September 2005

September 26, 2005

Searching for WS-Middleground

Filed under: webarch,webservices — Mark Baker @ 12:25 am

As you might expect from the brilliant, intrepid (and largely neutral) technologist/writer, Jon Udell, when it comes to Web services and the “REST vs. SOA(P)” debate, he looks for the middleground;

Today’s most visible exemplars of WS-Lite — Amazon and eBay — use Web services in a point-to-point way. In that mode there’s not much difference between SOAP/WSDL APIs and REST APIs, so it’s not surprising that developers who work with these platforms overwhelmingly prefer the REST flavor. But when you do need to flow your XML traffic through intermediaries, SOAP and WSDL suddenly make a lot more sense.

When a rich intermediary model is what is required, I agree wholeheartedly that SOAP’s is superior to that of vanilla HTTP. But I have two concerns with that last sentence.

The first is to point out that SOAP’s intermediary and processing models are RESTful, as they respect REST’s self-descriptive and layered constraints. I fear that Jon’s readers might interpret his statement as implying a false dichotomy; that the use of SOAP excludes the use of REST. I don’t know whether this was Jon’s intention, of course.

My second concern, related to the first, is the mention of WSDL. WSDL was designed primarily for the purpose of describing service interfaces, therefore implicitly suggesting that services need different interfaces. But what’s not often recognized is that an intermediary model such as SOAP’s (i.e. an application layer one) only makes sense with an architectural style where interface constraints are used, as only by constraining interfaces does composition become tractable.

I applaud Jon on his efforts to find a middle ground, primarily because in doing so he his giving the Web-friendly RESTful approach much needed respect and visibility. But I think that middle ground is to be found further from the WS-* end of the spectrum than he seems to believe.

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September 12, 2005

CommerceNet Tech Talk

Filed under: webarch,webservices — Mark Baker @ 6:58 pm

I’ve confirmed that I’ll be speaking at CommerceNet at their Thursday Tech Talk this week. However, the previously mentioned subject has changed; I’ll now be talking about the use of XForms for RESTful service description.

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September 7, 2005

Ian Foster on state and REST

Filed under: architecture,webservices — Mark Baker @ 2:31 pm

Ian Foster, well known Grid Illuminati, wrote an interesting draft paper titled
“How do I access state? Let me count the ways” (Google’s cached HTML version) earlier this year (which somehow evaded by radar), that discusses various approaches to handling state in four alternative architectural styles (or parts thereof) vying to be the base style to make the big picture Grid/SOA visions a reality.

It’s an interesting paper, with a fairly solid introduction to the issue that I agree with almost entirely. It’s also flattering (for the whole REST community, not just myself) that REST is even considered. Unfortunately, the conclusions fall short of the high standard set by the introduction, as REST is misrepresented.

The first indication of problems is the title of the first concluding sub-section; “Implementation Architecture as a Nonissue”. Prima facie this seems questionable, as the study of software architecture tells us that the architectural style one choices is critical in determining the resulting properties of a system. But let’s dig a little deeper;

It has been argued that some interfaces permit more reliable or efficient implementations than others. However, this argument is discredited by the fact that the messages transmitted across the wire in each approach contain essentially the same information.

The emphasis on “essentially” is mine, because the fact is that the messages described earlier in the paper do contain different information, and, as it turns out in this case, that information is critical to fully appreciating REST (and other document oriented architectural styles, though the paper doesn’t get into any of those).

Though there are several claims in the paper that misrepresent various aspects of REST, as well as confusing the Web, REST, and common practice using both, the following from the summary is probably the single most significant misrepresentation, and I’d guess the cause of the belief that lead to the problem described in the previous paragraph. It reads;

In contrast, what we call the State Id and REST approaches adopts a domain-specific encoding of operations, on top of SOAP and HTTP, respectively.

In a RESTful use of HTTP, HTTP provides the operations; there is nothing more to be encoded, domain-specifically or not. Only the data payload – the document representing the state of the job, or whatever it might be – is required, as I previously explained . It’s an odd oversight, because Table 6, which details the RESTful messages, makes no mention of any operation other than the HTTP ones.

I’d be interested in Ian’s comments about how this might impact the paper’s conclusions. I’d also be happy to provide a more detailed response to some of the other claims about the REST approach that I disagreed with, if requested.

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September 6, 2005

Bay Area visit

Filed under: Uncategorized — Mark Baker @ 4:34 pm

I’ll be down in the Bay Area for a few days, arriving next
Wednesday, September 14th.

On the 14th, I’ll be attending (though arriving late) the
SDForum “Web Based Architectures: Where do we go from here?” in Palo Alto. The next day, I might (still working it out) be giving a talk at CommerceNet on (working title), “RESTful advice for AJAX developers”. The following week, I’m at
Oracle OpenWorld at Moscone, for a client. I return the morning of the 23rd.

Please feel free to drop me a line if you’d like to get together to talk integration.

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