# Session & Cookie

<mark style="background-color:orange;">Sessions are not a part of the HTTP protocol itself. Sessions are a server-side technology</mark>, managed and implemented by server-side programming languages and frameworks.

HTTP is a stateless protocol, meaning each request is independent and does not have any knowledge or memory of previous requests.

Sessions are a method used to overcome this stateless nature and maintain a certain amount of information or state between different requests from the same client.&#x20;

<mark style="background-color:orange;">Sessions often leverage HTTP cookies to store a session ID</mark>:\
When a client makes a request, the server sets a cookie with a unique session ID in the response. The browser then includes this cookie in all subsequent requests, allowing the server to associate these requests with the same session. The actual session data is stored on the server, indexed by the session ID.

<mark style="background-color:orange;">Cookies are a part of the HTTP protocol and are managed by web browsers.</mark>

HTTP Protocol defines a way for servers to send small amounts of data to clients, which the clients should store and send back with subsequent requests. This data is included in HTTP headers: specifically, the `Set-Cookie` header from server to client, and the Cookie header from client to server. This is how cookies are defined and used in the HTTP protocol.

<mark style="background-color:orange;">While the HTTP protocol defines how cookies should be sent back and forth, it's the web browser that actually manages the cookies.</mark> When a browser receives a Set-Cookie header, it stores the cookie as defined by the server. Then, for subsequent requests to the same domain, the browser includes the cookie in the Cookie header. Browsers also manage when cookies should be deleted, either based on a provided expiration date or when the browser is closed, depending on the type of cookie.

A cookie is not a "part" of the server in the sense that it is not stored on or managed by the server. Instead, cookies are created by the server, sent to the browser and then stored and managed by the browser. <mark style="background-color:green;">% 服務器創建一個cookie，發送給瀏覽器。瀏覽器存儲並管理這個cookie。這塊餅，就是一個身份令牌。 20231118 %</mark>

Here's how it works:

1\. When responding to an HTTP Request, a Server create a cookie by including a Set-Cookie header in an HTTP Response. (This header contains a cookie name, a value, and several attributes such as a domain, path, expiration date, and security settings.)

2\. When a Browser receives this HTTP Response, it stores and manages the cookie. The browser then automatically includes this cookie in the Cookie header of all future HTTP Requests to the same domain and path, until it is deleted or expires.

3\. When the Server receives an HTTP Request that includes a Cookie header, it can use the information in this header to recognize returning users, manage sessions, or customize responses.



<mark style="color:green;">**WHL: How a cookie is created in Rails and sent to browser?**</mark>

ChatGPT: In Rails, you can set a cookie using the `cookies` method, which is available in both controllers and views.

```ruby
def create
    # some logic here
    cookies[:user_firstname] = "John"
    cookies[:user_lastname] = { value: "Bull", expires: 1.year.from_now, httponly: true }
end
```

When this controller action is called, it will send a `Set-Cookie` HTTP header to the browser with the key and value you specified. In this case, `:user_name` is the key for the cookie and `"John"` is the value.&#x20;

The `expires` attribute sets when the cookie should be deleted by the client's browser. \
The `httponly` attribute restricts the cookie from being accessed by JavaScript on the client side, which can provide a level of protection against certain cross-site scripting (XSS) attacks.

For sensitive data like user authentication, Rails provides a `session` method, which works similarly to `cookies` but stores the data on the server and only uses a session ID in the cookie, which is more secure.



<mark style="color:green;">**WHL: So the cookies method of a Rails controller will write the Set-Cookie part of the HTTP header?**</mark>

ChatGPT: Yes, that's correct. In a Rails application, when you set a cookie using the cookies method in your controller or view, Rails will automatically add a Set-Cookie header with the specified information to the HTTP response that it sends back to the client's browser.

When the client's browser receives the response, it will see the Set-Cookie header and store the cookie as instructed. Then, for all subsequent requests to the same domain, the browser will automatically include the stored cookie data in the Cookie header of the HTTP request.

This automatic handling of cookie data is part of what makes frameworks like Rails powerful tools for web development. They abstract away many of the lower-level details of the HTTP protocol, allowing developers to focus on the specific needs of their application.



<mark style="color:green;">**WHL: It seems the cookie ID if it has one and the Session ID are just kind of marks or identifiers during complex interaction process.**</mark>

ChatGPT: Yes, that's correct. <mark style="background-color:orange;">Both cookie IDs and session IDs are used as identifiers to maintain state during the interaction process between the browser and the server.</mark>

Cookie ID: Cookies are used to store data that persists across multiple requests. Each cookie has a name (which can be thought of as its ID) and a value. The name is used to retrieve the value later. A single browser could have many cookies stored, each with a different name, for a single website.

Session ID: A session is a way to store data between HTTP requests for a particular user. When a new session is created, it is given a unique session ID. This session ID is then stored in a cookie and sent to the client. On subsequent requests, the client sends back the session ID, which the server uses to find the correct session data. The session data itself is usually stored server-side.

So in essence, both the cookie ID and the session ID are identifiers used to maintain state and track user data across multiple requests. They are fundamental in creating a seamless and personalized user experience on the web, where the underlying HTTP protocol is stateless.



