What Is an HTTP Request?

Published April 09, 2024

HTTP requests are at the heart of communication on the World Wide Web. This article will provide an in-depth look at what HTTP requests are, how they work, and their role in enabling the web experiences we rely on every day.

HTTP Request Basics

An HTTP request is a request made by a client, such as a web browser, to a server to receive a resource. The client uses a URL (Uniform Resource Locator) that contains the information needed to access the server resources.

Structure of an HTTP Request

An HTTP request consists of three main parts:

  1. Request line: This includes the HTTP method (verb), the request-target (which can be a URI or URL), and the HTTP version. The HTTP method identifies the type of action to be performed on the resource.

  2. Headers: These give additional information about the request, such as the user-agent, cookies, content-length, and authorization token. Headers can also specify the types of data the client can accept.

  3. Message body: This part is optional and is used to send data from the client to the server or to deliver information from the server to the client. The message body is often used when creating or updating a resource on the server.

How HTTP Requests Work

HTTP requests are the main way of communication between a client and a server on the World Wide Web. When you enter a URL in your browser or click a link, the browser sends an HTTP request to the server hosting the website. The server then checks the request and sends back an HTTP response, which includes the requested resource (such as an HTML page, an image, or a JavaScript file). This transfer of resources happens using the HTTP protocol, which defines the structure and format of the requests and responses exchanged between the browser and the server.

HTTP Request Methods

HTTP request methods, also known as HTTP verbs, show the action that should be done on a resource. Each method has a specific purpose and can be used to get, create, update, or delete data on a server. Here are the most common HTTP request methods:

GET

The GET method is used to get a resource from a server. It is the most used HTTP method and is used every time you click a link or type a URL into your browser's address bar. GET requests should only be used to get data and should not change the state of the server.

POST

The POST method sends data to the server to create or update a resource. The data sent to the server is stored in the request body of the HTTP request. This is often used when submitting forms on websites.

PUT

The PUT method is similar to POST in that it can create or update a resource. However, calling a PUT request multiple times will always produce the same result, whereas multiple identical POST requests will create the same resource multiple times. PUT requests are idempotent, while POST requests are not.

DELETE

The DELETE method deletes a specific resource on the server. It contains all the information needed to find the resource to be removed.

HEAD

The HEAD method is identical to a GET request, but without the response body. This is useful for getting meta-information about a resource without transferring the entire content. HEAD requests are often used to test hyperlinks for validity or check if a resource has been modified.

PATCH

The PATCH method applies partial changes to a resource. It is similar to the PUT method but is used to make smaller changes, such as modifying a single field of a record.

OPTIONS

The OPTIONS method describes the communication options for the target resource. This allows a client to determine the options and requirements associated with a resource without getting it.

TRACE

The TRACE method performs a message loop-back test along the path to the target resource. It is used for debugging purposes.

Each of these methods serves a distinct purpose in communicating with servers and manipulating resources. By using the right method for each task, developers can make sure their applications interact with servers correctly.

Anatomy of an HTTP Request

An HTTP request has three main parts: the request line, request headers, and an optional request body.

Request Line

The request line is the first line of an HTTP request. It contains three important bits of information:

  1. The HTTP method (verb) - This shows the type of action to be performed on the resource. Common HTTP methods include GET, POST, PUT, DELETE, and others as discussed earlier.

  2. The request URL - This is the URL of the resource on which the action should be performed. It can be an absolute path or a full URL.

  3. The HTTP version - This specifies the version of the HTTP protocol being used, typically HTTP/1.1 or HTTP/2.

Here's an example of a request line:

GET /products HTTP/1.1

Request Headers

HTTP headers allow the client to pass additional information along with the request. Headers are key-value pairs separated by a colon. Some common headers include:

  • Host - The domain name of the web server.
  • User-Agent - Information about the client making the request, such as the browser type and version.
  • Content-Type - The MIME type of the request body.
  • Content-Length - The length of the request body in bytes.
  • Authorization - Contains credentials to authenticate a user agent with a server.
  • Cookie - Contains cookies previously sent by the server.

