Steve Vinoski’s Blog

As you may know, I’m a columnist for IEEE Internet Computing (IC), and I’m also on their editorial board. Our annual board meeting is coming up, so to help with planning, we’ve issued a call for special issue proposals.

The topics that typically come up in this blog and others it connects to are pretty much all fair game as special issue topics: REST and the programmatic web, service definition languages, scalability issues, intermediation, tools, reuse, development languages, back-end integration, etc. Putting together a special issue doesn’t take a lot of work, either. It requires you to find 3-4 authors each willing to contribute an article, reviewers to review those articles (and IC can help with that), and a couple others to work with you as editors. As editors you also have to write a brief introduction for the special issue. I’ve done a few special issues over the years and if you enlist the right authors, it’s a lot less work than you might think.

As far as technical magazines go, IC is typically one of the most cited, usually second only to IEEE Software, as measured by independent firms. I think one reason for this is that it has a nice balance of industry and academic articles, so its pages provide information relevant to both the practitioner and the researcher.

Regarding the REST and IDL discussion, Joe Gregorio already wrote an excellent explanation six months ago. Perhaps the rest of us should have just linked to it to begin with and avoided wasting our time rehashing it all.

But then again, rehashing is fun! :-) On the same topic, I agree with much of what Dare said, except for this:

When building services with WS-*, you have a WSDL to describe your methods & expected inputs/outputs and XSD schema(s) to describe the schemas for said inputs/outputs. When building a RESTful Web Service, the need for both of these documents does not go away regardless of how often you repeat the phrase “uniform interface”.

I still disagree. The HTTP verb set defines a RESTful uniform interface. When comparing CORBA IDL or WSDL to HTTP web services, that verb set must be considered because it’s the only thing that you can directly relate to IDL and WSDL interfaces. Otherwise, you’re talking apples and oranges. The uniform HTTP interface is the only way for applications to interact with web resources in the same sense that CORBA applications interact with CORBA objects, and WSDL applications interact with WSDL services.

Much of the typical IDL and WSDL definition is devoted to defining one or more specialized interfaces consisting of specialized methods/operations and specialized data types to pass across them. For web resources that use the uniform interface, however, there’s simply no need whatsoever to define interfaces like that. What are you going to do, define GET again and again for each of your resources, each time with exactly the same semantics already specified in RFC 2616? Obviously not.

It’s easy to see that Joe’s OpenSearch document bears no semantic resemblance to a CORBA IDL or WSDL definition. Similarly, an AtomPub service document is nothing like IDL or WSDL either. Both of those documents essentially inform you of service URIs and media types, but they don’t define methods or operations. They don’t have to, because of the HTTP uniform interface. That’s important, and it’s why I disagree with Dare’s point quoted above.

Consider the mindset of the CORBA and WS-* developer. They use systems that force them to always think about all their service endpoints in terms of specialized interfaces. Given how much time I’ve spent in that world (and don’t forget, I still use CORBA too), I know for certain that the concept of the uniform interface is one of the big items that trips these developers up when they try to figure out what REST is about. It’s therefore important to keep the uniform interface as part of the conversation.

Reusability is often promoted not only as a goal but also as a feature of all kinds of software architectures, designs, and systems. For example, in the CORBA, WS-*, and SOA worlds I formerly haunted, everyone spoke nonchalantly of reuse as if it were a given. You were supposed to simply identify the objects or services required to support your business processes, and then specify their interfaces. Then, anyone wanting to provide one or more of those objects or services was merely supposed to follow the appropriate interfaces and write implementations for them. Applications were written to the interfaces and thus were automatically decoupled from the implementations. As a result of these reusable interfaces, you could also potentially reuse the objects and services that implemented them, as well as the applications that consumed them.

Problem is, it never seemed to work out as easily as that. Most of the time, the interfaces people came up with were just too specific, and nobody could agree to apply them widely. Think of all the time people spent over the years in OMG, JSR, and W3C WS meetings trying to agree on just the infrastructure interfaces and not always succeeding. It’s therefore not surprising that there was never much success at defining broadly-accepted standard interfaces up at the application level; the scope is simply far too wide up there.

So, with technologies that promote interface variability, planned reuse is pretty hard. Consequently, serendipitous reuse, where services and facilities can be combined and reused beneficially in unforeseen ways, is virtually out of the question.

Stu Charlton’s blog was where I first saw those terms used, and I found them very enlightening. So did Bob Warfield.

One of the first things that attracted me to REST was the uniform interface constraint. Mark Baker first brought it to my attention nearly 8 years ago, and before I looked at it, I thought it was just a bad idea, like a totally generic doIt() interface, devoid of any meaningful semantics. But of course, there’s much more to it than that. The HTTP interface, for example, strikes a great balance that allows it to be efficiently reused across a very wide variety of applications. And when such a uniform interface is reused, the applications that use it stand a much better chance of being reusable themselves than if they were written against a non-uniform application-specific interface.

