Restful API - JSON response with single or all error(s) - json

Question

My current project, a restful API, validates a POST request to create a new user and multiple errors could occur (with HTTP status):
username not set (400 BadRequest)
username is taken (409 Conflict)
server can't establish db connection (500 Internal Server Error)
...
Should I immediatly send back a JSON response like this
{
"status": 400,
"Message": "No username is set"
}
if an error was detected or should is it better if I accumulate all errors like here:
{
"status": <HTTP STATUS CODE>,
"errors": [
{"message": "Username is not set."},
{"message": "Can't access the database."}
]
}
The last approach would not require multiple request to send a valid payload. But which status code should be used, if the username is not set (400 Bad Request) or the server can't access the database (500 Internal Server Error)?

Answer 1

I think if you foresee the need for multiple errors in one request, then the second JSON (with the multiple errors) is preferred. Another benefit of doing the multiple errors response is that as a user of your service, if i get back multiple errors, I can address them all at one shot, instead of addressing them one at a time as I get them.

Answer 2

After some research, the best (most standard) way to respond is a JSON structure of this form:
{
"error": {
"code": "400",
"message": "main error message here",
"target": "approx what the error came from",
"details": [
{
"code": "23-098a",
"message": "Disk drive has frozen up again. It needs to be replaced",
"target": "not sure what the target is"
}
],
"innererror": {
"trace": [ ... ],
"context": [ ... ]
}
}
}
The multiple errors you want to place would be individual elements in the "details" array. Using this structure you would still need some overall summary of the error, however the details would contain as many individual error messages as you want.
This is the format proposed by the OASIS data standard OASIS OData and seems to be the most standard option out there, however there does not seem to be high adoption rates of any standard at this point.
This also conforms to the JSON RPC 2.0 Spec as it requires that the error be an object in a "error" member, and that you have a code and message.
You can find the complete open source library that implements this at: Mendocino JSON Utilities. This library supports the JSON Objects as well as the exceptions.
The details are discussed in my blog post on Error Handling in JSON REST API