1. Re-Introduction to HTTP

A Quick Aside, Our Example

I think it’s a little easier to learn things when we have something concrete to tie it to
Throughout this topic, consider the following example:
- Suppose we want to create a REST API for a library system
- Our system needs to manage both books and users
- We must be able to get a list of all books/users, add new books/users, update existing books/users and remove books/users
As we discuss new pieces of REST APIs we’ll be relating them back to how they might apply to our library

HTTP (or Hypertext Transfer Protocol) is the protocol that the web, as we know it, is built upon
It’s a protocol used to transfer information between clients (such as web browsers) and servers (like the one’s we’ve written with Express)
- Traditionally HTTP was designed to transfer “hypertext” such as HTML, but it is now used as a general data transfer protocol for the internet

HTTP operates on a request-response model
This means:
- The client (often a web browser) sends a request to the server (such as asking the server for a specific web page)
- The server recieves this request, processes it, and sends back a response (such as sending back the actual requested web page)
Each request is standardized and includes the following components:
- A HTTP Method which tells the server what action the client is trying to perform
- A URL which tells the server the location of the resource the client is trying to interact with
- A collection of Headers which contain extra data about the request (such as what type of data the client is sending, or any cookies the client might have)
- Some requests also have a body. This is used when attempting to send data to the server, any data being sent would be placed in the body

As mentioned earlier, HTTP methods are used by the client to tell the server what action they are trying to perform on a server
Each HTTP request must have a method

Purpose: Retrieve, or get, data from a server
Example: Sending a GET request to https://google.com/ asks Google servers for the Google homepage
Query Parameters
- With a GET request, you’re only meant to retrieve data from a server and not provide it to the server
- But sometimes you do have specifics you need to provide about the data you want
- To work around this, we can use query parameters in the URL of our GET request to specify what data we want
- Example:
  - Sending a GET request to https://library.com/books might get you the entire list of books that library has
  - However, if we just wanted the information for books published in 2024 we might send a GET request to: https://library.com/books?publishYear=2024
  - The piece after books (?publishYear=2024) are the query parameters
  - In this case we just have the one parameter, publishYear, which has a value of 2024
- To send multiple parameters, we would separate each one with an & symbol
- Example:
  - If we just wanted the information for horror books published in 2024 we might send a GET request to: https://library.com/books?publishYear=2024&genre=horror
  - Now we have two parameters. publishYear with a value of 2024 and genre with a value of horror

Purpose: Create new data on the server
How is that new data provided?
- The data is sent in the body of the request, typically in a JSON format
- JSON (or Javascript Object Notation) is a lightweight format used to send and receive structured data
Example: To add a new book to our library, a client might send a POST request to /books with the book details in the body
```
{
  "title": "1984",
  "author": "George Orwell"
}
```
- When the server recieves the request it knows the client wants to create a new book
- Then it can read the data from the body of the request and create the corresponding book

Purpose: Replace existing data on the server
How is the data provided?
- Similar to a POST request, data is sent in the body (also frequently as JSON).
Example: To update the details of a book, the client sends a PUT request to /books/1 with new book data in the body
```
{
  "title": "Animal Farm",
  "author": "George Orwell"
}
```

Purpose: Remove data from the server.
Example: A DELETE request sent to /books/1 asks to remove the book with ID 1 from the library.