Important HTTP Status Codes for DevOps Engineers

HTTP status codes are more than just three-digit numbers - they are crucial indicators of how requests and responses are handled over the web. It’s important for effective problem-solving, performance optimization, and system management.

This comprehensive guide will dive deep into key HTTP status codes, offering practical strategies, real-world scenarios, reasons for occurrences, and actionable resolutions.

So read till end …

Understanding HTTP Status Codes : An Overview

HTTP Status Codes are categorized into five classes, each providing insights into different stages of the request-response lifecycle. Those status codes are -  Informational, Successful, Redirection, Client Error, and Server Error - providing practical strategies, detailed explanations, and resolutions. By learning these codes, we can enhance our ability to troubleshoot, optimize, and maintain robust web applications and services.

How Do HTTP Responses Work ?

An HTTP response is the message sent from a server to a client (usually a web browser or API consumer) after receiving an HTTP request. Its primary role is to communicate the result of the client's request, provide requested content, or explain why a request failed. Understanding HTTP responses is essential for developers, web administrators, and anyone dealing with web communication.

So When a client makes an HTTP request (for example, accessing a website URL), the server processes that request and sends back an HTTP response. The response informs the client whether the request succeeded, failed, or requires further action and can also deliver data such as HTML, JSON, files, or images.

Core Components of an HTTP Response

An HTTP response consists of three main parts :

1. Status Line
2. Headers
3. Body (optional)

1. Status Line

The status line is always the first line in an HTTP response and it includes :

• HTTP version : Specifies the protocol version (e.g., HTTP/1.1)
• Status code: A three-digit code indicating the outcome (e.g. 200 for success, 404 for not found)
• Reason phrase: A human-readable explanation of the status code (e.g., "OK", "Not Found")

Example :

HTTP/1.1 200 OK

2. Headers

Following the status line are HTTP response headers - key-value pairs that deliver metadata about the response. Headers can provide information on:

• Server details : e.g. Server : Apache/2.4.41 (Ubuntu)
• Content type : e.g. Content-Type : application/json or text/html
• Content length : Size of the response body, in bytes
• Cache control : How clients and intermediaries should cache the response
• Cookies : e.g. Set-Cookie: session_id=abc123; Expires=...
• Date : When the response was generated.

An empty line (CRLF) separates headers from the response body.

3. Body (Optional)

This section contains the actual data sent to the client, based on the request type and the server's response:

• For a successful GET request: The body returns the requested resource (like an HTML page, image, or JSON)
• For errors: It may contain an error message or instructions.
• For some responses (like a 204 No Content or a HEAD request): There is no body.

Examples of response bodies :

• HTML for web pages
• JSON or XML for APIs
• Media files (images, video, audio)
• Plain text or files for download

Complete Example of an HTTP Response

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 74
Cache-Control: max-age=3600
Date: Sun, 27 Jul 2025 13:30:00 GMT

{"message": "User created successfully", "user_id": 123, "status": "active"}

Why Understanding HTTP Responses Matters

• Debugging : Diagnose web and API errors with status codes and messages.
• Security: Ensure sensitive data is not leaked through headers or bodies.

So HTTP responses are structured messages from server to client, including a status line, headers, and often a body.

Now 

What Are 1xx HTTP Status Codes ?

1xx status codes represent "informational responses." They indicate that a client’s request was received and understood by the server, and that further processing is underway. The final response will arrive soon, but for now, the client should wait or continue.

• 1xx codes are NOT error codes.
• They are temporary, intermediary messages.
• Usually, browsers and most users never directly see them, but they’re crucial for optimizing advanced HTTP workflows (like chunked data transfer and protocol switching).

100 Status Code - Continue

• Indicates : Server received initial headers, client can send the rest (e.g. big upload).
• Why use it ? Prevents sending huge files if the server would just reject them (like due to bad authentication).
• How it works:
  1. Client sends headers with Expect: 100-continue.
  2. Server responds with HTTP/1.1 100 Continue.
  3. Client uploads the body (file, data, etc.).
  4. Server sends final status (e.g. 201 Created or 403 Forbidden)

101 Status Code - Switching Protocols

• Indicates : Server will change to a new communication protocol as requested by the client.
• When is it used ? Most commonly with protocols like WebSockets or HTTP/2 upgrades.

