HTTP Status Code 400 - Bad Request

This response means that server could not understand the request due to invalid syntax.

The server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.

Wikipedia

The request cannot be fulfilled due to bad syntax.

General error when fulfilling the request would cause an invalid state. Domain validation errors, missing data, etc. are some examples.

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HTTP Status Code: 200 - Ok

The request has succeeded. The 200 (OK) status code indicates that the request has succeeded. The payload sent in a 200 response depends on the request method. For the methods defined by this specification, the intended meaning of the payload can be summarized as: GET a representation of the target resource HEAD the same representation as GET, but without the representation data; POST a representation of the status of, or results obtained from, the action; PUT DELETE a representation of the status of the action; OPTIONS a representation of the communications options; TRACE a representation of the request message as received by the end server. Aside from responses to CONNECT, a 200 response always has a payload, though an origin server MAY generate a payload body of zero length. If no payload is desired, an origin server ought to send 204 No Content instead. For CONNECT, no payload is allowed because the successful result is a tunnel, which begin...

HTTP Status Code 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. The request has not been applied because it lacks valid authentication credentials for the target resource. The server generating a 401 response MUST send a WWW-Authenticate header field containing at least one challenge applicable to the target resource. If the request included authentication credentials, then the 401 response indicates that authorization has been refused for those credentials. The user agent MAY repeat the request with a new or replaced Authorization header field If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user agent SHOULD present the enclosed representation to the user, since it usually contains relevant diagnostic information. The request requ...

HTTP Status Code 304 - Not Modified

This is used for caching purposes. It is telling to client that response has not been modified. So, client can continue to use same cached version of response. A conditional GET or HEAD request has been received and would have resulted in a 200 OK response if it were not for the fact that the condition evaluated to false. If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The 304 response MUST NOT contain a message-body, and thus is always terminated by the first empty line after the header fields. The response MUST include the following header fields: If a clockless origin server obeys these rules, and proxies and clients add their own Date to any response received without one, caches will operate correctly. ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request Expires, Cache-Control, and/or Vary, if t...

HTTP Status Codes

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