Here's an example of some request headers:

Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/93.0.4577.82 Safari/537.36
Content-Type: application/json

Request Body

The request body is optional and contains the data being sent to the server. This is typically used with POST, PUT, PATCH, and sometimes DELETE requests. The data can be in various formats such as:

Here's an example of a JSON payload in a request body:

{
  "name": "John Doe",
  "age": 30,
  "city": "New York"
}

Understanding the anatomy of an HTTP request is important for web developers. It allows them to correctly structure requests and include all the necessary information for the server to process them correctly. The HTTP request contains key information like the request method, URL, headers, and optional body that the web server needs to understand and fulfill the request. Every HTTP request opens a TCP connection to send the request and receive the response. HTTP is an application layer protocol that defines this command language that the devices on both sides of the connection must follow in order to communicate. 

What Is an HTTPS Request?

Secure Extension of HTTP

HTTPS (Hypertext Transfer Protocol Secure) is an extension of the standard HTTP protocol that adds a layer of security. It uses bidirectional encryption to secure the communication between a client and a server. This means that all data exchanged between the client and the server is encrypted, making it much harder for third parties to intercept and read the data.

HTTPS uses SSL (Secure Sockets Layer) or TLS (Transport Layer Security) digital certificates on the server side to establish secure connections. When a client (usually a web browser) connects to an HTTPS server, the server presents its SSL/TLS certificate to prove its identity. The client then verifies this certificate before establishing an encrypted connection.

Role of Certificate Authorities

SSL/TLS certificates are issued by trusted third parties known as Certificate Authorities (CAs). The role of a CA is to verify the identity of the entity requesting the certificate (usually the owner of a website or web application) and to sign the certificate with the CA's own private key.

When a client receives an SSL/TLS certificate from a server, it can verify the signature using the CA's public key. If the signature is valid and the certificate has not expired, the client can be confident that it is communicating with the genuine server and not an imposter.

This process of verifying certificates is known as the SSL/TLS handshake. It happens automatically in the background when you connect to a website using HTTPS. The presence of a valid certificate and a successful handshake ensures the authenticity, integrity, and confidentiality of the data being exchanged between the client and the server.

CAs play an important role in the trust model of HTTPS. They are responsible for thoroughly vetting the entities they issue certificates to and for maintaining the security and integrity of their own signing keys. Web browsers and operating systems maintain a list of trusted CAs, and they will only establish trusted connections to servers with certificates issued by these CAs.

How HTTP Requests and Responses Work

Client-Server Communication

HTTP communication is based on a client-server model. The client, usually a web browser, sends an HTTP request to the server. The request includes the URL of the resource, the HTTP method (like GET, POST, etc.), request headers, and sometimes a request body.

The server then processes the request and returns an HTTP response. The response contains a status code indicating whether the request was successful (e.g., 200 OK) or if there was an error (e.g., 404 Not Found). The response also includes headers that provide additional information about the response, and usually a message body containing the requested data.

For example, when you enter a URL in your browser, the browser sends a GET request to the server. The server then sends back a response containing the HTML content of the web page. The browser then parses this HTML and sends additional requests for any other resources needed to display the page, such as images, CSS stylesheets, and JavaScript files.

Accessing Resources Hosted on Different Servers

A single web page often includes resources from multiple servers. For instance, a page might load its main content from one server, its images from another, and scripts or stylesheets from yet another server. This is because the internet is made of interconnected servers, each hosting their own resources.

When you visit a web page, your browser needs to make separate HTTP requests to each of these servers to get all the resources it needs to display the page. It starts by requesting the HTML document, which includes references to other resources like images, CSS, and JavaScript files. The browser then makes additional HTTP requests to the servers hosting these resources.