<mark style="color:green;">**WHL: As you previously mentioned, "cookies are created by the server, sent to the browser, and then stored and managed by the browser", a browser actually does not create a cookie but just has the mechanism to store a cookie?**</mark>

ChatGPT: Yes, that's correct. Web browsers do not create cookies themselves; rather, they are responsible for storing and managing cookies that are created and sent by web servers.

When a server responds to an HTTP request, it may include a `Set-Cookie` header. This header contains the information necessary to create a new cookie, including its name, value, and other attributes such as an expiration date or domain.

The browser receives this Set-Cookie header and creates the cookie according to the provided information. It then stores this cookie locally. For all subsequent requests to the same domain, the browser will automatically include the cookie data in the Cookie header of the HTTP request.

So while the browser doesn't create cookies, it plays a crucial role in storing and managing them, and in including them in subsequent requests to the server. It's also worth noting that while cookies are typically created by the server, JavaScript running in the browser can also create and manipulate cookies.



<mark style="color:green;">**WHL: You said "The browser receives this**</mark><mark style="color:green;">**&#x20;**</mark><mark style="color:green;">**`Set-Cookie`**</mark><mark style="color:green;">**&#x20;**</mark><mark style="color:green;">**header and creates the cookie according to the provided information.". It seems not an accurate description, since a browser does not "create " a cookie on its side as you mentioned previously. I guess a browser just store the the information of a cookie created by a server and store in the response header part.**</mark>

ChatGPT: You're correct, and I appreciate the clarification. In the context of server-side cookies, the web server determines the content of the cookie and includes it in the Set-Cookie HTTP response header. The web browser, upon receiving this response, stores the cookie data locally as provided, and automatically includes it in future HTTP requests to the same domain.

The term "create" might be misleading here. When a server sends a Set-Cookie header, the browser is effectively being instructed to store a cookie with certain parameters. The creation of the cookie content, such as its name, value, and attributes, is done server-side.

However, it's also worth noting that JavaScript running in the browser can create, read, and delete cookies. In this context, the browser, or more accurately, the JavaScript code running in the browser, does "create" cookies in the sense of determining the cookie content and instructing the browser to store it.

But in the context of your question and our discussion, yes, when it comes to server-side cookies, the browser's role is to store, manage, and send back the cookie data as instructed by the server. Thanks for your careful reading and attention to detail!



<mark style="color:green;">**WHL: What does a Session play in the whole process? Who handles a session, the Controller? How?**</mark>

ChatGPT: A session in a web application is a mechanism for storing user-specific data across multiple HTTP requests. This is necessary because the HTTP protocol, which underpins the web, is stateless: each request is independent and doesn't know anything about the previous or next request. However, for many applications, you need to maintain state across multiple requests, such as keeping a user logged in as they navigate between different pages. That's where sessions come in.

The session is typically managed by the application framework or server. ~~In an MVC architecture like Rails, both the Controller and the Model can interact with the session as needed.~~

**Here's how it works:**

When a user logs into your application, you might store their user ID in the session. This is often done in the Controller, in the action that handles the login request.

On subsequent requests, the Controller can check the session to see if the user is logged in and, if so, which user it is. This can be used to tailor the response to that specific user. For example, you could use the user ID in the session to fetch the user's data from the database and display their personalized homepage.

If the user logs out, you would clear their user ID from the session. This would also typically be done in the Controller, in the action that handles the logout request.

~~The Model might also interact with the session in some cases. For example, if you're using an ORM like ActiveRecord in Rails, it can use the session to manage database connections.~~

In terms of how sessions are implemented, it varies depending on the server and framework you're using. A session could be stored in a cookie, which is sent back and forth between the client and server with each request. Alternatively, the session could be stored on the server, with just a session ID stored in a cookie. There are also options for storing sessions in external data stores like databases or caching servers.



<mark style="color:green;">**WHL: Therefore, a session is only created and stored after a user logged in, namely passed the authentication? Without an authentication step, there is no session?**</mark>

ChatGPT: Not exactly. A session can be created as soon as a user visits a website, even if they are not authenticated (i.e., logged in). This allows the website to track certain information about the user's behaviour or preferences across multiple requests.

For example, an e-commerce website might use a session to keep track of items a user has added to their shopping cart, even if the user has not yet logged in. This allows the cart to persist as the user navigates between different pages on the site.

However, until a user has authenticated by logging in, the session will not be associated with a specific user account. Once the user logs in, you can store their user ID or other identifying information in the session, effectively linking the session to that user account. This allows you to tailor the user's experience based on their account information and preferences.

So while sessions are commonly used as part of the authentication process, they can also be used before authentication to maintain state across multiple requests for the same user. It's also worth noting that the specific behavior can vary depending on how your application and server are configured.



<mark style="color:green;">**WHL: You mentioned a Model sometimes interacts with a session in Rails, how can that be?**</mark>

ChatGPT: I apologize for any confusion earlier. In a typical Rails application, sessions are mostly handled at the Controller and View level, and not directly in the Model. Here's a bit more clarification:

Controller: Controllers often interact with session data. They're responsible for controlling the flow of data from the user to the application, and vice versa. For example, when a user logs into an application, the controller usually sets a session variable with the user's ID.

View: Views may use session data indirectly. For example, the application might use session data to decide which parts of a page to show to a user.

Models in Rails typically don't interact directly with sessions. Models generally don't have access to session data as they're supposed to represent and interact with application data in a stateless manner.
