How to return error messages and codes in RESTful Web Services

Consider the 4xx or 5xx HTTP status codes as error categories. You can elaborate the error in the body.

Failed to Connect to Database: / Incorrect Database Login: In general you should use a 500 error for these types of errors. This is a server-side error. The client did nothing wrong. 500 errors are normally considered “retryable”. i.e. the client can retry the same exact request, and expect it to succeed once the server’s troubles are resolved. Specify the details in the body, so that the client will be able to provide some context to us humans.

The other category of errors would be the 4xx family, which in general indicate that the client did something wrong. In particular, this category of errors normally indicate to the client that there is no need to retry the request as it is, because it will continue to fail permanently. i.e. the client needs to change something before retrying this request. For example, “Resource not found” (HTTP 404) or “Malformed Request” (HTTP 400) errors would fall in this category.