What are sessions? How do they work?

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.

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Explanation via Pictures:

https://i.stack.imgur.com/mYLGr.png

Simple Explanation by analogy

Imagine you are in a bank, trying to get some money out of your account. But it's dark; the bank is pitch black: there's no light and you can't see your hand in front of your face. You are surrounded by another 20 people. They all look the same. And everybody has the same voice. And everyone is a potential bad guy. In other words, HTTP is stateless.

This bank is a funny type of bank - for the sake of argument here's how things work:

you wait in line (or on-line) and you talk to the teller: you make a request to withdraw money, and then you have to wait briefly on the sofa, and 20 minutes later you have to go and actually collect your money from the teller.

But how will the teller tell you apart from everyone else?

The teller can't see or readily recognise you, remember, because the lights are all out. What if your teller gives your $10,000 withdrawal to someone else - the wrong person?! It's absolutely vital that the teller can recognise you as the one who made the withdrawal, so that you can get the money (or resource) that you asked for.

Solution:

When you first appear to the teller, he or she tells you something in secret:

"When ever you are talking to me," says the teller, "you should first identify yourself as GNASHEU329 - that way I know it's you".

Nobody else knows the secret passcode.

Example of How I Withdrew Cash:

So I decide to go to and chill out for 20 minutes and then later I go to the teller and say "I'd like to collect my withdrawal"

The teller asks me: "who are you??!"

"It's me, Mr. George Banks!"

"Prove it!"

And then I tell them my passcode: GNASHEU329

"Certainly Mr. Banks!"

That basically is how a session works. It allows one to be uniquely identified in a sea of millions of people. You need to identify yourself every time you deal with the teller.

If you got any questions or are unclear - please post comment and I will try to clear it up for you. The following is not strictly speaking, completely accurate in its terminology, but I hope it's helpful to you in understanding concepts.

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"Session" is the term used to refer to a user's time browsing a web site. It's meant to represent the time between their first arrival at a page in the site until the time they stop using the site. In practice, it's impossible to know when the user is done with the site. In most servers there's a timeout that automatically ends a session unless another page is requested by the same user.

The first time a user connects some kind of session ID is created (how it's done depends on the web server software and the type of authentication/login you're using on the site). Like cookies, this usually doesn't get sent in the URL anymore because it's a security problem. Instead it's stored along with a bunch of other stuff that collectively is also referred to as the session. Session variables are like cookies - they're name-value pairs sent along with a request for a page, and returned with the page from the server - but their names are defined in a web standard.

Some session variables are passed as HTTP headers. They're passed back and forth behind the scenes of every page browse so they don't show up in the browser and tell everybody something that may be private. Among them are the USER_AGENT, or type of browser requesting the page, the REFERRER or the page that linked to the page being requested, etc. Some web server software adds their own headers or transfer additional session data specific to the server software. But the standard ones are pretty well documented.

Hope that helps.

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HTTP is stateless connection protocol, that is, the server cannot differentiate between different connections of different users.

Hence comes cookie, once a client connects first time to a server, the server generates a new session id, which later will be sent to the client as cookie value. And from now on, this session id will identify that client connection, because within each HTTP request it will see the appropriate session id inside cookies.

Now for each session id, the server keeps some data structure, which enables him to store data specific to user, this data structure you can abstractly call session.

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Think of HTTP as a person(A) who has SHORT TERM MEMORY LOSS and forgets every person as soon as that person goes out of sight.

Now, to remember different persons, A takes a photo of that person and keeps it. Each Person's pic has an ID number. When that person comes again in sight, that person tells it's ID number to A and A finds their picture by ID number. And voila !!, A knows who is that person.

Same is with HTTP. It is suffering from SHORT TERM MEMORY LOSS. It uses Sessions to record everything you did while using a website, and then, when you come again, it identifies you with the help of Cookies(Cookie is like a token). Picture is the Session here, and ID is the Cookie here.

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Session is broad technical term which can be used to refer to a state which is stored either on server side using in-memory cache or on the client side using cookie, local storage or session storage.

There is nothing specific on the browser or server that is called session. Session is a kind of data which represents a user session on web. And that data can be stored on server or client.

And how it stored and shared is another topic. But the brief is when a user is logged in, the server creates a session data and generates a session ID. The session Id is sent back to user in custom header or set-cookie header which takes care of automatically storing it on user's browser. And then when next time the user revisits, the session ID is sent along the request and server check if there is existing session by that ID and processes accordingly.

You can store whatever you want in an session but the the main purpose is to remember the the user (browser) who have previously visit your site whether it's about login, shopping cart, or other activities.

And that's why it also important to protect the session ID from being intercepted by a hacker who will use it to identify himself as an another user.

By reading about Cookie, you will get the idea of session: (https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies)

Excerpt from MDN:

Cookies are mainly used for three purposes:

Session management

    Logins, shopping carts, game scores, or anything else the server should remember
Personalization

    User preferences, themes, and other settings
Tracking

    Recording and analyzing user behavior

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