Archive for April, 2008
Using WCF for REST, Part 1
Just because you can do something doesn’t mean it’s a good idea.
We decided to use the Windows Communication Foundation to drive our REST-based web service. In hindsight, it was a poor choice. REST support in WCF seems like it was a last-minute addition to .NET 3.5. You can certainly hack something together, but I’ve found few real-world examples on the Internet, and most of those sidestep the tricky issues.
Here’s the short of it: WCF was designed for RPC-SOAP. More importantly, it was designed to SOA-enable legacy services that used older communications channels like DCOM. If you’re starting from scratch, and have full control over the output of your web service and the design of your object model, I’d recommend using a different (and simpler) framework.
We’ve gotten a good bit of blog traffic from people looking for help with Windows Communication Foundation and the REST architectural pattern. (It’s good to know that we’re not the only ones needing advice.) Here are the better sources we’ve found so far.
Windows Communication Foundation documentation on MSDN
Good overview presentation on REST and Syndication using WCF
Microsoft’s Picture Services Sample
Justin Smith’s WCF articles on Cybertopian Chronicles
Assorted posts on Rick Strahl’s blog
If you find other useful places to look, let us know!
Firebird 2.1 Released
Firebird News just announced the release of Firebird 2.1.
We’ve been using Firebird as infovark’s SQL database for several months now. It’s an open source fork of Borland’s InterBase SQL server. So far, we’ve been extremely impressed by the database itself. Its companion database administration tool, FlameRobin, is shaping up nicely too. It’s currently in version 0.86, but it handles most of what we need.
Why did we choose Firebird? I’ve worked with a variety of databases during my career, including Microsoft’s SQL Server, Oracle, and MySQL. While we were strongly tempted use one of those, none of them met our criteria. We needed an embeddable, scalable, ANSI-compliant database that we could deploy easily. It also had to have liberal licensing arrangements and low (or no) royalties. That limited our available choices.
Two features persuaded us to give Firebird a try: It was free to use and the entire database is contained within a single file. I’m glad we did; it’s absolutely the right choice for us.
I wouldn’t recommend it for everyone, however. If you haven’t done much work with relational databases, Firebird is not the place to start. While the Firebird community is working on documentation and tools, they’re far behind what you’ll find at a commercial vendor or some other open source products. The Firebird website is tricky to navigate and lacks a search feature. Trying to get pointers from Google is tough, too, due to a certain American muscle car with the same name. (Tip: Search for Firebird SQL to get the database.) You’ll most likely need to rely on a combination of the Firebird website, InterBase documentation, and developer blogs to get specific information.
If you can look past those issues — and they’re common ones in the open source space — you’ll find FIrebird to be a solid database for development across a variety of platforms.
But keep Lorenzo Alberton’s cheat sheet handy, and bookmark Stefan Heymann’s reference site.
Review: Restful Web Services
As we mentioned recently, we’re building the infovark server using the REST pattern. Since REST is more a loose set of guidelines than a strict series of rules, it’s hard for implementers to know where to begin.
OK, you could go to the source, chapter 5 of Roy Fielding’s dissertation. Or you could check out the somewhat academic discussions on the REST wiki, though there hasn’t been much activity lately. You can occasionally find good advice from the odd blog post, like the REST for the Rest of Us article at Open Garden. But ironically, there’s not a whole lot of material about implementing REST web services available on the web yet. (If you know of good links, leave a comment.)
RESTful Web Services
For the practical, gritty details of how it’s actually done, you’ll need the Restful Web Services book by Sam Ruby and Leonard Richardson. They describe the principles that inform REST-ian (RESTafarian?) design in detail, taking you step-by-step through two different sample applications. If you’re a Ruby programmer using Rails, you’ll find the book especially valuable, since that’s the language and framework in which most of the examples are done. For those of us using different technology, it’s the thought process behind the examples that is most illuminating.
This is because the key challenge of the REST paradigm is the fact that it can’t really be implemented on today’s web without some workarounds. REST will come into its own with HTML 5. The book steers an interesting course between how REST web services might be done in HTML 5 with how they must be done today. I think the authors get the balance right, but at times it can make for a frustrating read for someone wanting practical advice about building a REST service right now.
But that’s less a criticism of the book than of the openness of the REST concept itself. The occasional what-if digression the authors make is a small price to pay for the amount of sound guidance you get. The appendixes alone, which discuss things like which HTTP status codes and headers are worth implementing and which are worth forgetting, will save you far more time than you’ll lose in reading how great things will be when HTML forms finally support the PUT method.
