HTTP status codes are an essential part of the REST concept. You can get familiar with all of them on Wikipedia.
The Magento API attempts to return appropriate HTTP status codes for all requests. Any information is returned in the form of a standard HTTP response with an HTTP status code describing the error and the body message.
HTTP Status Codes
The following table contains possible common HTTP status codes:
| Status Code | Message |
|---|---|
| 200 OK | - |
| 201 Created | Resource was partially created |
| 207 Multi-Status | - |
| 400 Bad Request | Resource data pre-validation error. Resource data invalid. Resource unknown error. The request data is invalid. Resource collection paging error. The paging limit exceeds the allowed number. Resource collection ordering error. Resource collection filtering error. Resource collection including additional attributes error. |
| 403 Forbidden | Access denied. |
| 404 Not Found | Resource not found. |
| 405 Method Not Allowed | Resource does not support method. Resource method not implemented yet. |
| 500 Internal Error | Unhandled simple errors. Resource internal error. |
Error Messages
When the Magento API returns an error message, it returns it in your requested format. For example, an error in the XML format might look like the following:
<?xml version="1.0"?>
<magento_api>
<messages>
<error>
<data_item>
<code>404</code>
<message>Resource not found.</message>
</data_item>
</error>
</messages>
</magento_api>
An error in the JSON format might look like the following:
{"messages":{"error":[{"code":404,"message":"Resource not found."}]}}