What should a JSON service return on failure error

Question

I m writing a JSON service in C    ashx file   On a successful request to the service I return some JSON data  If the request fails  either because an exception was thrown  e g  database timeout  or because the request was wrong in some way  e g  an ID that doesn t exist in the database was given as an argument  how should the service respond  What HTTP status codes are sensible  and should I return any data  if any   I m anticipating that service will mainly be called from jQuery using the jQuery form plugin  does jQuery or this plugin have any default way of handling an error response   EDIT  I ve decided I ll use jQuery    ashx   HTTP  status codes  on success I ll return JSON but on error I ll return a string  as it appears that that is what the error option for jQuery ajax expects

User · Answer

Rails scaffolds use 422 Unprocessable Entity for these kinds of errors  See RFC 4918 for more information

User · Answer

See this question for some insight into best-practices for your situation   The topline suggestion  from said link  is to standardize a response structure  for both success and failure  that your handler looks for  catching all Exceptions at the server layer and converting them to the same structure  For example  from this answer          success false      general message  You have reached your max number of Foos for the day       errors            last name  This field is required           mrn  Either SSN or MRN must be entered           zipcode  996852 is not in Bernalillo county  Only Bernalillo residents are eligible             This is the approach stackoverflow uses  in case you were wondering how others do this kind of thing   write operations like voting have  Success  and  Message  fields  regardless of if the vote was allowed or not     Success true  NewScore 1  Message     LastVoteTypeId 3     As  Phil H pointed out  you should be consistent in whatever you choose  This is easier said than done  as is everything in development      For example  if you submit comments too quickly on SO  instead of being consistent and returning    Success  false  Message   Can only comment once every blah         SO will throw a server exception  HTTP 500  and catch it in their error callback   As much as it  feels right  to use jQuery    ashx   HTTP  status codes  IMO it will add more complexity to your client-side code base than it s worth  Realize that jQuery does not  detect  error codes but rather the lack of a success code  This is an important distinction when trying to design a client around http response codes with jQuery  You only get two choices  was it a  success  or  error     which you have to branch further on your own  If you have a small number of WebServices driving a small number of pages then it might be okay  but anything larger scale may get messy   It s much more natural in a  asmx WebService  or WCF for that matter  to return a custom object than to customize the HTTP status code  Plus you get the JSON serialization for free

User · Answer

Using HTTP status codes would be a RESTful way to do it  but that would suggest you make the rest of the interface RESTful using resource URIs and so on    In truth  define the interface as you like  return an error object  for example  detailing the property with the error  and a chunk of HTML that explains it  etc   but once you ve decided on something that works in a prototype  be ruthlessly consistent

User · Answer

For server protocol errors I would try to be as REST HTTP as possible  Compare this with you typing in URL s in your browser     a non existing item is called   persons  non-existing-id-here    Return a 404  an unexpected error on the server  code bug  occured  Return a 500  the client user is not authorised to get the resource  Return a 401    For domain business logic specific errors I would say the protocol is used in the right way and there s no server internal error  so respond with an error JSON XML object or whatever you prefer to describe your data with  Compare this with you filling in forms on a website     a user wants to change its account name but the user did not yet verify its account by clicking a link in an email which was sent to the user  Return   error   Account not verified   or whatever  a user wants to order a book  but the book was sold  state changed in DB  and can t be ordered anymore  Return   error   Book already sold

User · Answer

Yes  you should use HTTP status codes  And also preferably return error descriptions in a somewhat standardized JSON format  like Nottingham   s proposal  see apigility Error Reporting      The payload of an API Problem has the following structure             type  a URL to a document describing the error condition  optional  and  about blank  is   assumed if none is provided  should resolve to a human-readable document  Apigility always   provides this        title  a brief title for the error condition  required  and should be the same for every   problem of the same type  Apigility always provides this        status  the HTTP status code for the current request  optional  Apigility always provides this        detail  error details specific to this request  optional  Apigility requires it for each   problem        instance  URI identifying the specific instance of this problem  optional  Apigility currently   does not provide this

User · Answer

The HTTP status code you return should depend on the type of error that has occurred  If an ID doesn t exist in the database  return a 404  if a user doesn t have enough privileges to make that Ajax call  return a 403  if the database times out before being able to find the record  return a 500  server error    jQuery automatically detects such error codes  and runs the callback function that you define in your Ajax call  Documentation  http   api jquery com jQuery ajax   Short example of a   ajax error callback     ajax     type   POST     url    some resource     success  function data  textStatus           Handle success        error  function xhr  textStatus  errorThrown           Handle error