Until that day comes, keep this book handy.
WCF Bad Request
I’ve just identified a horrible bug in WCF for the .NET Framework 3.5.
A caught exception in a WebInvoke operation will cause WCF to return an HTTP 400 Bad Request status code to the client. Any caught exception. Every time. Regardless of whatever error code you might want to send back.
I found the error by mistake. I’d used “BadGateway” instead of “BadRequest” in my code. If it weren’t for other odd WCF behavior, I wouldn’t have noticed that my status code was being ignored.
Consider the following example:
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// Read the Xml into our object and save.
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try
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{
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// The following line triggered the error.
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obj.ReadXml(reader);
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obj.Save();
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// Set HTTP Cache Options and MIME Type.
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Utilities.SetCaching(WebOperationContext.Current, obj.DateModified, 60);
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Utilities.SetMimeType(Format.Xml);
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return Utilities.GetXmlStream(obj);
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}
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catch (Exception e)
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{
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// Was it a schema validation error? If so, provide detail.
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if (!string.IsNullOrEmpty(_XmlValidationErrors))
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{
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// I slipped here, using BadGateway 502 instead of Bad Request 400.
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// But WCF doesn't care. If you enter the catch block it's _always_ 400.
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WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.BadGateway;
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WebOperationContext.Current.OutgoingResponse.StatusDescription = _XmlValidationErrors;
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WebOperationContext.Current.OutgoingResponse.SuppressEntityBody = false;
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return null;
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}
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}
If no error occurs, WCF will return the status code you specify. A try/finally block will work just fine; WCF returns whatever status code you specify. Enter a catch block, though, and it’s nothing but 400 Bad Request.
Hey, if there’s an error, it must be the client’s fault, right?
The Address Bar as the New Command Line
It’s been a common meme over the past few years that command-line interfaces (CLIs) are making a resurgence. Jeff Atwood noted in 2005 that the Google search box is a command line of sorts. In early 2007, Lifehacker’s Gina Trapani listed several examples of CLIs showing up in a variety of applications.
The return of the command-line interface is striking. It was pronounced dead, replaced by the graphical user interface (GUI) more than two decades ago, after usability research discovered that novice users were much more comfortable with GUIs. But a CLI has three big advantages over a GUI.
The first advantage is speed. Expert users find that command-line interfaces are significantly faster than GUIs. People can type much faster than they can identify a link or button, move the mouse to it, and click. This is especially true if the user’s hands are already positioned above the keyboard. People that do a significant amount of word processing (or writing code) often memorize keyboard shortcuts to avoid having to grab the mouse. It might save only a fraction of second, but if you do it many, many times over the course of the day, those split-second differences add up.
The second advantage is that a command line interface is a linear interface. Neither the user nor the application have to worry about the exact position of the pointing device over a two-dimensional surface. This makes command-line interfaces ideal for devices that have tiny screens, or whose pointing devices aren’t very accurate. It’s much easier to text a few commands on a cell phone or PDA than it is to use a stylus or manipulate a touch screen.
These first two advantages of CLIs are are well-known. The third advantage is less obvious: It turns out that CLIs are also much easier for computers to use.
Why do we care about making things easier on our silicon-based friends? Well, one reason is that it indirectly makes things easier on those of us humans that give instructions to computers. (As a programmer myself, that’s a big selling point.) Another reason is that if it’s easy enough for a computer to figure out, then it ought to be brain-dead simple for a human to understand. As I’ve pointed out before, computers just aren’t very bright.
So the advantage of having a command-line interface is that if you, a human, get bored of typing the same silly commands over and over, you can easily write a small program that instructs a computer to do it for you. This is the reason why every office suite has some kind of macro language. It allows humans to hand off repetitive tasks to a machine so that people can get on with the important creative stuff.
So what’s this about using a browser’s address bar as a command line? With AJAX technologies, the URI is no longer merely something that a human types into the address bar to get a web page. It’s also an interface that computers use to interact with data. This means the humble web URL shares most of the key characteristics and advantages of a command line interface.
It’s this insight that’s led to increased interest in the REST architectural style. REST has begun moving into the mainstream because the patterns it promotes allow the URI to serve an audience of computers as well as an audience of humans.
