James Strachan responds to my SDO comments suggesting I missed the point. I don’t think I did. Perhaps James missed the point of my comments. 8-)

He writes;

To help set Mark straight; Servlets are a Java API for implementing HTTP operations. (Incidentally Servlets don’t have a client side – for that you need to either use the URL class or the commons-httpclient library). So Servlets take care of the server-side part of HTTP protocol for you. To use REST speak, Servlets implement the server side of the transfer part of REST.

HttpServlets do expose a client side API, they just do it in a language neutral way via the HTTP protocol. I could have also compared SDO to java.net or HttpClient I suppose, but my argument would have been the same.

SDO on the other hand is a generic Java API for any arbitrary typesafe blob of state. It says nothing of how you fetch the state, where it comes from, what transfer/marshalling mechanism etc. The state could come from anywhere; from parsing any XML document, calling some RESTful service, a web service, an RMI call, an EJB / JDO / JDBC / JNDI / JMX query etc.

Yes, true, just as a RESTful service could front for an EJB, JDO, JDBC, JNDI, JMX query etc.

When I first reviewed it, I looked for a “smoking gun”; that thing that would stand out as evidence that the authors hadn’t studied the Web in sufficient detail. I didn’t see any at the time, but after having a second look I’d say it’s the use of XPath; SDO should have used URIs (or perhaps both) as the means of relating objects. Perhaps that might help you see where I’m coming from with my objections, James.

Thanks for the feed/pushback.

Update: James responds again. I think his last paragraph is all I really need to comment on;

Incidentally, the interesting thing about SDO is that the navigation mechanism is pluggable. So Mark, an SDO implementation could use URIs (or XPointer, or XQuery etc) to navigate through the data graph.

Yes, understood, but that misses the point of the Web. URIs aren’t just another identification mechanism, they are the most general identification mechanism, enabling other mechanisms to be defined in its terms (e.g. FTP file names engulfed via the ftp URI scheme, email addresses engulfed via mailto: etc..).

Sure, the Web can be treated as just another system, but as soon as you do that, any attempt to build a general model that includes it ends up being an attempt to reinvent it, because that’s what the Web is; the system of all systems.

Stefan Tilkov thoughtfully responded to my “Building the Web with Web services” blog. Here are some of his comments.

In response to my assertion that documents/state don’t ask anything of anyone, he writes;

Er … well, I think I can see the point, but I can’t really make sense of the examples. So, a purchase order isn’t asking anything from anyone? That can’t be true – so obviously, these are counter examples – a purchase order is asking something. Looking at the paragraph, even for some length of time, I have problems finding out what exactly Mark is getting at – I fail to see the similarity between a pure piece of data like current time and a very concrete, business level document like purchase order. Arguably, it’s just not phrased very clearly, or maybe it’s because I’m not a native English speaker.

A purchase order isn’t asking anything of anyone; it’s just capturing the state of some order. It can be used to ask something of somebody. For example, I could give it to a skywriter, and they could write the XML out in smoke overhead. Or I could give it to an archiving service which promises to keep a copy around for some period of time. Or, I could give it to a purchasing department with the intent that they fulfill the order. But all those actions are where I’m asking somebody to do something. What would you do if you found a paper purchase order lying in the street? Do you know the intent of its author? No.

He also added this;

While Mark obviously was thinking of content negotiation, support for Expired headers and so on, he doesn’t talk about it at all. How is anybody supposed to make sense of that?

I quite intentionally (though I slipped some in at the last minute, sigh) tried to avoid mentioning Web terms where I could, and just talk about the general approach. I did this because in my experience, once you start using Web terms, people automatically import their preconceived views of the Web … which is what I’m trying to fix.

And then this;

OK, I’m a bit puzzled by now, but surely things are going to be clarified in the next few sections. (Seriously, that’s what I thought when I first read the article). Imagine my surprise when I saw that the next paragraph’s title is Conclusions:

Well, that’s all there is! 8-) The intent of the essay was to describe how the Web relates to document style Web services; that if you add identification of the things whose state is represented by those documents, make semantics uniform and explicit on the wire in the form of an application protocol, and dereferencing of identifiers to request a document, that you’ve got the Web.

Thanks for the constructive comments, Stefan. I know how easy it is to get all kneejerky on this. It just seems that the industry is SO close to finally understanding the Web, that I feel an obligation to jump up and down and yell “Yo, over here, we’ve already got one of those!”.