OK, it's time to talk about SOAP and web-services (and many other things ;-). Why now? Well, I want to write about it before I start reading Nicolai Josuttis' book about SOA, just to record my current (and possibly superficious) impressions.
Beacause Nicolai is known to me as author of fine C++ books, I expect rather much from this one, and suspect that its lecture will shutter some of my safe and secure beliefs. If thad be so, I'l write a new blog to restore the honour of SOAP. But now, ad rem! Down to business!
I remember as a couple of years ago I was receiving one email after another: new SOAP-based protocol A invented, new SOAP-based protocol B approved, new SOAP-based protocol C proposed. And I was wondering - why are the people inventing the wheel for the n-th time afte RPC, DCE and CORBA? Why so much protocols? One is daunted by the sheer number of needed specs, regulative bodies, and documents. Wasn't is supposed to be about simplicity? It's Web! Here I'll cite my first source - "Dave Podnar’s 5 Stages of Dealing with Web Services"*:
- Denial - It's Simple Object Access Protocol, right?
- Over Involvement - OK, I'll read the SOAP, WSDL, WS-I BP, JAX-RPC, SAAJ, JAX-P,... specs. next,I'll check the Wiki and finally follow an example showing service and client sides.
- Anger - I can't believe those #$%&*@s made it so difficult!
- Guilt - Everyone is using Web Services, it must be me, I must be missing something.
- Acceptance - It is what it is, Web Services aren't simple or easy.
Well, of course it's about distributed computing, but let me make my point here: web-services are got so complicated because they are too low-level a model of distributed computing! SOAP is basically RPC done with XML, and PRC deals with location-transparent procedure calls. This paradigm never had a lot of success - look at the demise of IIOP and RMI - because it's too fine-grained. Compare this remote call model to other model of distrinuted computing - Google's Map-Reduce system (to my mind the most successful distributed computing system today)***. It works at totally different level: it's about gathering and transforming information in a distributed manner, hiding away details like remote calls and node failures!
Now enter REST (representational state transfer)****, one another model for distributed computing. Let's explain it to those of you doing embedded programming or databases: REST is the principle the Internet is acting upon. There is a set of ressources which we can acces using URL's, i.e. only knowing their location. So, instead of using UDDI+WSDL+SOAP we just send:
- an HTTP GET request for an URI like: www.marek-here.de/mareks_blog?title=first%20blog
- or, as Ruby on Rails uses it: : www.marek-here.de/mareks_blog/1
- or a POST with: www.marek-here.de/mareks_blog?&title=new%20post&text=bla%20bla%20bla
Well, one must admit, there's a lot of public SOAP interfaces in the Web at the moment: look at Google's and Yahoo's maps, Amazon's bookstore and Ebay auctions. But these interfaces are increasingly perceived as old-fashioned, because RESTful services scale better and are simpler to use. For example Google discontinued its SOAP-based search API and is rumored to be striving to get away from SOAP-based APIs entirely. As for Yahoo and Amazon, which offer SOAP and REST based API in parallel, 85% of their users allegedly choose REST.
It is perhaps a good place now for some well-known citation. It has aleady made its round over the Web, but I'll use it here nevertheless, as it is simultaneously fun and instructive*****:
"WS-* is North Korea and REST is South Korea. While REST will go on to become an economic powerhouse with steadily increasing standards of living for all its citizens, WS-* is doomed to sixty years of starvation, poverty, tyranny, and defections until it eventually collapses from its own fundamental inadequacies and is absorbed into the more sensible policies of its neighbor to the South.Let me close this post with a little story. At the BEA's Arch2Arch Summit in Frankfurt, Germany, this summer I attended the panel discussion about SOA with some of high-level BEA people. BEA is a middleware company, Java-centric and very much into web-services. Of of the questions at the panel discussion was the simple and succint: "Why do you think SOA is so good?". The answer of the president of the German subsidiary was: "Because it's about customer orientation!". The answer of BEA's chief architect was: "Because there's no programming language binding (as in CORBA) - the protocol is language-neutral!".
The analogy isn’t as silly as it sounds either. North Korean/Soviet style “communism” fails because it believes massive central planning works better than the individual decisions of millions of economic actors. WS-* fails because it believes massive central planning works better than the individual decisions of millions of web sites."
After hearing that I asked myself: is that something really new? Isn't industry repeating itself? It was the same feeling I described at the start of this post, when I heard about new SOAP protocols: are the people trying to solve something, or to generate more buzz and revenues? But my gut feeling may be wrong - admittedly SOAP is too low level as an abstraction, but SOA is building a layer on top of it. On the other hand SOA is normally implemented using SOAP. So SOA=SOAP=WS* in the end? And why is corporate world loving it? Is it doomed to failure like North Corea? Could we do SOA with RESTful services?
PS: I updated my August-07 "iPhone Presentation" post. After I've read Nicolai's book, I'll probably update this one too.
* see http://rmh.blogs.com/weblog/2006/04/dave_podnars_5_.html and discussion http://www.theserverside.com/news/thread.tss?thread_id=40064
** books homepage (http://soabook.com/), and excerpt http://soabook.com/blog/?p=5
*** for example see the presentation: "Using mapreduce on Large Geographic Datasets (Barry Brumitt, Google)", http://video.google.com/videoplay?docid=6202268628085731280 "
**** defined in Roy Fielding's dissertation: "Architectural Styles and the Design of Network based Software Architectures, 2000", www.ics.uci.edu/~fielding/pubs/dissertation/top.htm
***** see here for the post and discussion: http://cafe.elharo.com/xml/north-and-south/