Knowledge Base

The internet is a complex system in constant communication between a client (usually a web browser) and a server. During this communication, the server sends numerical responses called HTTP status codes to indicate whether the request was successful or encountered a problem. These codes are vital for web developers, system administrators, and even end-users. When interpreted correctly, they help identify and resolve the source of problems. This comprehensive guide will delve into the depths of HTTP error codes, examining the meaning, possible causes, and solutions of each in detail.

1. Introduction: Overview of HTTP Status Codes

HTTP (Hypertext Transfer Protocol) is the protocol that forms the foundation of data exchange on the web. When a web browser (client) sends a request to a web server, the server responds to this request with an HTTP status code. This code provides information about the outcome of the request. HTTP status codes are divided into five main classes:

• 1xx (Informational): Request received and processing continues.
• 2xx (Successful): The request was successfully received, understood, and accepted.
• 3xx (Redirection): Further action needs to be taken to complete the request.
• 4xx (Client Error): There is an error on the client side.
• 5xx (Server Error): There is an error on the server side.

Each code has its own meaning and provides important clues in the troubleshooting process. In this guide, we will examine the most common HTTP error codes and their solutions in detail.

2. 2xx Success Status Codes

2xx status codes indicate that the request was completed successfully. These codes generally do not pose a problem, but it is important to know their meanings.

2.1. 200 OK

Meaning: The request was completed successfully. The server successfully found the requested resource and sent it to the client.

Possible Causes: There are no errors. The request was processed successfully.

Solution: No action is required. The request was successful.

2.2. 201 Created

Meaning: The request was completed successfully and a new resource was created.

Possible Causes: Usually returned in POST requests when the server creates a new resource.

Solution: No action is required. The request was successful.

2.3. 204 No Content

Meaning: The request was completed successfully, but the server has no content to return.

Possible Causes: Usually returned in DELETE or PUT requests when the server performs an action but does not need to send a response.

Solution: No action is required. The request was successful.

3. 3xx Redirection Status Codes

3xx status codes indicate that the client needs to take additional action to complete the request. This usually means that the requested resource has been moved to a different address or is temporarily located elsewhere.

3.1. 301 Moved Permanently

Meaning: The requested resource has been permanently moved to a different address. Browsers cache this information and automatically use the new address in future requests.

Possible Reasons: Changes in website structure, domain name changes.

Solution:

• Web Developers: Set up permanent redirects via .htaccess file or server configuration files.
• Users: You usually don't need to take any action. The browser will automatically redirect to the new address.

        # .htaccess example (Apache)
        Redirect 301 /eski-sayfa.html /yeni-sayfa.html
    

3.2. 302 Found (Temporary Redirect)

Meaning: The requested resource has been temporarily moved to a different address. Browsers do not cache this information and use the original address for each request.

Possible Reasons: Website maintenance, A/B testing, temporary promotions.

Solution:

• Web Developers: Set up temporary redirects via .htaccess file or server configuration files.
• Users: You usually don't need to take any action. The browser will automatically redirect to the new address.

        # .htaccess example (Apache)
        Redirect 302 /eski-sayfa.html /yeni-sayfa.html
    

3.3. 304 Not Modified

Meaning: The requested resource has not changed. The browser can use the cached version.

Possible Reasons: Browser cache is being used and the resource on the server has not changed.

Solution: No action is required. The browser will use the cache.

4. 4xx Client Error Status Codes

4xx status codes indicate that there is an error on the client side. These errors are usually caused by reasons such as incorrect URLs, insufficient permissions, or missing parameters.

4.1. 400 Bad Request

Meaning: The server could not understand the request sent by the client.

Possible Reasons:

• Incorrect syntax
• Invalid request parameters
• Corrupted request headers

Solution:

• Check the request parameters and make sure they are correct.
• Check the request headers and make sure they are in the correct format.
• Review the API documentation and make sure you are using the correct request format.

4.2. 401 Unauthorized

Meaning: Authentication is required to access the requested resource.

Possible Reasons:

• Incorrect username or password
• Missing or invalid API key
• Session expired

Solution:

• Log in with the correct username and password.
• Check your API key and make sure it is correct.
• Make sure your session has not expired.

4.3. 403 Forbidden

Meaning: The client is authenticated but does not have permission to access the requested resource.

Possible Reasons:

• The client does not have permission to access the resource.
• The server decided to refuse the request.

Solution:

• Make sure you have permission to access the requested resource.
• Contact the system administrator and request access permission.

4.4. 404 Not Found

Meaning: The server could not find the requested resource.

Possible Reasons:

• Incorrect URL
• Resource deleted or moved
• Web server misconfigured

Solution:

• Check the URL and make sure it is correct.
• Check the website structure and make sure the resource still exists.
• Make sure the web server is configured correctly.