102 Status Code - Processing (WebDAV Specific)

• Indicates : The request may take a long time. Server acknowledges and is still working.
• Example Usage : When many sub-operations (like file moves) are required, so the server prevents the client from timing out by giving this notice.

103 Status Code - Early Hints

• Indicates : Server provides some headers before the final response.
• Real World Use : Lets browsers start preloading assets (CSS, JS, images) before the main response body is ready; improves perceived performance.

.

"1xx Informational status codes are temporary responses that help coordinate communication and optimize real-time web operations before a final response is sent to the client."

References :

- Wikipedia – List of HTTP status codes

Successful HTTP Status Codes (200–299)

HTTP status codes in the 200 range-commonly known as Successful Status Codes , indicate that a client’s request was received, understood, and processed successfully by the server. These codes are crucial in the client-server communication cycle because they confirm that the intended action was completed without errors.

What Are Successful Status Codes ?

Whenever your browser requests a webpage, submits a form, or interacts with an API, the web server responds with a status code. When that code falls between 200 and 299, it means the server has successfully processed the request.

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200 Status Code - OK

• The most common success status.
• Means everything worked as intended.
• The response body contains the requested resource.
• Example: Someone opening a website, the server responds with status 200 and the HTML content.

201 Status Code - Created

• Implies that a new resource (like a new user account or document) has been created.
• The server response usually contains a Location header with the URI of the new resource.
• This is mostly used in RESTful APIs.

202 Status Code - Accepted

• The request was valid but still under processing.
• Suitable for asynchronous server operations where the result is not immediate.
• Example: Queueing a message for processing, where the client is informed that the request is valid but work continues.

204 Status Code - No Content

• The server successfully processed the request, but there is nothing to send back.
• Useful in cases like deleting a resource or form submissions where refreshing the page or updating the UI manually is enough.
• No response body is returned.

206 Status Code - Partial Content

• Used when only a portion of a resource is sent back.
• Supports resumable downloads or streaming by serving byte ranges.
• The client requests a range, and the server delivers that part with this status.

Why Are 2xx Codes Important ?

• They inform the client that their request worked as expected, allowing proper application behavior without guesswork.
• Applications and browsers depend on these codes to decide next actions: render data, show success messages, or continue streams.
• APIs use them to confirm creation or acceptance of data, enabling robust automation and interaction.

So he 2xx range of HTTP status codes represents a successful interaction between client and server. 

Redirection Status Codes (300-399) in HTTP

When you enter a URL in your browser or click on a link, you expect to be taken seamlessly to the correct webpage or resource. But behind the scenes, web servers often need to communicate that the requested resource has moved, changed location or requires some additional steps before delivering the content. This communication happens through redirection status codes, which fall within the 300 to 399 range in the HTTP protocol.

Redirection codes are a way for the server to tell your browser or HTTP client: "The resource you want isn’t here anymore but here’s what to do next." Instead of ending the request abruptly, these codes provide instructions that help clients follow new paths or reuse cached content efficiently.

Unlike success or error codes, redirection codes imply that the client needs to take some further action.

300 Status Code – Multiple Choices

• Indicates multiple options are available for the requested resource (e.g., different formats or languages).
• The server tells the client to choose one option.
• Rarely used, as most clients do not prompt users to make a selection.

301 Status Code – Moved Permanently

• Signals the resource has been permanently moved to a new URL.
• Browsers and search engines should update bookmarks and index the new address.
• Commonly used for domain changes or switching from HTTP to HTTPS.
• Helps preserve SEO rankings and ensures user requests reach the updated location.

