Cookie in server and client

HTTP COOKIES

Cookies are key/value pairs used by websites to store state informations on the browser. Say you have a website (example.com), when the browser requests a webpage the website can send cookies to store informations on the browser.

Browser request example:

GET /index.html HTTP/1.1
Host: www.example.com

Example answer from the server:

HTTP/1.1 200 OK
Content-type: text/html
Set-Cookie: foo=10
Set-Cookie: bar=20; Expires=Fri, 30 Sep 2011 11:48:00 GMT
... rest  of the response

Here two cookies foo=10 and bar=20 are stored on the browser. The second one will expire on 30 September. In each subsequent request the browser will send the cookies back to the server.

GET /spec.html HTTP/1.1
Host: www.example.com
Cookie: foo=10; bar=20
Accept: */*

SESSIONS: Server side cookies

Server side cookies are known as "sessions". The website in this case stores a single cookie on the browser containing a unique Session Identifier. Status information (foo=10 and bar=20 above) are stored on the server and the Session Identifier is used to match the request with the data stored on the server.

Examples of usage

You can use both sessions and cookies to store: authentication data, user preferences, the content of a chart in an e-commerce website, etc...

Pros and Cons

Below pros and cons of the solutions. These are the first that comes to my mind, there are surely others.

Cookie Pros:

• scalability: all the data is stored in the browser so each request can go through a load balancer to different webservers and you have all the informations needed to fullfill the request;
• they can be accessed via javascript on the browser;
• not being on the server they will survive server restarts;
• RESTful: requests don't depend on server state

Cookie Cons:

• storage is limited to 80 KB (20 cookies, 4 KB each)
• secure cookies are not easy to implement: take a look at the paper A secure cookie protocol

Session Pros:

• generally easier to use, in PHP there's probably not much difference.
• unlimited storage

Session Cons:

• more difficult to scale
• on web server restarts you can lose all sessions or not depending on the implementation
• not RESTful