RE: Choreography and REST

Firstly lets not argue the usefulness of the factory pattern:-)

Secondly in your example, I could get a c_obj from a
trader/uddi/directory, without 
there being any explicit make_c. There is  no systematic way to know
whether a c_obj provider must provide the factory 
or whether that is a private act and the object can be
advertised/discovered seperatley; nor whether once I create it it can be

shared or not. Without some syntax that ties them together you really do
have to look at comments/documentation (heaven forbid).

Just my personal opinion but I think design by contract/path experession
type thingy is a pre-cursor to a
choreography/orchestration/collaboration solution.

Martin.

> -----Original Message-----
> From: Paul Prescod [mailto:[email protected]] 
> Sent: Tuesday, August 13, 2002 12:23 PM
> To: Martin Chapman; [email protected]
> Subject: Re: Choreography and REST
> 
> 
> Martin Chapman wrote:
> > 
> >...
> > 
> > Maybe I have a different definition of choreography, but CORBA (and 
> > COM and c++ and java and c#) does have this problem. Given  the 
> > following:
> > 
> > Interface example {
> >     void a (in x:string);
> >     short b (inout y:short);
> >     boolean c(out temp:long);
> > };
> > 
> > How can anything know, and hence statically check, the correct 
> > invocation order(s)?
> 
> Thank you for coming up with some demonstration code. 
> Consider the following decomposition which statically and 
> explicitly enforces a particular correct ordering of method calls:
> 
> Interface example {
>   b_obj make_b(in x: string):
> }
> 
> Interface b_obj {
>   c_obj make_c(inout y:short);
>   example back_to_example(in blah:int);
> }
> 
> Interface c_obj {
>   boolean doit(out temp:long);
> }
> 
> This decomposition isn't just good because it removes the 
> need for a choreography language. Indeed, this is a minor 
> benefit. Actually the decomposition has many more important 
> benefits. Most crucial, the consumer of the service has much 
> more flexibility about its own composition. Because b_obj and 
> c_obj are now first-class objects, I can pass those to any 
> other component anywhere on the Internet and have it work 
> with them. I don't have to pass a "state" as some kind of 
> flag. To use WS buzzwords, my relationship with that other 
> component can be more "loosely bound" because I have less to 
> tell him about what's going on in the conversation.
> 
> Some other c_obj producer could proceed like this:
> 
> Interface z_obj {
>   c_obj short_cut_to_c_obj();
> }
> 
> So the relationship between the object types and the 
> choreography is very flexible and other components brought 
> into the mix care only about the former and not at all about 
> the latter.
> 
> In other words, by decomposing I have better opportunities 
> for reuse of the c_obj abstraction which translates in 
> networked systems into higher interoperability and less "glue code".
> 
> > ... Aside from unparsable comments
> > and documentation (if you're lucky), there is no extra information. 
> > The fact that the corba community didn't address this 
> problem doesn't 
> > mean to say it doesn't exist.
> 
> The CORBA community doesn't have the problem because it is so 
> easy to design around and in fact it is common to "solve" it 
> merely as a side effect of good object design. If you feel 
> you have a concrete case where this model does not work 
> properly, I would be interested in seeing it.
> 
> > What you need to do is augment interface definitions with extra 
> > information. Design by contract is one example, but also 
> there is the 
> > excellent work done by ANSA in the early 90s called  Path 
> Expressions 
> > [1], which people are only just re-inventing  (or is that 
> > re-discovering:)
> 
> Design by contract and path expressions are different than 
> choreography. I would be much more enthusiastic about "design 
> by contract for web services" than choreography for web services.
> -- 
> "When I walk on the floor for the final execution, I'll wear a denim 
> suit. I'll walk in there like Willie Nelson, John Wayne, Will Smith 
> -- Men in Black -- James Brown. Maybe do a Michael Jackson 
> moonwalk." Congressman James Traficant.
>