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BurdettML

From: Yaron Goland <ygoland@bea.com>
Date: Wed, 10 Sep 2003 21:33:44 -0700
To: "WS Chor Public (E-mail)" <public-ws-chor@w3.org>
Message-ID: <029201c3781d$e32fc810$bccacf0c@bea.com>

Executive Summary - I think the group owes a debt of gratitude to David
Burdett for his excellent spec. I think his spec should become the core of
what will eventually be the WS-Chor spec.
	Most of my comments below have to do with removing features. I see that as
a positive thing. Pruning is a healthy activity for a spec. It's when you
find yourself having to re-engineer that you need to worry. Thankfully, that
isn't the case here.

	I do think we need to add a few things to David's spec, including but not
limited to:
		* Make signals visible in the choreography,
		* Figure out how to do runtime versus static role binding,
		* Adding a scope/function call functionality (maybe
ExtendsChoreography/DependsOnChoreography achieves this goal, I'm not sure)
and
		* Adding a true composition/segmentation mechanism (although I think some
of the hooks we need for this are already in the spec).

	But mostly, I think features need to be removed. Such as:
		* Remove all the choreography progress checking features mentioned in
section 2.2.5,
		* Collapse name and URN attributes into a single attribute,
		* Get rid of ConditionalEnd, Precondition & Process,
		* Make InteractionDef, MessageFamily and Role much easier to use in the
'easy' case and
		* Simplify start and end states

Long Winded Version:

Section 2.2.3 - Interactions, Reliable Messaging and Signals - My
understanding of the group consensus is that we do need to surface signal
information in a choreography. For example, a Buyer, upon receiving a
message processing complete signal might then send a message to an 'invoice
tracking' service telling it about the order and asking that the order be
tracked. The challenge before the group is to figure out how we can surface
signal based information in a manner that does not require us to adopt any
particular signaling mechanism. This is especially important given that
there is no standard Web Service signaling mechanism nor does one appear to
be on the horizon. So users of WS-Chor are most likely going to have to
contend with non-standard solutions for the foreseeable future.

Section 2.2.5 - Checking Choreography Progress - Per our last phone
conference I do not believe that we need to define a standard way to specify
what choreography/state a message belongs to at this time. I think this
feature fails the 'the spec is done when there is nothing left to cut' test.
After all, one can clearly implement a choreography without this feature and
even better yet it is easy to add this feature later in a manner that is
backwards compatible with existing WS-Chor systems.
	Although we didn't discuss this issue on the last phone call I would like
to signal a formal objection to introducing a mechanism for one role to
query another role's choreography state for the same reasons that I object
to putting choreography information into messages. The existence proof for
the ability to survive without this feature is self-evident and, again, we
can always add it later.

Section 3 - Scopes/Functions - At some point we will end up having to create
the equivalent of a scope/function call. This will need to be able to define
things like local roles.

Section 5 - Organization - While organizing the element definitions
alphabetically makes it easier to find things in the document it makes it
much harder to understand how everything works together. It would perhaps be
easier on the reader if the elements were organized with a focus on
pedagogy.

Section 5.2.1 & 5.2.2 - Name and URN - I'm vaguely worried about the idea
that a choreography has two names. The 'name' attribute that pretty much can
only be used inside of a single choreography definition file and a 'URN'
that can be used universally. Having two ways of doing the same thing in a
standard is dangerous, it leads to mistakes. Can we just reduce this to only
have the URN? The time honored hack around having to type in the whole URN
is to define the URN name using XML namespace notation, e.g. X:foo. In that
case so long as the default namespace is X then you can just say 'foo'
within the choreography definition and in more general circumstances give
the full name.

Section 5.3 - ConditionalEnd - It seems to me that explicitly identifying
ConditionalEnd states isn't necessary. This is something that can always be
determined by examining the choreography definition.

Section 5.4 - DependsOnChoreography - I agree with the intent of the
'DependsOnChoreography' feature but I think we will need to accomplish it in
another way. For example, does 'following' another choreography mean that
reaching any end state, even one reached as a consequence of a fault,
consists of 'successful completion'? I suspect this feature needs to be
subsumed into a more generic segmentation/composition feature.

Section 5.5.2 - Description@Ref - In general I council against the use of
attributes
(http://lists.w3.org/Archives/Public/w3c-dist-auth/1998JulSep/0084.html) and
this is a good example of why. One can quite possibly want to put multiple
Refs on a description. The first reason is that there may be multiple
documents one wants to point to. The second reason is that there may be
multiple alternate mirrors for the document one wants to provide. Trying to
express all of this in an attribute is a nightmare. Why not just switch to
using elements which can handle both scenarios easily?

Section 5.8 - Import - One of the things that is really annoying about WSDL
is that it only allows a WSDL definition to contain definitions for a single
namespace. This is also a really annoying problem with XML schema. There is
no inherent reason to have this limitation. So I would suggest that the
import element should not specify a namespace for the items to be imported.
Instead the imported items themselves should specify whatever namespace(s)
they are using (which the suggestion for sections 5.2.1/5.2.2 would provide
for). What I would like (and what I suspect was actually intended) is the
ability to provide a unique identifier (in the form of a URL) of the
document to be imported along with the location hint(s).

