookie:1678"
server -> database: Access
Note over server, database: Cookie specific action
server ->> client: Usual HTTP response msg
note left of client: One week later
client ->> server: Usual HTTP request msg & "cookie:1678"
server -> database: Access
Note over server, database: Cookie specific action
server ->> client: Usual HTTP response msg

HTTP Cookies

Cookies can be used for:

  • Authorisation
  • Shopping Carts
  • Recommendations
  • User Session State (Web E-Mail)

Cookies permit sites to learn a lot about you. Third party cookies allow a common identity to be tracked across multiple sites.

Web Caches (Proxy Servers)

Proxy servers allow a client request to be satisfied without involving the origin server.

  • If the object is in the cache - The cache returns the object to the client.
  • Else - The cache requests the object from the origin server, caches the object and sends the object to the client.

Web caching allows the following:

  • Less response time for the client.
  • Reducing traffic on an institutions access link.
  • Enables poor content providers to operate more effectively.

Conditional GET

This allows servers to check if their cached object is up to date. This allows:

  • No object transmission delay.
  • Lower link utilisation.

You can specify the date of a cached copy in the request:

If-modified-since: <date>

If the copy is up to date the server will not return the object but the following response:

HTTP/1.0 304 Not Modified

Here is the sequence of interactions:

sequenceDiagram
client ->> server: Request & "If-modified-since:<date>"
alt Object not modified
	server ->> client: "HTTP/1.0 304 Not Modified"
else Object modified
	server ->> client: "HTTP/1.0 200 OK <data>"
end