4.5. 405 Method Not Allowed

Meaning: The requested HTTP method (GET, POST, PUT, DELETE, etc.) is not supported for the resource.

Possible Reasons:

• Incorrect HTTP method being used.
• The server does not support the requested method.

Solution:

• Make sure you are using the correct HTTP method.
• Review the API documentation and check which methods the resource supports.

4.6. 429 Too Many Requests

Meaning: The client has sent too many requests in a given amount of time (rate limiting). The server has temporarily blocked the client.

Possible Reasons:

• You have exceeded the API usage limit.
• You are excessively scraping a website.

Solution:

• Slow down your requests and try again.
• Check your API usage limit and make sure you are not exceeding it.
• Wait for the "Retry-After" time specified in the server's response headers and then try again.

5. 5xx Server Error Status Codes

5xx status codes indicate an error on the server side. These errors are usually caused by problems in server configuration, overloading, or software errors.

5.1. 500 Internal Server Error

Meaning: The server encountered an unexpected error and could not complete the request.

Possible Reasons:

• A software error on the server side.
• Database connection problems.
• Errors in server configuration.

Solution:

• Examine the server logs and identify the source of the error.
• Fix software bugs.
• Check the database connection and troubleshoot any issues.
• Check the server configuration and fix any errors.
• Contact the system administrator.

5.2. 502 Bad Gateway

Meaning: The server received an invalid response from another server.

Possible Reasons:

• An upstream server (e.g., a database server or another web server) is not responding or is returning an invalid response.
• Network connectivity issues.

Solution:

• Ensure that the upstream server is running and responding correctly.
• Check the network connection and troubleshoot any issues.
• If you are using a proxy server or load balancer, check their configurations.
• Contact the system administrator.

5.3. 503 Service Unavailable

Meaning: The server is currently unable to handle the request (e.g., due to overload or maintenance).

Possible Reasons:

• The server is overloaded.
• The server is undergoing maintenance.
• Server resources are insufficient.

Solution:

• Try again later.
• Increase server resources (e.g., more memory or CPU).
• Distribute the load using a load balancer.
• Contact the system administrator.

5.4. 504 Gateway Timeout

Meaning: The server did not receive a timely response from another server.

Possible Reasons:

• The upstream server is responding too slowly.
• Network connectivity issues.
• The server is overloaded.

Solution:

• Check and improve the performance of the upstream server.
• Check the network connection and troubleshoot any issues.
• Increase server resources (e.g., more memory or CPU).
• Contact the system administrator.

6. HTTP Status Codes Table

The following table summarizes the most common HTTP status codes and their meanings:

7. HTTP Status Codes: Comparison Table (4xx vs 5xx)

The table below summarizes the key differences between client errors (4xx) and server errors (5xx):

8. Real-Life Examples and Case Studies

8.1. Case Study 1: 404 Errors on an E-commerce Site

An e-commerce site experienced a sudden increase in 404 errors on product pages. Investigations revealed that some URLs were not updated during a category restructuring and no redirects were made from the old URLs. As a solution, 301 redirects were made from the old URLs to the new URLs, and the 404 errors were eliminated.

8.2. Case Study 2: 429 Errors in API Integration

A mobile application was integrated with a third-party API. However, application users frequently received 429 (Too Many Requests) errors. Analysis revealed that the application was sending requests to the API too frequently and exceeding the API usage limit. As a solution, requests were optimized, caching mechanisms were used, and the number of requests to the API was reduced.

9. Frequently Asked Questions (FAQ)

1. Why are HTTP status codes important?

   HTTP status codes indicate whether requests are successful or encounter a problem. These codes provide important clues in the troubleshooting process and help web applications function correctly.

2. Which HTTP status codes are most common?

   The most common HTTP status codes are: 200 OK, 301 Moved Permanently, 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable, 504 Gateway Timeout.

3. What is the difference between 4xx and 5xx errors?

   4xx errors indicate a problem on the client side, while 5xx errors indicate a problem on the server side.

4. How can I fix a 404 error?

   Check the URL, check the website structure, make sure the web server is configured correctly.

5. How can I fix a 500 error?

   Examine server logs, fix software errors, check the database connection, check the server configuration, contact the system administrator.

10. Conclusion and Summary

HTTP status codes are an integral part of communication on the web. Understanding the meanings of these codes and their possible solutions is vital for web developers, system administrators, and end-users. In this guide, we have examined the most common HTTP status codes and their solutions in detail. I hope this information helps your website and applications run smoothly.

Important Notes:

• Check HTTP status codes regularly.
• Examine server logs and identify the source of errors.
• Optimize the performance of your website and applications.
• Take security measures and protect your website from malicious attacks.
• Review the API documentation and make sure you are using the correct request format.