400 BAD request HTTP error code meaning

Question

I have a JSON request which I m posting to a HTTP URL    Should this be treated as 400 where requestedResource field exists but  Roman  is an invalid value for this field      requestedResource  Roman       Should this be treated as 400 where  blah  field doesn t exist at all     blah  Roman

User · Answer

From w3 org     10 4 1 400 Bad Request      The request could not be understood by the server due to malformed   syntax  The client SHOULD NOT repeat the request without   modifications

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Selecting a HTTP response code is quite an easy task and can be described by simple rules  The only tricky part which is often forgotten is paragraph 6 5 from RFC 7231      Except when responding to a HEAD request  the server SHOULD send a   representation containing an explanation of the error situation    and whether it is a temporary or permanent condition    Rules are as following    If request was successful  then return 2xx code  3xx for redirect   If there was an internal logic error on a server  then return 5xx  If there is anything wrong in client request  then return 4xx code  Look through available response code from selected category  If one of them has a name which matches well to your situation  you can use it  Otherwise just fallback to x00 code  200  400  500   If you doubt  fallback to x00 code  Return error description in response body  For 4xx codes it must contain enough information for client developer to understand the reason and fix the client  For 5xx because of security reasons no details must be revealed  If client needs to distinguish different errors and have different reaction depending on it  define a machine readable and extendible error format and use it everywhere in your API  It is good practice to make that from very beginning  Keep in mind that client developer may do strange things and try to parse strings which you return as human readable description  And by changing the strings you will break such badly written clients  So always provide machine readable description and try to avoid reporting additional information in text    So in your case I d returned 400 error and something like this if  Roman  is obtained from user input and client must have specific reaction          error type     unsupported resource        error description       Roman   is not supported      or a more generic error  if such situation is a bad logic error in a client and is not expected  unless developer made something wrong          error type     malformed json        error description       Roman   is not supported for   requestedResource   field

User · Answer

Think about expectations   As a client app  you expect to know if something goes wrong on the server side  If the server needs to throw an error when blah is missing or the requestedResource value is incorrect than a 400 error would be appropriate

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First check the URL it might be wrong  if it is correct then check the request body which you are sending  the possible cause is request that you are sending is missing right syntax   To elaborate   check for special characters in the request string  If it is  special char  being used this is the root cause of this error   try copying the request and analyze each and every tags data

User · Answer

A 400 means that the request was malformed  In other words  the data stream sent by the client to the server didn t follow the rules  In the case of a REST API with a JSON payload  400 s are typically  and correctly I would say  used to indicate that the JSON is invalid in some way according to the API specification for the service  By that logic  both the scenarios you provided should be 400s  Imagine instead this were XML rather than JSON  In both cases  the XML would never pass schema validation--either because of an undefined element or an improper element value  That would be a bad request  Same deal here

User · Answer

As a complementary  for those who might meet the same issue as mine  I m using   ajax to post form data to server and I also got the 400 error at first   Assume I have a javascript variable   var formData          name   Gearon        hobby   Be different            Do not use variable formData directly as the value of key data like below     ajax       type   post       dataType   json       url   http   localhost user add       contentType   application json       data  formData      success  function data  textStatus           alert  Data      data     nStatus      status                Instead  use JSON stringify to encapsulate the formData as below     ajax       type   post       dataType   json       url   http   localhost user add       contentType   application json       data  JSON stringify formData       success  function data  textStatus           alert  Data      data     nStatus      status                Anyway  as others have illustrated  the error is because the server could not recognize the request cause malformed syntax  I m just raising a instance at practice  Hope it would be helpful to someone

User · Answer

Using 400 status codes for any other purpose than indicating that the request is malformed is just plain wrong   If the request payload contains a byte-sequence that could not be parsed as application json  if the server expects that dataformat   the appropriate status code is 415      The server is refusing to service the request because the entity of   the request is in a format not supported by the requested resource for   the requested method    If the request payload is syntactically correct but semantically incorrect  the non-standard 422 response code may be used  or the standard 403 status code      The server understood the request  but is refusing to fulfill it    Authorization will not help and the request SHOULD NOT be repeated

User · Answer

In neither case is the  syntax malformed   It s the semantics that are wrong  Hence  IMHO a 400 is inappropriate  Instead  it would be appropriate to return a 200 along with some kind of error object such as    error      message    Unknown request keyword      or whatever   Consider the client processing path s   An error in syntax  e g  invalid JSON  is an error in the logic of the program  in other words a bug of some sort  and should be handled accordingly  in a way similar to a 403  say  in other words  something bad has gone wrong   An error in a parameter value  on the other hand  is an error of semantics  perhaps due to say poorly validated user input  It is not an HTTP error  although I suppose it could be a 422   The processing path would be different   For instance  in jQuery  I would prefer not to have to write a single error handler that deals with both things like 500 and some app-specific semantic error  Other frameworks  Ember for one  also treat HTTP errors like 400s and 500s identically as big fat failures  requiring the programmer to detect what s going on and branch depending on whether it s a  real  error or not

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