Archive for the ‘Web Services’ Category
Looking for an IIS Alternative
One year ago it became clear that Infovark had outgrown the Windows Communication Foundation (WCF).
We’d decided to use WCF because we wanted Infovark to provide web services, and we liked the fact that we could deploy WCF to client machines. Since WCF is built directly on top of HttpListener, a core part of the Microsoft .NET Framework, we wouldn’t need to use System.Net or Microsoft IIS.
But we’ve struggled with WCF for a variety of reasons. First, we wanted to use a REST model for our web services, and WCF’s support for REST architectures lags behind its SOAP support.
Second, there’s no easy way to return HTML from WCF. We tried transforming our XML with XSLT and returning the XHTML results as a Stream. This works, but the programming experience is frustrating.
Last, because of the previous two reasons, we were left with a website that was way too rigid and programmer-like. It didn’t feel like an organic website. The tool we’d picked was forcing us to compromise on our website design goals.
Infovark’s primary mission is to help human beings, not other computers. That means that the look and feel of the web interface should be our number one priority. Awesome web services are nice to have, but happy users are more important.
Web server alternatives
So for the past few months, we’ve been hunting for an alternative web server. We can’t use IIS because its footprint is too heavy. Most IT departments won’t allow us to install IIS on client machines.
We could use Apache. It has a nice embeddable version, but interacting with it via C# is tricky. We’d prefer something a little more Microsoft-native.
That basically leaves us with one commercial option and two open source options.
- UltiDev Cassini is a commercial product that’s been around for some time. We’re sure it could do the job, but the licensing model is cost prohibitive.
- C# WebServer is an open source project on CodePlex. It’s been around for two years, but the pace of development seems slow.
- Kayak is a promising open source project, but it hasn’t reached its first public release yet.
(If you know of other web servers worth investigating, please let us know in the comments!)
Making the switch
More important than picking an alternative web hosting framework for Infovark is the timing of the switch. We don’t want to impede future development.
As a stopgap, we might try plugging in the Spark View Engine to replace our current XML-XSLT-XHTML rendering path. Who knows? If it improves our web development flow, we might be able to keep our WCF base after all.
REST: How to respond to an HTTP POST
I ran into a problem yesterday. I’d sent a HTTP POST request to a collection of resources on our RESTful web service. Our server responded with an HTTP 201: Created status code and the URI of the new resource in the Location header.
And then… nothing happened.
This was not what I was expecting. I expected my web browser to follow up with a GET request to the URI I’d provided. But Firefox 3 wasn’t biting. A problem with Firefox? I checked in IE8 and Google Chrome and got the same behavior.
Had I misread the HTTP spec? Did I misunderstand the REST pattern? I grabbed for my worn copy of RESTful Web Services. Nope. HTTP 201 seemed to be the right status for this situation.
Had I blundered into some common error? I checked Stefan Tilkov’s useful list of REST Antipatterns. But I couldn’t find anything that quite matched my situation.
I started Googling, but couldn’t find much apart from this question about HTTP Post on Stack Overflow. There were some cryptic responses (to which I’ve added my own answer now).
Eventually, I discovered what I needed to know from Ben Ramsey’s article on HTTP redirection. It’s part of his series discussing RFC 2616, which describes the HTTP/1.1 protocol.
The answer is that while web service clients will often “take the hint” provided by a HTTP 201: Created response, web browsers won’t. If you actually want a web browser to go somewhere else, you need to send a status code in the 3xx series. In this situation, the status code you want is HTTP 303: See Other.
Once I changed the status code returned by the server, all the web browsers followed up the response with a GET to the new URI.
Put the “Web” Back in Web Services
I know it’s possible to transmit Internet Protocol by carrier pigeon, but I’m not sure I could recommend it to our customers with a straight face. By the same token, I’m continually surprised to hear vendors and consultants insist that Web Services can be done without the web.
Yet I’ve seen that claim repeated all over the Internet. Sure, it’s true in the academic sense. Technically, there’s nothing wrong with “webless” web services, just like IP by avian carrier will eventually get the job done (though latency and packet loss are a challenge). But practically speaking, why would you use anything other than HTTP?
The “webless web services” meme has infected a number of good ideas. The SOAP standard is a case in point. At first, SOAP was a simple XML wrapper around an XML payload. (Remember when the S in SOAP stood for Simple?) Then a variety of industry heavyweights jumped on the bandwagon, and soon SOAP could be used with any protocol.
With, uh, a few changes. Like five or six additional specifications. And schema. Lots of schema. Maybe a diagram would help? No?
Why not use REST instead?
Nicholas Allen recently shared a great presentation on REST and SOA given by Stefan Tilkov at QCon. Stefan makes some great points about how RESTful web service design aligns well with the goals of Service Oriented Architecture (SOA).
Sure, you could use the WS-* stack if you like, but RESTful web architecture is a proven, scalable and truly simple approach, as long as you don’t mind having to use HTTP.
Much of the added plumbing in the WS-* stack is there to help transition older computer-to-computer network communication technologies. It allows applications that depend on CORBA or DCOM to tunnel through firewalls. But unless you’re trying to retrofit a system built before, say, 1998, you can skip all that stuff.
Wasn’t using web protocol the whole point of web services anyway? As Stefan said in his closing comments: “Protocol independence is a bug, not a feature.”
Use a RESTful architecture for new development. Put the web back in web services.
WCF Instance Context
I finally figured out the source of my HTTP 400 problem. Apparently the Windows Communication Foundation deals with exceptions differently depending on your InstanceContextMode settings. I had been using the Single setting but I should have used the PerCall setting. In PerCall mode, the try/catch block works as expected.
I think it has something to do with the way that WCF distinguishes between channel exceptions and message faults.
Anyway, if you’re building a REST web service, you’ll want to make sure your class is decorated with the following ServiceBehavior attribute.
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[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
Using WCF for REST, Part 2
In part one of this series, I listed several websites and blogs that had useful information on the Windows Communication Foundation (WCF) and REST. I also mentioned that if I was stating again, I’d probably use something other than WCF. Perhaps deriving my own REST server from System.Net.HTTPListener, for example.
Vish asked for some additional details in his comment to that post. He works on the Microsoft WCF development team and was curious about our experience.
I had just begun putting together my response when I noticed Scott Guthrie’s post about Service Pack 1 for the .NET Framework 3.5 beta release. Steve Maine also posted specifics about the ADO.Net Data Services and WCF changes.
So, Vish, it seems your team’s beaten me to the punch on some of these issues! Many of the difficulties I was having with WCF and REST were addressed by the service pack. Here’s an overview of our key stumbling blocks:
- REST requires much greater control over the URI than SOAP does, and the URITemplate class just wasn’t up to the task. We had to hardcode most of our endpoints to compensate. (Fixed in SP1. Hooray!)
- Supporting multiple formats, such as serving both XML and JSON, either require you to program against Stream or require twice the number of endpoints.
- Existing serializers had trouble with complicated object graphs, forcing us to perform serialization/deserialization ourselves. (This seems greatly improved in SP1.)
- WCF allows only one contract/interface per endpoint. This makes it tricky to factor out common contract patterns.
- Good REST practice would have you return many kinds of errors as HTTP status messages. SOAP embeds all error information in the returned XML. WCF is closely aligned with the SOAP approach, which means that you’ve got to be very careful distinguishing exceptions from faults when implementing REST in WCF. It was an unpleasant surprise, and we had to do quite a bit of work to deal with that.
I’ll talk about all five of these areas in more detail in upcoming posts in this series. And I’ll be sure check out the SP1 beta once we get our Infovark Alpha release out the door.
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!
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.
