Yaron Y. Goland wrote:
Ron Ten-Hove wrote:
This is why <assign> is atomic. The
intermediate states are never exposed.
Even trivial XML manipulation tends to involve some forms of
computation during the manipulation process. A classic example is a
while loop that goes through a document pulling out information in
order to create a new document. During the time the while loop is
iterating it is likely that the new document will be in a schema
inconsistent state. Assign's atomicity is of no use here since there is
no way to shove a while loop into an assign.
Looping to construct a variable is a good example of where the
atomicity of <assign> is insufficient to hide the intermediate
stages you are concerned about. This could be addressed in several ways:
Use serializable scopes.
Introduce a concept of "compound assignment" which is also
atomic. This could contain the <while> loop of concern.
Learn to live with the problem. This is nothing new; let the
programmer beware, as be already must in countless other languages.
More generally, any XML manipulation which is not strictly linear and
involves schema inconsistent intermediate states cannot be dealt with
in BPEL since assign cannot contain decision or iteration logic.
Is there a requirement that variables be schema-valid during all stages
of variable manipulation? Or should the requirement be the looser
"variables should be schema-valid when used (directly or indirectly) in
a message sent by <invoke> or <reply> activities? Or is
there a more useful requirement in the arena?
Therefore if one wants to enable even simple XML manipulation in BPEL
one inevitably ends up having to create some kind of transacted schema
free zone. I'm suggesting we don't want to go there.
The assign mechanism, in all its various forms that we have seen in
this forum, is essentially a piece of imperative code embedded in a
declarative process model. This does introduce some form of
"impedance" mismatch. However, I think you are overstating the problem
by suggesting that this is too difficult to address. Imperative logic
that manipulates any kind of structure will always introduce transient
states that are illegal / nonsensical; this is a problem that has been
with us since t = 0. :-) This is also a problem that has been
addressed in the past, where t ≥ 0.
Is it your assertion that we should create a
standard that MUST be
supplemented by an unspecified companion language, in order to create
executable processes?
Not at all. There are many ways to make BPEL work on its own. But I am
asserting that BPEL should focus on the areas that it adds value and
not re-invent functionality that is widely available in a standardized
form.
Leveraging existing standards is a Good Thing, but being insufficiently
prescriptive can harm portability. Leaving this completely open to the
whim of BPEL engine providers harms the utility of the BPEL spec in
this regard. (I believe the proposal Danny has put forward leverages
the existing W3C DOM work to a high degree.)
-Ron
