HTTP Status codes help categorize the response. Sometimes they are self-explanatory (e.g. 404) and sometimes they are backed by information (e.g. 201). A REST API MUST implement these status codes well to convey the right information to its clients. Correct status codes help client app developers handle responses better. Here will focus more on 4XX Status Codes for communicating errors with the REST clients.
2XX Content
Browsers generate a success indicator for the 2XX status code. So 2XX status codes should be used to specify a successful request.
201 Created - Used for POST request to create resource.
202 Accepted - Request accepted by server, but cannot respond immediately.
203 Non-Authoritative Information - Retrieve information expected from 202 request.
204 No Content - Response doesn't have a payload.
When the server has a long operation to be performed, it responds with 202. But since REST is stateless, it cannot respond to the request later. The client, therefore after a certain interval, requests again for the same resource and gets the data intended for request with 202 response. This follow-up request is replied with status 203 by the server.
3XX Redirect
The REST API developer should maintain old resources, in case he is migrating to new ones. 3XX series codes are displayed as errors in the browser console and should be used to indicate resource relocation.
301 Moved Permanently - Server changed the URI and asking the client to use a new URI.
302 Found - Server wants to retain old URI, providing an alternate URI. Since Cool URIs don't change
303 See Other - Moving a request from PUT request to GET request.
304 Not Modified - Server instructing the client to use its cached results.
307 Temporary Redirect - This should be used instead of 302 for a temporary redirect. When compared with 303, this doesn't require a change in method.
308 Permanent Redirect - This is similar to 307 but permanent. The HTTP method cannot be changed in the request like in 301.
Interestingly 304 is considered a redirect because the server is redirecting the client to its own cache for the response and not to another URI.
4XX Status Codes (Client Errors)
When the client makes a mistake, the server should notify the client of 4XX error. The most popular being 404. When the server cannot find the resource the client requested. Browsers show errors in their console for 4XX series, even when they necessarily are errors. For example, when the resource is deleted, the server SHOULD return 410 instead of 200 stating that the resource has been deleted.
400 Bad Request - Request is missing some critical information. Some important Headers.
401 Unauthorised - Client doesn't have valid credentials/token (Authentication).
402 Payment Required - Paid Service
403 Forbidden - Client has requested a forbidden resource. This error can be sent even when the client has the correct credentials. (Authorization).
404 Not Found - Resource does not exist on the server.
405 Method not Found - Request method (HTTP Verb) sent by the client is not supported.
406 - Accept Header sent by the client is not supported.
408 request timeout - Server didn't receive a complete request from the client
409 Conflict - Client attempting to create a duplicate record, which is not allowed.
410 Gone - The requested resource has been deleted.
411 Length Required - The server will not accept the request without the Content-Length Header.
412 Precondition Failed - The server understands the request, but the format of the request is incorrect.
415 unsupported media type - Server doesn't understand the payload format. e.g Server parses xml, json but payload contains yml. Content-Type or Content-Encoding may also mislead the server and it will send 415 error without checking payload.
419 Too Many Requests - The server is unable to handle further requests temporarily.
451 Unavailable For Legal Reasons - This status code can be used to adhere to compliance and regulatory requirements. For example, some APIs cannot be across in certain geographies.
The server throws 400 when the user is sending an invalid request. Something that's not even HTTP request. For example, the client is sending just plaintext, with no reference to method or protocol. It throws 422 when Content-Type header says application/JSON, but XML is being sent.
There are several other 4XX status codes that can be used as well like 407 proxy authentication required,
416 requested range not satisfiable, 417 expectation failed but they are less common.
These client codes are the most common and best used by REST APIs to convey client errors. There are other status codes in 4XX series and you can roll out your own for something specific.
5XX Server Errors
500 Internal Server Error is the generic server error which the server shouldn't send. Because it gives very little information to the REST client about the error on the server, we only use it when we don't know the cause of the error. But the server is taking the responsibility of the error.
