What is the difference between server side cookie and client side cookie?
What is the difference between creating cookies on the server and on the client? Are these called server side cookies and client side cookies? Is there a way to create cookies that can only be read on the server or on the client?
Solution 1:
HTTP COOKIES
Cookies are key/value pairs used by websites to store state information on the browser. Say you have a website (example.com), when the browser requests a webpage the website can send cookies to store information 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 information 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
Solution 2:
You probably mean the difference between Http Only cookies and their counter part?
Http Only cookies cannot be accessed (read from or written to) in client side JavaScript, only server side. If the Http Only flag is not set, or the cookie is created in (client side) JavaScript, the cookie can be read from and written to in (client side) JavaScript as well as server side.
Solution 3:
All cookies are client and server
There is no difference. A regular cookie can be set server side or client side. The 'classic' cookie will be sent back with each request. A cookie that is set by the server, will be sent to the client in a response. The server only sends the cookie when it is explicitly set or changed, while the client sends the cookie on each request.
But essentially it's the same cookie.
But, behavior can change
A cookie is basically a name=value pair, but after the value can be a bunch of semi-colon separated attributes that affect the behavior of the cookie if it is so implemented by the client (or server).
Those attributes can be about lifetime, context and various security settings.
HTTP-only (is not server-only)
One of those attributes can be set by a server to indicate that it's an HTTP-only cookie. This means that the cookie is still sent back and forth, but it won't be available in JavaScript. Do note, though, that the cookie is still there! It's only a built in protection in the browser, but if somebody would use a ridiculously old browser like IE5, or some custom client, they can actually read the cookie!
So it seems like there are 'server cookies', but there are actually not. Those cookies are still sent to the client. On the client there is no way to prevent a cookie from being sent to the server.
Alternatives to achieve 'only-ness'
If you want to store a value only on the server, or only on the client, then you'd need some other kind of storage, like a file or database on the server, or Local Storage on the client.