User · Answer

I think if you just bubble an exception  it should be handled in the jQuery callback that is passed in for the  error  option    We also log this exception on the server side to a central log    No special HTTP error code required  but I m curious to see what other folks do  too   This is what I do  but that s just my   02    If you are going to be RESTful and return error codes  try to stick to the standard codes set forth by the W3C  http   www w3 org Protocols rfc2616 rfc2616-sec10 html

User · Answer

I would definitely return a 500 error with a JSON object describing the error condition  similar to how an ASP NET AJAX  ScriptService  error returns   I believe this is fairly standard   It s definitely nice to have that consistency when handling potentially unexpected error conditions   Aside  why not just use the built in functionality in  NET  if you re writing it in C    WCF and ASMX services make it easy to serialize data as JSON  without reinventing the wheel

User · Answer

I don t think you should be returning any http error codes  rather custom exceptions that are useful to the client end of the application so the interface knows what had actually occurred   I wouldn t try and mask real issues with 404 error codes or something to that nature

User · Answer

If the user supplies invalid data  it should definitely be a 400 Bad Request  The request contains bad syntax or cannot be fulfilled

User · Answer

I ve spend some hours solving this problem  My solution is based on the following wishes requirements    Don t have repetitive boilerplate error handling code in all JSON controller actions  Preserve HTTP  error  status codes  Why  Because higher level concerns should not affect lower level implementation  Be able to get JSON data when an error exception occur on the server  Why  Because I might want rich error information  E g  error message  domain specific error status code  stack trace  in debug development environment   Ease of use client side - preferable using jQuery    I create a HandleErrorAttribute  see code comments for explanation of the details   A few details including  usings  has been left out  so the code might not compile  I add the filter to the global filters during application initialization in Global asax cs like this   GlobalFilters Filters Add new UnikHandleErrorAttribute       Attribute   namespace Foo     using System    using System Diagnostics    using System Linq    using System Net    using System Reflection    using System Web    using System Web Mvc          lt summary gt        Generel error handler attribute for Foo MVC solutions        It handles uncaught exceptions from controller actions        It outputs trace information        If custom errors are enabled then the following is performed         lt ul gt           lt li gt If the controller action return type is  lt see cref  JsonResult   gt  then a  lt see cref  JsonResult   gt  object with a  lt c gt message lt  c gt  property is returned              If the exception is of type  lt see cref  MySpecialExceptionWithUserMessage   gt  it s message will be used as the  lt see cref  JsonResult   gt   lt c gt message lt  c gt  property value              Otherwise a localized resource text will be used  lt  li gt         lt  ul gt        Otherwise the exception will pass through unhandled         lt  summary gt     AttributeUsage AttributeTargets Class   AttributeTargets Method     public sealed class FooHandleErrorAttribute   HandleErrorAttribute         private readonly TraceSource  TraceSource            lt summary gt           lt paramref name  traceSource   gt  must not be null           lt  summary gt           lt param name  traceSource  gt  lt  param gt      public FooHandleErrorAttribute TraceSource traceSource              if  traceSource    null          throw new ArgumentNullException   traceSource           TraceSource   traceSource             public TraceSource TraceSource             get                 return  TraceSource                          lt summary gt          Ctor           lt  summary gt      public FooHandleErrorAttribute               var className   typeof FooHandleErrorAttribute  FullName    typeof FooHandleErrorAttribute  Name         TraceSource   new TraceSource className              public override void OnException ExceptionContext filterContext              var actionMethodInfo   GetControllerAction filterContext Exception            It s probably an error if we cannot find a controller action  But  hey  what should we do about it here        if actionMethodInfo    null  return         var controllerName   filterContext Controller GetType   FullName     filterContext RouteData Values   controller          var actionName   actionMethodInfo Name     filterContext RouteData Values   action              Log the exception to the trace source       var traceMessage   string Format   Unhandled exception from  0   1  handled in  2   Exception   3    controllerName  actionName  typeof FooHandleErrorAttribute  FullName  filterContext Exception          TraceSource TraceEvent TraceEventType Error  TraceEventId UnhandledException  traceMessage             Don t modify result if custom errors not enabled         if   filterContext HttpContext IsCustomErrorEnabled            return            We only handle actions with return type of JsonResult - I don t use AjaxRequestExtensions IsAjaxRequest   because ajax requests does NOT imply JSON result            The downside is that you cannot just specify the return type as ActionResult - however I don t consider this a bad thing        if  actionMethodInfo ReturnType    