302 Status Code – Found / Moved Temporarily

• Indicates the resource is temporarily located at a different URI.
• Clients should continue to use the original URL for future requests.
• Commonly used during site maintenance or A/B testing.
• Does not clarify whether the HTTP method should change, which can cause inconsistencies with POST requests.

303 Status Code – See Other

• Instructs the client that the requested resource can be found at another URI and should be retrieved via a GET request.
• Often used after processing a POST (such as a form submission), redirecting to a results or confirmation page.
• Prevents accidental resubmission of form data when refreshing or bookmarking.

304 Status Code – Not Modified

• Used for efficient web caching.
• Indicates the resource has not changed since the client last requested it—client should use its cached version.
• Reduces bandwidth usage and improves load times.

305 Status Code – Use Proxy (Deprecated)

• Previously directed clients to access the resource through a specified proxy server.
• Deprecated due to security concerns and inconsistent support.
• Rarely used today.

307 Status Code – Temporary Redirect

• Indicates a temporary URI for the resource.
• Specifies that the client must use the same HTTP method (e.g., POST stays POST) when repeating the request.
• Ensures reliable redirection for sensitive operations, like form submissions or API calls.

308 Status Code – Permanent Redirect

• Similar to 301 but explicitly requires the same HTTP method and request body to be used after redirection.
• Useful for API endpoints and situations needing strict method consistency.
• Ensures clients do not accidentally alter the request method during a permanent redirect.

Why Are These Redirects Important ?

Redirection codes keep the web experience smooth despite the constant evolution of websites. They enable webmasters and developers to :

• Move content without breaking links.
• Implement SEO-friendly URL changes.
• Manage caching effectively to speed up access.
• Safely redirect form submissions and API calls.
• Handle traffic load balancing and A/B testing.

How Redirection Works: A Typical Flow

Here’s a simplified interaction :

1. Client requests: GET /old-page
2. Server responds: 301 Moved Permanently + Location: /new-page
3. Client requests: GET /new-page
4. Server responds: 200 OK + content of /new-page

Client Error Status Codes (400-499) in HTTP

When interacting with the web, not every request goes as planned. Sometimes, clients like your browser or an API consumer send requests that the server cannot or will not process due to errors on the client side. The HTTP protocol communicates such problems using Client Error Status Codes, which fall within the 400 to 499 range.

These codes are essential because they inform clients about the exact nature of the problem.

What Are Client Error Status Codes?

Client error status codes indicate that there's an issue with the client's request. Unlike server errors (5xx codes), these errors are meant to warn clients that either:

• The syntax of the request is wrong.
• The client lacks necessary permissions.
• The resource requested does not exist or is unreachable.
• The client’s request cannot be fulfilled for some other reason related to client behavior or data.

By providing a precise HTTP status within this range, clients can take appropriate action such as retrying, correcting input, or authenticating.

Key Client Error Status Codes Explained

400 Bad Request

This is a generic client error indicating that the server cannot process the request due to malformed syntax. The client should not repeat the request without modification. It typically arises from invalid request formatting, such as:

• Invalid URL encoding
• Missing required headers
• Invalid JSON or XML body in the request

Practical Example:

Submitting a form where the JSON payload is incorrectly structured or the request misses required parameters.

401 Unauthorized

This status indicates that authentication is required and the client has either not provided any credentials or provided invalid credentials. It means the requested resource needs valid user authentication.

Note:

Despite the name, it actually means "unauthenticated," not "unauthorized." The server expects credentials via headers like Authorization.

Practical Example:

Trying to access a private API endpoint without a valid token or after token expiration.

403 Forbidden

This means the client is authenticated but does not have permission to access the requested resource. Unlike 401, credentials won’t help here; the client simply lacks necessary rights.

Practical Example:

Attempting to access an admin page without admin privileges.

404 Not Found

One of the most commonly encountered HTTP codes, 404 means the server cannot find the requested resource. This could mean:

• The URL is incorrect.
• The resource never existed.
• It was deleted or moved without redirection.

Practical Example:

Trying to visit an outdated or broken link on a website.

405 Method Not Allowed

This status indicates that the HTTP method used in the request (GET, POST, DELETE, etc.) is not supported for the targeted resource.

Practical Example:

Sending a POST request to a URL that only accepts GET requests.

406 Not Acceptable

The requested resource is capable of generating only content not acceptable according to the Accept headers sent by the client.

Practical Example:

Client requests a Content-Type that the server cannot provide, such as requesting XML when the server only supports JSON.

407 Proxy Authentication Required

Similar to 401 but specific to proxy servers. This means the client must authenticate itself with the proxy.

Practical Example:

Accessing the internet through a corporate proxy that requires login.

408 Request Timeout

The client took too long to send the full request. This happens when the server times out waiting for the client’s data.

Practical Example:

Slow network connections where the POST body is incomplete for extended periods.

409 Conflict

The request could not be completed because of a conflict with the current state of the resource. This is common in version control or editing scenarios.

Practical Example:

Two users simultaneously trying to update the same document, resulting in a conflict.

410 Gone

Indicates that the resource requested is no longer available and has been permanently removed. Unlike 404, which may suggest the resource is temporarily missing, 410 is permanent.

411 Length Required

The server refuses to accept the request without a valid Content-Length header.

413 Payload Too Large

The request entity is larger than the server is willing or able to process.

414 URI Too Long

The URI requested is longer than the server is willing to interpret. This happens with excessively long query strings or URL-encoded data.

415 Unsupported Media Type

The server refuses to accept the request because the payload’s media format is not supported.

429 Too Many Requests

This means the client has sent too many requests in a given amount of time (rate limiting).

How Clients Should Handle 4xx Errors

• 400-level errors signify issues with the request, so clients must inspect and often modify their request before retrying.
• For authentication-related errors (401, 407), clients should obtain or refresh credentials.
• For permission errors (403), clients may need to contact admins or request elevated rights.
• For resources missing (404, 410), clients should check URLs or handle “not found” gracefully.
• In cases of rate limiting (429), clients must wait or back-off before retrying.

Visual Diagram: Understanding Client Error Flow

text
Client              Server
  |  -- Request -->  |
  |                  |
  |  <-- 4xx Error ---|  (Client-side error in request)
  |                  |
(Adjust request or credentials accordingly)

Summary

Client error status codes (400–499) provide precise feedback about issues with client requests. They enable developers and users to identify what went wrong—from syntax errors, authentication failures, and permission issues, to non-existent resources and too many requests.

Understanding these codes improves debugging, error handling, and user experience in web applications and APIs. By appropriately responding to 4xx errors, clients and servers create smoother interactions and more reliable communications.

References for Further Reading

• MDN Web Docs — HTTP Status Codes
• RFC 7231 — HTTP/1.1 Semantics and Content
• Wikipedia — List of HTTP status codes

If you'd like, I can also create an infographic or visual summary of these client error codes for a clearer understanding. Just let me know!

🔸Server Error Status Codes (500-599)

Server error status codes indicate that the server encountered an issue while processing the request.

→ 500 Internal Server Error 🖥️❌

Technical Explanation :

Practical Strategy :

Real-World Scenario :

Resolution :

→ 502 Bad Gateway 🛤️❌

Technical Explanation :

Practical Strategy :

Real-World Scenario :

Resolution :

→ 503 Service Unavailable 🛠️

Technical Explanation :

Practical Strategy :

Real-World Scenario :

Resolution :

→ 504 Gateway Timeout ⏳

Technical Explanation :

The `504 Gateway Timeout` status code signifies that a server, acting as a gateway or proxy, did not receive a timely response from an upstream server. This typically occurs when the upstream server is slow or unresponsive.

Practical Strategy :

Real-World Scenario :

Resolution :

Conclusion — HTTP status codes

HTTP status codes are more than just technical details—they are integral to effective web communication, problem-solving, and system management. For software developers and DevOps engineers, mastering these codes is essential for diagnosing issues, optimizing performance, and maintaining a robust web infrastructure.

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