language agnostic - What are sessions? How do they work?

Question

I am just beginning to start learning web application development, using python. I am coming across the terms 'cookies' and 'sessions'. I understand cookies in that they store some info in a key value pair on the browser. But I have a little confusion regarding sessions, in a session too we store data in a cookie on the user's browser.

For example - I login using username='rasmus' and password='default'. In such a case the data will be posted to the server which is supposed to check and log me in if authenticated. However during the entire process the server also generates a session ID which will be stored in a cookie on my browser. Now the server also stores this session ID in its file system or datastore.

But based on just the session ID, how would it be able to know my username during my subsequent traversal through the site? Does it store the data on the server as a dict where the key would be a session ID and details like username, email etc. be the values?

I am getting quite confused here. Need help.

Answer 1

Because HTTP is stateless, in order to associate a request to any other request, you need a way to store user data between HTTP requests.

Cookies or URL parameters ( for ex. like http://example.com/myPage?asd=lol&boo=no ) are both suitable ways to transport data between 2 or more request. However they are not good in case you don't want that data to be readable/editable on client side.

The solution is to store that data server side, give it an "id", and let the client only know (and pass back at every http request) that id. There you go, sessions implemented. Or you can use the client as a convenient remote storage, but you would encrypt the data and keep the secret server-side.

Of course there are other aspects to consider, like you don't want people to hijack other's sessions, you want sessions to not last forever but to expire, and so on.

In your specific example, the user id (could be username or another unique ID in your user database) is stored in the session data, server-side, after successful identification. Then for every HTTP request you get from the client, the session id (given by the client) will point you to the correct session data (stored by the server) that contains the authenticated user id - that way your code will know what user it is talking to.