RESTful Web APIs

Appendix A. The Status Codex

An HTTP status code is a three-digit number attached to an HTTP response. It’s a bit of protocol semantics that lets the client know, on the most basic level, what happened when the server tried to handle the request. The 41 HTTP response codes defined in the HTTP specification form a set of basic protocol semantics that any API can use.

Apart from HTTP redirects, and the famous “404 Not Found” error page notwithstanding, we don’t really use status codes on the World Wide Web. A human learns what happened to a request by reading the entity-body served as part of the response, not by looking up a numeric code in the HTTP standard. When you fill out a form on a website, but you forget to fill in one of the required fields, the server sends back an error message, but the response code associated with the error message is 200 (OK).

That’s fine. You don’t even see the response code. You read the error message and correct the problem. But an API that behaved that way would be lying to its client! Computer programs are very good at looking up numeric codes, and very bad at understanding prose. When you serve the 200 status code on an error condition, you must write extra documentation explaining that in your API, OK doesn’t necessarily mean OK. That extra documentation means more work for your users.

In the world of APIs, then, HTTP response codes become very important. They tell a client how to regard the document in the entity-body—whether ...