Jakob Ƙstergaard Hegelund

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Archive for September 2014

Unrestful XMLHTTPRequest

2014-09-02

The RESTful model for web service APIs preaches simplicity and brings back the sanity into web service design. It has, once again, become acceptable to solve simple problems with simple solutions. I have on several occations been the principal designer behind an internet facing RESful API, and I truly enjoy the power and expressiveness of plain simple HTTP/1.1 as the vehicle that transports requests and responses to and from the APIs.

A little background

I need to point out some obvious things to set the scene for this post. HTTP defines status codes. Every response has one. The ones we all know are "200 OK", "404 Not Found" and perhaps "500 Internal Server Error". But HTTP defines many other status codes - in fact, with a little creative thinking, HTTP will have error codes for pretty much any situation you will run into when building an API. Don't believe me? Take a look for yourself - it is RFC 2616 (yes I know it has been split now, but 2616 is still a good document). The most important aspect is, that when designing an API, you can actually return a real error code in the HTTP response, and you can put a descriptive error message in the body of the response document.

GET /our/stuff HTTP/1.1
host: mine.local

-------------------------
HTTP/1.1 403 Forbidden
content-length: ...
content-type: text/plain

Hi there. We simply won't allow people from your
side of the internet access to OUR stuff.

Sincerely,
 US

The second killer feature of HTTP is its brutal simplicity and consistency. Every request consists of a request line (GET /our/stuff HTTP/1.1 in the above example), some headers and a body. Every response consists of a status line (HTTP/1.1 403 Forbidden in the above example), some headers and a body. There are very few exceptions to this rule. One such exception, is, that a response to a HEAD request must not include a body - because the whole point of the HEAD request is to do a GET without actually getting the body data. HTTP does however mandate some specific semantics on the methods - a GET request, for example, must be idempotent (meaning, multiple identical invocations of the method must have the same effect as one invocation) where a POST doesn't have to be. This of course makes perfect sense - multiple requests to retrieve a particular document should of course all yield the same result, whereas it would only be natural if the initial creation of a document succeeds while subsequent creations of the same document would fail (with a 409 Conflict error of course).

Fail 1: error codes

When issuing API requests from JavaScript in a browser using XMLHTTPRequest directly or indirectly through the myriad of JS libraries out there, one would like to issue a request and receive the response. This is what the method facilitates - except if the response status code is 401 Unauthorized. If that status code is sent, the browser will intercept the method return and pop up a horific looking pop-up, prompting the user to authenticate. What's the point?!? Sure, if the user himself entered the URI in the browser address bar, it makes sense. But the user should not be troubled with return codes from the internal workings of a JS application running in his browser. If the application wishes to authenticate, it could ask the user directly, or the browser could expose a method for this purpose.

The workaround I devised for this, was to accept a header "authentication-failure-code" which can be set to any integer from 400 to 499. If the API wishes to return a 401 status and this header is set, the API will return the given integer instead of 401. This is a simple way to keep the API clean while providing a workaround to the misguided implementations of XMLHTTPRequest out there.

Fail 2: GET requests

Let us assume that I have an API which can transform a document from a simple mark-down style text, to a full XHTML document with fancy formatting and graphics according to some theme configured in the system. When implementing an editor, I want to execute this API request whenever the user has edited his document, so that we can present an up-to-date view of what the finalized messaeg will look like. Which method would we us for that?

SOME-MEHOD /render/welcome-message HTTP/1.1
host: api.local
content-length: ...
content-type: text/xml

<render>
 <lang>en-GB</lang>
 <content>Dear ${fullname}

Once upon a time there was an RFC, but nobody could
be bothered to read it.

The end.
 </content>
</render>
-------------------------
HTTP/1.1 200 OK
content-length: ...
content-type: text/html

<!DOCYPE...>
<html>
 ...
<body>
<h1>Dear John D. Anyuser</h1>

<p>Once upon a time...

Well, this method would not change anything on the server, so it is definitely idempotent. First off, neither POST nor PUT would be suitable. The request returns a body, so HEAD is also a no-go. In fact, the only reasonable method to use, is GET. It even makes perfect sense - we execute a "static" method on the server (a pre- configured rendering routine) with no side effects, just like when we request a static document, or a search result. Instead of encoding the document we wish to have transformed in the path of the URI (which would be inconvenient and even impossible with larger texts), we simply supply it in the body of the request - which is perfectly valid and well defined by the HTTP 1.1 RFC. So what is all the fuss about you may ask? Why not just go ahead and do this and be done with it? Well, I did, and as it turns out, XMLHTTPRequest will ignore the body in a request if the method is GET. No I am not kidding and this is not a joke (or at least it is not a very funny one). It is right here.

Yet again I was forced to implement a workaround in an otherwise fairly clean API, to allow for something which I this time completely fail to see the explanation for. I mean, they specifically went to all the trouble of special casing GET so that XMLHTTPRequest specifically would not allow a standard HTTP request - what for? To help us? Please, if that is the case, stop helping. Please just remove the special case from the standard, remove the code necessary to implement this breakage of HTTP support, and thereby allow plain simple RESTful APIs to be used from the browsers that people have. Anyway, the workaround was simple; add another handler so that users can use the "PUT" method (even though that makes NO sense, as nothing gets updated on the server) instead of "GET", thereby bypassing the special case in XMLHTTPRequest that breaks protocol.

Ah... glad I got all this off my chest. I hope you find the workarounds useful, and if you are a browser vendor or otherwise have leverage to influence things, please consider if it would be possible to work towards supporting HTTP in all its beautiful simplicity in the browsers of the future. Thank you.