HTTP 400 bad request for logical error not malformed request syntax

Question

The HTTP 1 1 specification  RFC 2616  has the following to say on the meaning of status code 400  Bad Request    10 4 1       The request could not be understood by   the server due to malformed syntax    The client SHOULD NOT repeat the   request without modifications    There seems to be a general practice among a few HTTP-based APIs these days to use 400 to mean a logical rather than a syntax error with a request  My guess is that APIs are doing this to distinguish between 400  client-induced  and 500  server-induced   Is it acceptable or incorrect to use 400 to indicate non-syntactic errors  If it is acceptable  is there an annotated reference on RFC 2616 that provides more insight into the intended use of 400   Examples    Google Data Protocol  Protocol Reference  HTTP Status Codes

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In my case   I am getting 400 bad request because I set content-type wrongly  I changed content type then able to get response successfully   Before  Issue    ClientResponse response   Client create   resource requestUrl  queryParam  noOfDates   String valueOf limit                    header SecurityConstants AUTHORIZATION  formatedToken   header  Content-Type      application json     get ClientResponse class     After  Fixed    ClientResponse response   Client create   resource requestUrl  queryParam  noOfDates   String valueOf limit                    header SecurityConstants AUTHORIZATION  formatedToken   header  Content-Type      application x-www-form-urlencoded     get ClientResponse class

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Status 422  RFC 4918  Section 11 2  comes to mind      The 422  Unprocessable Entity  status code means the server understands the content type of the request entity  hence a 415 Unsupported Media Type  status code is inappropriate   and the syntax of the request entity is correct  thus a 400  Bad Request  status code is inappropriate  but was unable to process the contained instructions  For example  this error condition may occur if an XML request body contains well-formed  i e   syntactically correct   but semantically erroneous  XML instructions

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It could be argued that having incorrect data in your request is a syntax error  even if your actual request at the HTTP level  request line  headers etc  is syntactically valid   For example  if a Restful web service is documented as accepting POSTs with a custom XML  Content Type of application vnd example com widget xml  and you instead send some gibberish plain text or a binary file  it seems resasonable to treat that as a syntax error - your request body is not in the expected form   I don t know of any official references to back this up though  as usual it seems to be down to interpreting RFC 2616   Update  Note the revised wording in RFC 7231   6 5 1      The 400  Bad Request  status code indicates that 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     seems to support this argument more than the now obsoleted RFC 2616   10 4 1 which said just      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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As of this time  the latest draft of the HTTPbis specification  which is intended to replace and make RFC 2616 obsolete  states      The 400  Bad Request  status code indicates that the server cannot or   will not process the request because the received syntax is invalid     nonsensical  or exceeds some limitation on what the server is willing   to process    This definition  while of course still subject to change  ratifies the widely used practice of responding to logical errors with a 400

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This is difficult   I think we should    Return 4xx errors only when the client has the power to make a change to the request  headers or body  that will result in the request succeeding with the same intent  Return error range codes when the expected mutation has not occured  i e  a DELETE didn t happen or a PUT didn t change anything  However  a POST is more interesting because the spec says it should be used to either create resources at a new location  or just process a payload    Using the example in Vish s answer  if the request intends to add employee Priya to a department Marketing but Priya wasn t found or her account is archived  then this is an application error   The request worked fine  it got to your application rules  the client did everything properly  the ETags matched etc  etc   Because we re using HTTP we must respond based on the effect of the request on the state of the resource  And that depends on your API design   Perhaps you designed this   PUT   updated members list    marketing members   Returning a success code would indicate that the  replacement  of the resource worked  a GET on the resource would reflect your changes  but it wouldn t   So now you have to choose a suitable negative HTTP code  and that s the tricky part  since the codes are strongly intended for the HTTP protocol  not your application   When I read the official HTTP codes  these two look suitable      The 409  Conflict  status code indicates that the request could not be completed due to a conflict with the current state of the target resource   This code is used in situations where the user might be able to resolve the conflict and resubmit the request   The server SHOULD generate a payload that includes enough information for a user to recognize the source of the conflict    And     The 500  Internal Server Error  status code indicates that the server encountered an unexpected condition that prevented it from fulfilling the request    Though we ve traditionally considered the 500 to be like an unhandled exception  -   I don t think its unreasonable to invent your own status code so long as its consistently applied and designed   This design is easier to deal with   PUT   membership add command    accounts groups memberships instructions 1739119   Then you could design your API to always succeed in creating the instruction  it returns 201 Created and a Location header and any problems with the instruction are held within that new resource   A POST is more like that last PUT to a new location  A POST allows for any kind of server processing of a message  which opens up designs that say something like  The action successfully failed    Probably you already wrote an API that does this  a website  You POST the payment form and it was successfully rejected because the credit card number was wrong   With a POST  whether you return 200 or 201 along with your rejection message depends on whether a new resource was created and is available to GET at another location  or not   With that all said  I d be inclined to design APIs that need fewer PUTs  perhaps just updating data fields  and actions and stuff that invokes rules and processing or just have a higher chance of expected failures  can be designed to POST an instruction form

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HTTPbis will address the phrasing of 400 Bad Request so that it covers logical errors as well  So 400 will incorporate 422   From https   tools ietf org html draft-ietf-httpbis-p2-semantics-18 section-7 4 1  The server cannot or will not process the request  due to a client error  e g   malformed syntax

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On Java EE servers a 400 is returned if your URL refers to a non-existent  web -application   Is that a  syntax error   Depends on what you mean by syntax error  I would say yes   In English syntax rules prescribe certain relationships between parts of speech  For instance  Bob marries Mary  is syntactically correct   because it follows the pattern  Noun   Verb   Noun    Whereas  Bob marriage Mary  would be syntactically incorrect   Noun   Noun   Noun    The syntax of a simple URLis   protocol            server       port    According to this  http   www google com 80  is syntactically correct     But what about  abc   www google com 80   It seems to follow the exact same pattern  But really it is a syntax error  Why  Because  abc  is not a DEFINED protocol   The point is that determining whether or not we have a 400 situation requires more  than parsing the characters and spaces and delimiters  It must also recognize what are the valid  parts of speech

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Even though  I have been using 400 to represent logical errors also  I have to say that returning 400 is wrong in this case because of the way the spec reads  Here is why i think so  the logical error could be that a relationship with another entity was failing or not satisfied and making changes to the other entity could cause the same exact to pass later  Like trying to  completely hypothetical  add an employee as a member of a department when that employee does not exist  logical error   Adding employee as member request could fail because employee does not exist  But the same exact request could pass after the employee has been added to the system   Just my 2 cents     We need lawyers  amp  judges to interpret the language in the RFC these days     Thank You  Vish

[http] HTTP 400 (bad request) for logical error, not malformed request syntax

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