Section 5.10 - InteractionDef - I'm a big believer in the idea that easy
things should be easy and hard things should be possible. WSDL fails this
test because even easy things are hard. Part of its complexity is derived
from its insistence on forcing every aspect of a port definition (operation,
portType, Service, Port, etc.) to have its own name even when a definition
is only used in a single context and so could be defined implicitly by
containment rather than by an explicit naming. The result is that people
trying to use WSDL end up coming up with tons of names that make the file
really hard to read.
	I fear that InteractionDef is leading us down a similar road. I agree that
having re-usable interaction definitions is a good thing. One can easily
imagine extremely complex interaction definitions with lots of configuration
information that one would rather re-use than have to constantly re-define.
So I like the feature.
	But I think we need to make easy things easy by providing a way to define
an interaction 'inline' so that it doesn't need its own name and is
referenced 'by value' (as a virtue of its location) rather than 'by
reference' as the spec would currently require.
	I realize the road I'm walking down will lead to some non-trivial changes
in the syntax (but not semantics) of the specification but I think it's
worth going down that road in order to make it easy to do easy things.

Section 5.12 - MessageFamily - I support the concept of a Message Family.
Minimally one could imagine using Message Families to allow  a choreography
definition to be used with either WSDL 1.1 or WSDL 1.2 (remember, our
charter mandates support for 1.2 but it doesn't prevent us from supporting
1.1). The fact that Message Families also provide the potential to support
other messaging systems like ebXML is a major bonus.
	But I do think we need to tweak it a bit.
	First, I think the it should be possible to directly reference a WSDL 1.2
operation if one wants to in an interaction definition without having to use
the abstraction layer offered by MessageFamily. This is another example of
'easy things should be easy' issue discussed in section 5.10. If all someone
wants to do is just hardcode in a WSDL 1.2 operation then they should be
free to do so. We shouldn't force extra layers of abstraction on those who
don't need them.
	Second, I think that Message Families should be seen not as a way to refer
to multiple messages that 'could' meet the definition but rather as a true
placeholder in the same sense that a WSDL portType is a place holder. A WSDL
portType doesn't say 'here are the bindings I could be used with', it just
says 'I need to be bound'. I think in the same way an instance of a
choreography is defined by a binding that binds a Message Family to a
particular message. This means that when one creates a Message Family there
is no need to say anything about what messages it 'could' be bound to in the
same way that a WSDL portType doesn't say 'well I could be bound to SOAP'. I
think this actually calls for us to change the name of MessageFamily to
something a bit more singular and generic like MessagePlaceHolder or
MessageType.

Section 5.13 - Precondition - Providing for preconditions is fundamentally
dangerous and actively harmful to the interests of interoperability. Please
see http://lists.w3.org/Archives/Public/public-ws-chor/2003Jul/0135.html for
my full argument. Therefore I would like to see this feature removed.

Section 5.14 - Process - I don't think we need processes. A process is a
strictly internal action that does not need to be visible in a choreography.
So Figure 1 could look like:


   Buyer                           Seller

[Order Sent]--->Send Order--->[Order Received]--->(End Choreography)
                                   |
                                   |
[Order Error <--Send Order Error----
 Received]

	The choreography definition still provides a complete definition of all
shared behavior, thus illustrating why processes are not necessary.
	Another way to think of this is that WS-Choreography should only express
those parts of the choreography which are externally 'knowable'. If you can
change a piece of information in the choreography definition in a way that
an external observer could never discover then that information does not
need to be in the choreography definition. In this case one could change the
processes and 'internal states' within the Seller and so long as the
response to a Send Order is either nothing or a Send Order Error then the
Buyer (an external observer) would be none the wiser. This is why processes
can be safely dropped.
	A nice side benefit of dropping processes is that we can get rid of section
2.2.4 which will prevent us from having to have a really nasty argument
about what exactly a 'domain of control' is.

Section 5.16 - Role - This is another 'easy things should be easy' argument.
I can see the potential benefit of having roles defined separately from
states but this requires a whole bunch of linking. If someone just wants to
throw together states with hard coded roles they should be free to do so.
They should then have the option, much like I discuss in my comments about
MessageFamily, to put in a layer of abstraction and allow for late binding
of roles. But they shouldn't be required to late bind in all cases.

Section 5.18 - StartEndStates - Why are start and end states defined
separately from the states themselves? I would prefer having explicit
'start' and 'end' states (essentially reserved state names) and then use
interactions to hook other states into those explicit 'start' and 'end'
states rather than using the StartEndStates declaration to turn an existing
state into a hybrid that is both an existing state and a start/end state.
BTW, I realize this issue is dangerously close to 'tastes great - less
filling'.
Received on Thursday, 11 September 2003 00:34:21 GMT

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