Without HTTP (Hypertext Transfer Protocol), the World Wide Web as we know it today would not exist. HTTP is the protocol that enables the transfer of data over the internet, allowing users to access websites and other online resources.
There are various ways to design web applications, including GraphQL, SOAP, Falcor, gRPC, WebSockets, and Serverless Functions, with REST being the most popular (according to the 2021 State-of-API survey by Postman). And at the heart of all these architectures is HTTP.
To understand HTTP, it is helpful to have a basic understanding of the following concepts:
By the end of the article, readers should have a basic understanding of HTTP, as well as the tools and resources available for testing and troubleshooting applications.
HTTP (Hypertext Transfer Protocol) is a protocol used for exchanging information over the internet. HTTP is like the delivery system for information on the internet. It makes sure information goes from one place to another, like how ships carry goods across the ocean. It's the foundation of the World Wide Web.
HTTP is what makes the internet work. It's a way for web browsers and servers to talk to each other and send things like web pages back and forth. It's important for people who build websites and web applications to know how it works.
Without HTTP, it would be difficult to imagine how the internet would work. There would be no web pages, no URLs, and no hyperlinks. Instead, users would need to know the exact IP address of the server hosting the information they want to access, and they would need to use a low-level protocol like TCP/IP to transfer data.
Communication in HTTP centers around a concept called the Request-Response Cycle.
The request-response cycle is the process by which a client (such as a web browser or a mobile app) communicates with a server to retrieve resources or perform actions. The cycle involves several steps:
To create a valid HTTP request, you need the following:
Here's an example of an HTTP request header, with three lines:
GET /watch?v=8PoQpnlBXD0 HTTP/1.1 Host: www.youtube.com Cookie: GPS=1; VISITOR_INFO1_LIVE=kOe2UTUyPmw; YSC=Jt6s9YVWMd4 
When a web browser attempts to access an image on the internet, it sends a request to the server using a URL. This URL is unique and points to a specific resource on the server.
A resource can be anything that has a name and can be accessed with a unique identifier like a user, product, article, document, or image. You can think of resources as nouns.
The request method tells the server what kind of action the client wants the server to take. The most common methods are:
| HTTP Methods | Definition |
|---|---|
| HEAD | Asks the server for status (size, availability) of a resource. |
| GET | Asks the server to retrieve a resource. |
| POST | Asks the server to create a new resource. |
| PUT | Asks the server to edit/update an existing resource. |
| DELETE | Asks the server to delete a resource. |
HTTP request headers are additional pieces of information that are sent by the client as part of an HTTP request. They have a name/value format. That is:
Name: Value These headers provide context and additional instructions to the server, which can be used to process the request or customize the response.
Here's an example of an HTTP request header:
GET /api/data HTTP/1.1 Host: example.com User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.82 Safari/537.36 Accept: application/json Accept-Language: en-US,en;q=0.5 Authorization: Token abc123 Cache-Control: no-cache Connection: keep-alive Referer: https://www.google.com/ Pragma: no-cache In this example, the GET method is used to send a request to the /api/data endpoint on the example.com server using HTTP/1.1 protocol. The request includes ten headers:
| Headers | Definition |
| Host | This header specifies the hostname of the server that the client is trying to connect to. |
| User-Agent | This header provides information about the client that is making the request (in this case, a version of the Chrome browser). |
| Accept | This header specifies the MIME types of data that the client is willing to accept in the response. |
| Accept-Language | This header specifies the preferred language(s) for the response. |
| Authorization | This header provides an access token (in this case, Token abc123) for authentication purposes. |
| Cache-Control | This header specifies caching directives for both requests and responses. |
| Connection | This header specifies options for the connection handling between client and server. |
| Referer | This header specifies the URL of the page that linked to the current page. |
| Pragma | This header specifies implementation-specific directives that might apply to any agent along the request-response chain. |
| Content-Type | This header specifies the MIME type of the data that is being sent in the body of the request, but it is not used in this example because this is a GET request without a request body. |
In HTTP, the request body is the data that is sent from the client to the server as part of an HTTP request. The example below shows how to upload an image to the Cat API Server:
And here's what the request looks like:
POST /v1/images/upload HTTP/1.1 Host: api.thecatapi.com x-api-key: API_KEY Content-Length: 232 Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW ------WebKitFormBoundary7MA4YWxkTrZu0gW Content-Disposition: form-data; name="file"; filename="/C:/Users/USER/Downloads/cat1.jpg" Content-Type: image/jpeg (data) ------WebKitFormBoundary7MA4YWxkTrZu0gW-- The request includes these headers:
When the server receives this request, it will parse the request body and use it to create a new entry in the Cat API database. The server will then return a response that includes information about the new entry, such as the image URL and the database ID.
An HTTP response is the message that a server sends back to a client in response to an HTTP request. It usually consists of a status line, headers, and a message body:
HTTP/1.1 200 OK Date: Sun, 28 Mar 2023 10:15:00 GMT Content-Type: application/json Server: Apache/2.4.39 (Unix) OpenSSL/1.1.1c PHP/7.3.6 Content-Length: 1024 < "name": "John Doe", "email": "johndoe@example.com", "age": 30, "address": < "street": "123 Main St", "city": "Anytown", "state": "CA", "zip": "12345" > > The status line contains the HTTP version, a status code indicating the outcome of the request, and a corresponding message.
The headers provide additional information about the response, such as the content type of the message body or the date and time that the response was sent.
The message body contains the actual response data, such as HTML, JSON, or XML content.
Some common HTTP status codes include:
| Code | Meaning |
| 100 | Continue |
| 101 | Switching Protocols |
| 200 | OK |
| 201 | Created |
| 202 | Accepted |
| 203 | Non-Authoritative Information |
| 404 | Not Found : The requested resource was not found on the server. |
| 500 | Internal Server Error : The server encountered an error while processing the request. |
| 301 | Moved Permanently: The requested resource has been permanently moved to a new URL. |
Here's an example of a JSON response from the Cat API:
< "id": "ErDd1JRRT", "url": "https://cdn2.thecatapi.com/images/ErDd1JRRT.jpg", "width": 4282, "height": 6424, "original_filename": "cat1.jpg", "pending": 0, "approved": 1 > This is a JSON response from a Cat API request that appears to provide metadata about an image that has been uploaded or retrieved from the API. Here's what each field means:
HTTP (Hypertext Transfer Protocol) is a protocol used for exchanging information over the internet. It forms the foundation of the World Wide Web and allows communication between web browsers and servers.
This article is a short introduction to HTTP. If you are interested in learning more, check out these textbook recommendations:
