200 OK

The "200 OK" status code is a standard response in the HTTP protocol that indicates a request has succeeded. The meaning of success varies depending on the HTTP method:

• GET: The resource has been fetched and is transmitted in the message body.
• HEAD: The entity headers are in the message body.
• POST: The resource describing the result of the action is transmitted in the message body.
• TRACE: The message body contains the request message as received by the server.

This status code is the most common and is the default status for a successful HTTP response. When you visit a web page and everything loads properly, your browser has likely received a 200 OK status code from the server, indicating that the page exists and was retrieved without issue.

In API interactions, a 200 OK status signifies that the API successfully processed the request and is returning any requested information in the response body, such as data in a REST API call.

When building web services or APIs, properly handling HTTP status codes is essential to provide clear communication between the client (e.g., a web browser or an API consumer) and the server.

4XX

The `4XX` class of HTTP status codes is intended to indicate errors caused by the client. When a server responds with a `4XX` code, it suggests that the client made an error, such as requesting a resource that doesn't exist or failing to authenticate. Below are some of the most common `4XX` status codes:

• 400 Bad Request: The server cannot or will not process the request due to an apparent client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).
• 401 Unauthorized: Although the HTTP standard specifies "unauthorized," semantically this response means "unauthenticated." That is, the client must authenticate itself to get the requested response.
• 403 Forbidden: The client does not have access rights to the content; that is, it is unauthorized, so the server is refusing to give the requested resource. Unlike 401, the client's identity is known to the server.
• 404 Not Found: The server can't find the requested resource. In the browser, this means the URL is not recognized. In an API, it can also mean that the endpoint is valid but the resource itself does not exist. Servers may also send this response instead of 403 to hide the existence of a resource from an unauthorized client.
• 405 Method Not Allowed: The request method is known by the server but has been disabled and cannot be used. For example, an API may not allow calling DELETE on a particular resource.
• 406 Not Acceptable: This response is sent when the web server, after performing server-driven content negotiation, doesn't find any content following the criteria given by the user agent.
• 409 Conflict: This response is sent when a request conflicts with the current state of the server.
• 410 Gone: This response is sent when the requested content has been permanently deleted from the server, with no forwarding address.
• 429 Too Many Requests: The user has sent too many requests in a given amount of time ("rate limiting").

These responses inform the client that it should change the request or the process to repeat the request. The specific action depends on the error code and the details of the response. For instance, if a client receives a 401, it should repeat the request with proper authentication headers. If it gets a 404, it might need to correct the resource URL or accept that the resource is no longer available.

5XX

The `5XX` class of HTTP status codes indicates that the server is aware it has encountered an error or is otherwise incapable of performing the request. In contrast to `4XX` codes, which signal client-side errors, `5XX` errors suggest that the client's request was valid, but the server failed to fulfill it. Here are some of the common `5XX` status codes:

• 500 Internal Server Error: A generic error message, given when no more specific message is suitable. This is a catch-all response for when the server encounters an unexpected condition that prevented it from fulfilling the request.
• 501 Not Implemented: The server either does not recognize the request method, or it lacks the ability to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.
• 502 Bad Gateway: The server was acting as a gateway or proxy and received an invalid response from the upstream server. This often occurs when one server receives an invalid or unexpected response from another server it is communicating with.
• 503 Service Unavailable: The server is not ready to handle the request. Common causes are a server that is down for maintenance or that is overloaded. This error implies that this is a temporary condition.
• 504 Gateway Timeout: The server was acting as a gateway or proxy and did not receive a timely response from the upstream server. This can occur when an upstream server is down or not functioning correctly.
• 505 HTTP Version Not Supported: The server does not support the HTTP protocol version used in the request. This is typically a technical issue where the client is trying to use a newer or older HTTP version than the server supports.

When a server issues a `5XX` error, it's generally a sign that something has gone wrong on the server side. For the user or client making the request, there isn't much they can do except perhaps try the request again later, as these errors imply an issue that needs to be resolved by the server administrator or the individual responsible for maintaining the web server or application.

301

The HTTP `301 Moved Permanently` status code indicates that the URL of the requested resource has been changed permanently, and any further inquiries should be directed to the new URL. It is an important tool for maintaining SEO rankings when moving content from one URL to another.

When a server returns a `301` status code, it also provides a new URL in the Location header field of the HTTP response. The client (a web browser or a search engine crawler, for example) is expected to automatically follow the redirection to the new URL provided.

Here's what typically happens when a `301` status code is involved:

1. Browser Request: A user's browser requests a page at `http://oldsite.com/page`.

2. Server Response: The server at `oldsite.com` returns a `301` status code and a `Location` header with `http://newsite.com/page`.

3. Browser Follows: The user's browser automatically makes a new request to the URL provided in the `Location` header.

4. Server Responds Again: The server at `newsite.com` responds with the content of the page, typically with a `200 OK` status, assuming no other redirection or errors occur.

Use cases for a `301` redirect include:

• Redirecting from an old website to a new one.
• Ensuring that users and search engines are directed to the correct URL when accessing a site that is accessible via multiple URLs (e.g., redirecting from `www.example.com` to `example.com`).
• Migrating to a new domain name while preserving search engine rankings.
• Redirecting traffic from old or deprecated product pages to newer versions or alternatives on an e-commerce website.

It's important to use `301` redirects wisely and to remember that they are cached by browsers, meaning that once a user hits a `301` redirect, their browser will remember the new location and automatically go there in the future without checking the original location. This is why it is termed "permanent."

302

The HTTP `302 Found` status code indicates that the requested resource is temporarily located at a different URI. Since the redirection might be altered on occasion, the client should continue to use the effective request URI for future requests.

Here's how it generally works:

1. Client Request: A user's browser or client software makes a request for a resource at `http://example.com/page`.

2. Server Response: The server at `example.com` responds with a `302` status code and includes a `Location` header in the response specifying the temporary URI where the resource can be found, such as `http://example.com/temporary-page`.

3. Client Follows Redirect: The client acknowledges the `302` status and makes a new request to the URI provided in the `Location` header.

4. Temporary Content Delivery: The server at the temporary URI then serves the requested resource, typically responding with a `200 OK` status if everything is fine.

The key feature of a `302` redirect is that it's meant to be temporary. It tells clients and search engines that the original URL is still valid and important, and they should check back next time instead of replacing the old URL with the new one in their databases.

Use cases for a `302` redirect include:

• A/B testing of a new page to measure how it performs in comparison to the original.
• User-specific redirection, where different information is presented to different users, but the original page should remain indexed by search engines.
• Situations where content is temporarily moved, such as site maintenance or a temporary promotion, but the original URL will be restored soon.

It's important to use the `302` status code correctly and not to confuse it with `301 Moved Permanently`, which indicates a permanent change and can affect search engine indexing. Using a `302` when content has permanently moved may prevent search engines from updating their indexes to reference the new location.

500

The HTTP `500 Internal Server Error` status code is a generic error message indicating that the server encountered an unexpected condition that prevented it from fulfilling the request. This error is a catch-all for when the server throws an exception that is not specifically handled or when there is no more specific error message suitable.

When a `500 Internal Server Error` occurs, the problem lies with the server's software or hardware or an issue on the server's end that is outside the client's control. It is often a server-side script error, a server misconfiguration, or a problem with the server's resources.

Here’s what typically happens:

1. Client Request: The client, which could be a browser, application, or search engine, sends a request to the server.

2. Server Processing: The server tries to process the request but encounters an unexpected condition or error.

3. Error Response: Instead of returning the expected content, the server sends back an HTTP response with the `500` status code.

4. Client Reaction: The client displays an error message to the user, such as "500 Internal Server Error". The specifics of the message can vary depending on the client and the context of the request.

From a webmaster or operator perspective, when a `500` error occurs, logs should be checked to identify the cause of the error. This might involve:

• Checking server error logs to see what caused the error.
• Reviewing server code for unhandled exceptions or errors.
• Ensuring that all server software is up-to-date and properly configured.
• Investigating any server resource issues, such as database availability or server load. 

For end-users, a `500` error can be frustrating because it is typically not something they can fix. Usually, the best course of action for users is to refresh the page or come back later when the server issue has likely been resolved. If the issue persists, users might contact the website owner or support team to notify them of the problem.

404

The HTTP `404 Not Found` status code is an error message indicating that the server could not find the requested resource. This response is one of the most recognized due to its frequent occurrence on the web. It signifies that the client was able to communicate with the server, but the server could not find what was requested.

Here's the typical flow when a `404` error occurs:

1. Client Request: A user requests a URL or a resource from a server by using a web browser, an API call, or another method.

2. Server Response: The server processes the request and looks for the resource.

3. Resource Missing: The server cannot find the requested resource. It may be missing, or the URL may be incorrect or typed wrongly by the user.

4. 404 Error: The server sends back a `404 Not Found` status code in the HTTP response, indicating the absence of the requested resource.

A `404 Not Found` error could occur due to various reasons:

• The URL or its content (files or images) might have been removed or moved to another URL.
• There could be a mistake in the URL entered by the user.
• The server might be configured not to fulfill the request without revealing the reason why, typically for security reasons.

What should be done after encountering a `404` error varies depending on the context:

• As a User: Check the URL in the address bar for any spelling mistakes, try navigating to the homepage and looking for the content, use a search function if available, or reach out to the website for assistance.
• As a Webmaster or Developer: Investigate the reason for the `404` error. Ensure that the content exists at the specified URL, check for broken links, and consider implementing user-friendly custom `404` error pages that can guide users back to active parts of your website.

For SEO and user experience, it's generally advised to fix `404` errors where possible, by either restoring the missing content or by redirecting the old URL to a new, relevant destination using a `301` or `302` status code if the move is permanent or temporary, respectively.