typeof JsonResult   return            Handle JsonResult action exception by creating a useful JSON object which can be used client side          Only provide error message if we have an MySpecialExceptionWithUserMessage        var jsonMessage   FooHandleErrorAttributeResources Error Occured        if  filterContext Exception is MySpecialExceptionWithUserMessage  jsonMessage   filterContext Exception Message        filterContext Result   new JsonResult                     Data   new                             message   jsonMessage                   Only include stacktrace information in development environment               stacktrace   MyEnvironmentHelper IsDebugging   filterContext Exception StackTrace   null                             Allow JSON get requests because we are already using this approach  However  we should consider avoiding this habit            JsonRequestBehavior   JsonRequestBehavior AllowGet                      Exception is now  being  handled - set the HTTP error status code and prevent caching  Otherwise you ll get an HTTP 200 status code and running the risc of the browser caching the result        filterContext ExceptionHandled   true        filterContext HttpContext Response StatusCode    int HttpStatusCode InternalServerError     Consider using more error status codes depending on the type of exception       filterContext HttpContext Response Cache SetCacheability HttpCacheability NoCache             Call the overrided method       base OnException filterContext                   lt summary gt          Does anybody know a better way to obtain the controller action method info          See http   stackoverflow com questions 2770303 how-to-find-in-which-controller-action-an-error-occurred           lt  summary gt           lt param name  exception  gt  lt  param gt           lt returns gt  lt  returns gt      private static MethodInfo GetControllerAction Exception exception              var stackTrace   new StackTrace exception         var frames   stackTrace GetFrames          if frames    null  return null        var frame   frames FirstOrDefault f   gt  typeof IController  IsAssignableFrom f GetMethod   DeclaringType          if  frame    null  return null        var actionMethod   frame GetMethod          return actionMethod as MethodInfo                I ve developed the following jQuery plugin for client side ease of use    function     undefined       using strict        FooGetJSON   function  url  data  success  error             lt summary gt                                                                               UNIK GET JSON JQUERY PLUGIN                                                                                                         This plugin is a wrapper for jQuery getJSON          The reason is that jQuery getJSON success handler doesn t provides access to the JSON object returned from the url         when a HTTP status code different from 200 is encountered  However  please note that whether there is JSON         data or not depends on the requested service  if there is no JSON data  i e  response responseText cannot be         parsed as JSON  then the data parameter will be undefined                  This plugin solves this problem by providing a new error handler signature which includes a data parameter          Usage of the plugin is much equal to using the jQuery getJSON method  Handlers can be added etc  However          the only way to obtain an error handler with the signature specified below with a JSON data parameter is         to call the plugin with the error handler parameter directly specified in the call to the plugin                  success  function data  textStatus  jqXHR          error  function data  jqXHR  textStatus  errorThrown                  Example usage                      FooGetJSON   foo     id  42    function data    alert  Name      data name      function data    alert  Error      data message                lt  summary gt          Call the ordinary jQuery method     var jqxhr     getJSON url  data  success           Do the error handler wrapping stuff to provide an error handler with a JSON object - if the response contains JSON object data     if  typeof error      undefined           jqxhr error function response  textStatus  errorThrown            try             var json     parseJSON response responseText             error json  response  textStatus  errorThrown             catch e              error undefined  response  textStatus  errorThrown                                     Return the jQueryXmlHttpResponse object     return jqxhr          jQuery     What do I get from all this  The final result is that   None of my controller actions has requirements on HandleErrorAttributes  None of my controller actions contains any repetitive boiler plate error handling code  I have a single point of error handling code allowing me to easily change logging and other error handling related stuff  A simple requirement  Controller actions returning JsonResult s must have return type JsonResult and not some base type like ActionResult  Reason  See code comment in FooHandleErrorAttribute    Client side example   var success   function data      alert data myjsonobject foo      var onError   function data      var message    Error     if typeof data      undefined       message           data message    alert message        FooGetJSON url  params  onSuccess  onError     Comments are most welcome  I ll probably blog about this solution some day

[jquery] What should a JSON service return on failure / error

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