Once the browser has received all the necessary resources, it renders the web page by parsing the HTML, applying the CSS styles, and executing any JavaScript code. This JavaScript code can further interact with the page, modifying the HTML and CSS, and even making additional HTTP requests to servers (this is how AJAX works).

So, even though it may seem like you're interacting with a single entity when you use a website, under the hood there's a complex network of client-server interactions happening over HTTP.

Role of Web Browsers in HTTP Requests

Sending Requests and Rendering Responses

Web browsers are the most common HTTP clients. When you enter a URL into a browser's address bar or click a link on a web page, the browser sends an HTTP request to the server hosting the website. This request usually includes the URL of the page, the HTTP method (usually GET for normal web browsing), and headers giving information about the browser.

Once the server responds with the requested resources, the browser's job is to render the content for the user. For a typical web page, this means parsing the HTML to build the Document Object Model (DOM), applying CSS styles to lay out and decorate the page elements, and running any JavaScript code to add interactivity or dynamically update the page content.

Browsers also send more HTTP requests for any embedded resources within the HTML document, such as images, CSS files, JavaScript files, or other linked content. Each of these resources needs a separate HTTP request and response cycle.

Browser Features and HTTP Requests

Modern web browsers have many features that optimize HTTP communication:

  1. Caching: Browsers can store recently fetched resources in a local cache. If the same resource is needed again, the browser can serve it from the cache instead of making another HTTP request. This reduces network traffic and makes pages load faster.

  2. Conditional Requests: Browsers can send conditional HTTP requests, which include headers like "If-Modified-Since" or "If-None-Match". These let the server send back a full response only if the resource has changed since the last request, saving bandwidth if the resource is unchanged.

  3. Different Request Methods: While most browser requests use the GET method, browsers can send other types of requests based on user actions. For example, submitting a form may send a POST request, and some AJAX interactions may use PUT, DELETE, or other methods.

  4. Parallel Requests: Modern browsers can send multiple HTTP requests in parallel, allowing faster loading of pages with many resources. However, there are limits to the number of concurrent connections a browser will make to a single server to avoid overwhelming the server.

  5. Security Features: Browsers have built-in security features related to HTTP, such as blocking mixed content (HTTP resources on HTTPS pages), enforcing CORS (Cross-Origin Resource Sharing) policies, and handling cookies and other authentication mechanisms.

HTTP Status Codes

HTTP status codes are returned by a server in response to a client's request. These codes show whether an HTTP request has been completed, and if not, they give information about the cause of the failure. Knowing HTTP status codes helps web developers debug and handle different situations in web applications.

Types of Status Codes

HTTP status codes are put into five classes, with each class representing a specific type of response:

  1. 1xx (Informational): These status codes show that the request has been received and the process is continuing. The client can expect a final response once the request processing completes. An example is 100 Continue, which means the server has received the request headers and the client should send the request body.

  2. 2xx (Success): These status codes mean that the request was successfully received, understood, and accepted by the server. The most common code in this class is 200 OK, which shows that the request has succeeded.

  3. 3xx (Redirection): These status codes show that further action needs to be taken by the client to complete the request. This usually involves redirecting the client to another URL. For example, 301 Moved Permanently means the requested resource has been permanently moved to a new URL, and the client should use this new URL for future requests.

  4. 4xx (Client Error): These status codes are used when the request contains bad syntax or cannot be fulfilled due to a client-side issue. The well-known 404 Not Found code belongs to this class, showing that the server could not find the requested resource.

  5. 5xx (Server Error): These status codes mean that the server failed to fulfill a valid request. This usually shows a problem with the server itself. The most common code in this class is 500 Internal Server Error, which is a generic response when the server encounters an unexpected condition that prevents it from fulfilling the request.

Common Status Codes

While there are many HTTP status codes, here are some of the most common ones:

  • 200 OK: The request has succeeded. This is the standard response for successful HTTP requests.