My latest Internet Computing column, entitled Serendipitous Reuse (and here’s the pdf if you prefer), explores how the uniform interface contributes to reuse of both the planned and serendipitous kinds.

Don says he agrees with a lot of what James has to say about Stefan’s recaps of the QCon SOA Track, listed below:

• Dan’s talk
• Pete’s talk
• Sanjiva’s talk
• my talk

I too agree with many of James’s insights. If you don’t regularly read his blog, you should subscribe immediately. I especially like this quote:

When I consider SOA and REST today, I do not see two competing visions, I see an evolution that has brought the industry back to a core set of fundamental design principles that we seemed to have lost sight of for a while.

How very true. However, I disagree with just one small point: his critique of the part of my talk where I said SOA has no constraints (as Mark Baker has been pointing out for years now).

James says that SOA has constraints, it’s just they’re just not well-documented. I know exactly what he’s saying, but I have to stick with what I said. The only official definition of SOA that I know of is the OASIS SOA Reference Model, which mentions several desirable properties for SOA systems, but it documents no constraints for inducing them AFAICS. Later in my talk, as Stefan’s notes indicate, I did say that I was willing to concede that the OASIS SOA RM promotes the client-server constraint. However, I was being really, really generous, since all the specification really does is hint that SOA systems should be distributed, and “distributed” has a lot more meanings than just “client-server.”

Since there are no official standard SOA constraints, the only constraints you get are the ones that each vendor or supplier of SOA platforms and systems decides to give you. Naturally, each different solution in this space will differ in those constraints.

As the quote from James above implies, pitting SOA and REST against each other doesn’t make a lot of sense. The reason I went to the trouble of pointing out in my talk that SOA is missing critical architectural requirements like constraints was to help illustrate the significant differences between SOA and REST. Fundamentally, as I also said in my talk, SOA isn’t really about architecture in the way that REST is. Rather, it’s about IT culture, best practices, breaking down the enterprise organizational and political barriers that needlessly increase IT costs, and following good fundamental software engineering principles like reuse, minimizing coupling, and maximizing cohesion.

I returned home from QCon San Francisco last night. In the SOA track, it was most excellent to finally get to meet Patrick Logan, Stefan Tilkov, Pete Lacey, Jim Webber, Stu Charlton (thanks again for the Dew, Stu!), and Mike Herrick in person, as well as to get to meet up again with Sanjiva Weerawarana, Glen Daniels, and Dan Diephouse. All in all the SOA/REST track was highly informative, with solid presentations through the whole day. Pete’s was especially cool due to the demo he gave of the extreme flexibility and integrability of REST-oriented solutions. It was also reasonably fun, given how much the attendees got into it with their questions, and especially due to Jim’s presentation at the end in which he mixed some serious technical chops with very funny one-liners popping out every second or third sentence.

My talk, “REST Eye for the SOA Guy” (title taken from here), went reasonably well. The only time it didn’t flow as smoothly as I would have liked was when some semi-anti-REST folk started questioning my assertions about specialized interfaces inhibiting scalability, and wondering whether REST just pushes all the coupling problems to the data. I felt like I was being pulled backwards in time by 5 years to the old W3C Web Services Architecture Working Group meetings, where very little was ever accomplished due to arguments like those. I’ll have more to say on those topics in some near-future blog postings.

An interesting thought struck me as the track progressed: in my experience it seems that most SOA/WS proponents who argue against REST on technical merits have never actually tried to build any RESTful applications. Many of their objections seem to be heavily based on emotional reactions, or based on conclusions reached only by blowing issues out of proportion (for example, I felt that many of the points in Sanjiva’s talk fell into this category). So it led me to wonder about how many, if any, dyed-in-the-wool WS advocates ever seriously, honestly, and fairly looked at REST, actually tried it, but then decided it didn’t work and so willingly chose to go back to WS. I don’t want to count those who had to go back to WS only to integrate with something else already implemented using WS, because that doesn’t fit the “willingly” part. (For example, I recently had to write a client to access a SOAP service, and I was definitely not in the “willingly” camp because the service was way more complicated than it needed to be, only because of SOAP.) I personally know of nobody who has ditched REST for WS like this, but if you have, or if you know of someone who has, I’d love to hear the whole tale, so feel free to leave it in a comment.

The conference itself was much like JAOO, which is not surprising given its JAOO roots. It was smaller, of course, given that this was the first QCon held in the U.S. And just like JAOO, there were times where I wanted to be in multiple places at the same time to see two or three different talks all scheduled concurrently. For example, I very much would have liked to sit all day in the Thursday “Architecture Quality” track, but couldn’t since it ran parallel to the track I was in.

In short, if you didn’t go to QCon, you missed a great conference. Stefan, thanks again for inviting me.