Learn HTTP, damn it

I recently had to debug an issue which required a lot of familiarity with HTTP to debug. There were a lot of random workarounds to the problem (“if I disable feature X it works”) but only when a developer clearly articulated the exact problem with the HTTP response was it possible to trace the error to a consistent reproduction case. The underlying lesson is one I’ve learned at least a few times now. When working with web APIs there will be times where things break at a level where you will be completely helpless unless you know how things work underneath all of the frameworks, toolkits, and client libraries in your application. So learn HTTP, damn it.

Yes, I know. Not exactly controversial. Yet consider the abstraction that .NET uses to create a HTTP request containing a Basic Authorization header:

String URL = "http://www.example.com/";
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(URL);
req.Credentials = new NetworkCredential(UserName, Password);
using (HttpWebResponse res = (HttpWebResponse)req.GetResponse())
    StreamReader sr = new StreamReader(res.GetResponseStream());

What exactly is a NetworkCredential? Certainly nothing defined by the HTTP spec - it’s an abstraction introduced by Microsoft so that developers can specify a username and password and access protected resources over HTTP. Like most abstractions, it makes a specific use case very easy, at the cost of understanding what the hell is going on under the covers. As a method of interacting with HTTP, it’s leaky as hell.

How is any .NET code jockey, furiously copying and pasting from the MSDN examples, going to understand that the entire point of that code block is to add a base64-encoded Username:Password HTTP header to the request? How are they going to understand that the username and password are sent in plaintext over non-HTTPS requests? How is that developer going to use a non-basic Authorization scheme? How is such a developer going to know what a HTTP header is?

Lest you think I’m biased (I am, but that’s immaterial) I’ll say that this isn’t endemic to .NET folks either. Every day I get support requests from programmers who should know how this stuff works, but don’t seem to grasp things correctly. In my experience, poor HTTP abstractions are typically to blame. The sad thing is that HTTP is not very difficult to learn. Spending some time to cover the fundamentals is well worth it.

Let’s hypothesize that I have the magical ability to open a connection to any web server, and that every key press I make on my keyboard will be sent to the remote computer. I want to get the Google homepage, so I connect to one of their many web servers.

If the server spoke English, I might type:

Hello!  Please give me the content located at the path /
on the domain www.google.com.
I'm using the Chrome web browser, version 26.
I would like to receive the content as HTML,
zipped to save bandwidth,
in US English,
with UTF-8 characters (like a snowman ☃).
This is my only request.

Of course, computers prefer structured data since it’s easier to parse. What HTTP does is give me a way to convey the above information (and more, if I wanted) in a manner which any HTTP-compatible server will be able to understand. In fact, the exact same request is expressed as the following in version 1.1 of the HTTP protocol:

GET / HTTP/1.1
Host: www.google.com
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_3) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.65 Safari/537.31
Accept: text/html
Accept-Encoding: gzip
Accept-Language: en-US
Accept-Charset: utf-8
Connection: close

And that’s it! All I’d have to do is open up my hypothetical program which would let me type to a web server, type the lines above, and pipe the response to a file or other destination (probably a decompression utility since the response would likely be compressed). The great thing about HTTP is that it’s human-readable. It’s probably as low as one can go in the whole web stack and still have a chance of constructing a query and response from hand. Which is, of course, why I think that understanding raw HTTP is still a reasonable requirement to be expected of any programmer intent on using web services.

And that hypothetical program which allows me to type out HTTP requests? It’s called Netcat, and it’s awesome for testing HTTP servers. If you’re lucky enough to use a computer with a decent command line, you probably already have this installed, and if you use Windows, there are ports available too.

If you’re on Linux, Unix or OSX, try the following from a command prompt:

nc maps.googleapis.com 80

That opens a connection to a Google Maps API server on port 80, which is not-coincidentally, the standard port for HTTP requests. Everything you type after pressing enter is sent to the server, and everything sent back is printed to the console (there is no other output). Since the server won’t send anything until it gets a request, the ball is in your court. Copy and paste:

GET /maps/api/geocode/json?address=San+Francisco,+CA&sensor=false HTTP/1.1
Host: maps.googleapis.com
Connection: close

Then press enter twice and watch the magic happen. You are now COMMUNICATING WITH MACHINES. The response will be a JSON-encoded representation of how Google treats an address of “San Francisco, CA”.

You can pretty much play with any URL you’d access in your browser in this way. One thing to keep in mind is that if the url begins with https (note the trailing “s”) the connection needs to be encrypted, which Netcat isn’t so good at. Luckily, there’s a function in a utility named openssl which allows you to do the same thing as Netcat, but encrypted. For example, connecting to port 443 (standard for HTTPS) of a Twitter API server:

openssl s_client -connect api.twitter.com:443

Then you can issue commands as usual:

GET /1.1/statuses/show.json?id=119220039382614016 HTTP/1.1
Host: api.twitter.com
Connection: close

Note that you’ll see an error response - Twitter expects an extra level of authorization which I won’t cover here. But the concept is very similar, and once you understand this level of the protocol, understanding auth becomes a lot easier.

What you typed into Netcat was practically as simple an HTTP request as you could make. There’s a wealth of other stuff you could pass, all of which is specified in this document, known as RFC2616. It looks crazy but it’s actually pretty understandable if you spend a little bit of time working through it, particularly sections 5 and 6, which deal with requests and response structures. The specification was not meant to be cryptic. It is supposed to be clear, unambiguous, and implementable. The specification is your friend.

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