  • 201 Created: The request has been fulfilled, and a new resource has been created as a result. This is typically the response sent after a POST request.

  • 301 Moved Permanently: The requested resource has been permanently moved to a new URL. Future references to this resource should use the returned URL.

  • 400 Bad Request: The server cannot process the request due to a client error, such as a malformed request syntax.

  • 401 Unauthorized: The request requires user authentication. The client can repeat the request with a suitable Authorization header.

  • 403 Forbidden: The server understood the request but refuses to authorize it. This status is similar to 401 but is used when authentication will not help.

  • 404 Not Found: The server cannot find the requested resource. This might be due to a wrong URL or a resource that has been deleted.

  • 500 Internal Server Error: The server encountered an unexpected condition that prevented it from fulfilling the request. This is a generic response.

  • 502 Bad Gateway: The server, while acting as a gateway or proxy, received an invalid response from an upstream server it needed to access to complete the request.

  • 503 Service Unavailable: The server is currently unable to handle the request due to a temporary overload or scheduled maintenance. This condition is usually temporary.

Understanding these status codes can greatly help in fixing issues and handling different situations in your web applications. When designing APIs, it's important to use the right status codes to clearly communicate the result of a client's request. This makes your API more understandable and easier to work with for other developers.

When a client sends an HTTP request to a server, it opens a TCP connection. The server then returns a response that contains an HTTP status code, indicating the desired action to be performed by the browser. The response data is stored in the body of the HTTP request, and the Content-Length header specifies the size of this data. Different HTTP methods like GET and PUT are used to indicate the desired action to be executed by the server. While GET requests are used to retrieve data, PUT requests are idempotent and can be used to update or create resources on the server.

How to Monitor HTTP Requests

Importance of Monitoring

Monitoring HTTP requests is important to make sure your web service is running well and can fulfill client requests. By keeping track of HTTP requests and responses, you can quickly find and fix any issues that may happen. Monitoring helps you maintain the health and performance of your web application, providing a better user experience for your clients.

Using Uptimia for Monitoring

Uptimia is a useful tool for monitoring HTTP requests. It allows you to set up single HTTP request monitors that record detailed response data. With Uptimia, you can:

  • Select the HTTP request method (GET, POST, PUT, DELETE, etc.) for your monitor
  • Specify the URL you want to monitor
  • Choose the test interval at which the monitor will run
  • Select the test locations from where the HTTP requests will be sent

You can also set up advanced request settings such as headers, query parameters, cookies, and request body. This flexibility lets you simulate real-world scenarios and test your web service thoroughly. You can read more about website monitoring here.

Setting Up Alerts

To find and fix issues quickly, it's important to set up alerts in your monitoring system. With Uptimia, you can create alerts that are triggered when certain conditions are met, such as:

By setting up alerts, you can make sure you are quickly notified when issues occur, allowing you to fix them fast. This helps minimize downtime and maintain a high quality of service for your users.

Monitoring HTTP requests is not only important for finding and fixing problems but also for learning about the performance and usage patterns of your web application. By analyzing the data collected by your monitoring tool, you can make informed decisions to optimize your application, such as:

  • Finding and removing performance bottlenecks
  • Scaling infrastructure to handle increased traffic
  • Improving caching strategies to reduce response times
  • Optimizing database queries or API calls

Key Takeaways

  • HTTP requests are the main way clients (like web browsers) communicate with servers on the World Wide Web.
  • An HTTP request consists of a request line (including the HTTP method, URL, and HTTP version), headers, and an optional message body.
  • HTTPS is an extension of HTTP that adds a layer of security by encrypting the data exchanged between clients and servers.
  • Web browsers play a key role in the HTTP ecosystem by sending requests, rendering responses, and providing features to optimize and secure these interactions.
  • HTTP status codes are returned by servers to indicate whether a request has been successful or if there was an error, with different codes representing different types of responses.