REST: Mapping application errors to HTTP Status codes

Question

Is it to be considered good practice to reuse RFC HTTP Status codes like this, or should we be making up new ones that map exactly to our specific error reasons?

We're designing a web service API around a couple of legacy applications.

In addition to JSON/XML data structures in the Response Body, we aim to return HTTP Status Codes that make sense to web caches and developers.

But how do you go about mapping different classes of errors onto appropriate HTTP Status codes? Everyone on the team agrees on the following:

GET /package/1234 returns 404 Not Found if 1234 doesn't exist

GET /package/1234/next_checkpoint returns 400 Bad Request if "next_checkpoint" and 1234 are valid to ask for but next_checkpont here doesn't make sense...

and so on... but, in some cases, things needs to be more specific than just "400" - for example:

POST /dispatch/?for_package=1234 returns 412 Precondition Failed if /dispatch and package 1234 both exist, BUT 1234 isn't ready for dispatch just yet.

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(Edit: Status codes in HTTP/1.1 and Status codes in WebDAV ext.)

Answer 1

RESTful use of HTTP means that you must keep the API uniform. This means that you cannot add domain specific methods (ala GET_STOCK_QUOTE) but it also means that you cannot add domain specific error codes (ala 499 Product Out Of Stock).

In fact, the HTTP client error codes are a good design check because if you design your resource semantics properly, the HTTP error code meanings will correctly express any errors. If you feel you need additional error codes, your resource design is likely wrong.

Jan