Edwin,
Thanks very much for the thoughtful reply. You've raised some very
good points; the one I most appreciate is the reminder that there are
costs and benefits for all the language features we create, or, I would
add, exclude. Users will bear some of the costs, after all.
My comments are in-line with yours, below:
Edwin Khodabakchian wrote:
We should be driven by uses
cases NOT analogies. In this case case, the use case is about re-use.
As pointed out, there are multiple ways to address that need:
(1) Encapsulation of the logic
to be re-used in a seperate process
(2) Let the tooling address
this need through templates
(3) Create a new construct at
the language level (called subfunction)
It seems that the only
benefits of (3) over (1), (2) or a combination of (1) and (2) is that
the use of subfunction signatures can having to avoid creating a new
message type and the assign activities needs to initialize and read
data from the message types.
This is not an insignificant advantage. Crafting new message types,
and writing "gnarly" assign activities are not simple tasks, and can be
regarded as "overhead" getting in the way of the process author. Put
another way, our domain-specific language requires a lot of solution
domain elements; sub-functions could serve reduce the number of
solution domain elements the author need worry about.
There might be also an
argument about (3) offering better support for privacy and packaging
and deployment.
But there are also
drawbacks/costs with (3) that need to be weighted:
(1) Complexity:
to really deliver on the benefit of sub-functions, we need to replicate
a very large number of concept and constructs already existing in the
process and scope elements.
I think your general point about increased complexity is true, but I'm
not sure about the need to replicate much of what already exists.
Yaron's proposal seemed a very straight forward extension of those
concepts, not duplicative. Perhaps you were referring to
implementations, rather than the language itself?
(2) Lack of
communication channel: we need to create a communication
mechanism between the subfunction and the calling activity so that the
subfunction can report its status. This last need is much better
addressed with option (1) because already supported by scope event
handlers and receive activities.
An interesting point. I regard the proposed sub-function invocation
mechanism as a simple extension of that used by existing activity
types, such as <invoke>. Variables are passed by reference,
either as input, or output. The chief difference is cardinality --
subfunctions allow multiple input (and output?) variables. The general
mechanism, from the user's perspective, is the same.
I'm not sure what you mean by a subfunction reporting its status. Are
you envisioning some sort of asynchronous subfunction invocation?
This is an area where BPEL is
very different from traditional flow languages and this is why I am not
sure that your "other process language do this" argument is valid.
I don't think BPEL is so different from existing languages, just
simplified. The adoption of an abstract messaging model for work
performers and requesters does eliminate (or move elsewhere) some of
the complexities found in other languages. In some other languages it
is theoretically possible to model other processes as work
performers/requesters, but this, as far as I know, has never been
carried to the extreme of requiring that it be done so. Regardless, I
don't think BPEL is so different that the need for for a compositional
mechanism supported by the language itself has been eliminated.
(3) Lack of
consistency:
This is a software point but I
find the passing arguments to the subfunction not consistent with the
rest of the spec: you need to create a new subfunction interface
definition language (not WSDL), you need to learn that sometime you can
do assign (when using invoke, receive, pick) for data manipulation and
sometime you can plug expressions directly in the signature (this is at
least what Yaron is suggesting).
Right. There would be a new concept, the subfunction, which is invoked
in a syntactically different, seemingly more direct way than services.
This distinguishes subfunctions from (external) services, but at the
cost of the user knowing the difference. The actual "pass by reference"
semantics would remain the same, keeping things consistent at that
level.
I believe that the single most
important success factor for BPEL to succeed is to remain simple (which
is not trivial given that we inherit the complexity of XML Schema and
some of the exotic variations of WSDL). Given that there is a significant complexity "cost"
associated with adding this feature to the language, I would like to
recommend following Ugo's advice and finding use cases where the
problem can not be addressed using a combination of (1) and (2) and
therefore require us to go the extra mile of spec'ing out (3).
Unfortunately we have already established that we will not simply
reduce BPEL to the simplest language possible. (I find this minimalist
approach very attractive, but this has been rejected by the TC. BPEL is
a modelling language as well as an executable one. This will increase
language complexity / clutter.)
I still find Ugo's suggestion (that you refer to) as being decidedly
biased, and it certainly misses the mark in this issue. As you
mentioned earlier in your note, the question is about reuse: what
language support for reuse should we have? This affects readability,
composability, and (less directly) the granularity of reusable
"components" that are feasible. None of these factors are examined when
simply looking for use cases that cannot be addressed.
Your point about cost/benefit trade-off is well taken. Adding more
domain-specific concepts to a language makes it more complex to
implement, while making it easier for end-users to utilize. Conversely,
reducing the number of domain-specific concepts makes it harder for
end-users to use the language, but makes implementations easier to
realize. There is a balance to be achieved, not by always favouring
minimal language features, but, as Satish reminded us, by using our
judgement. How are we to judge the cost/benefit of language-based
support for reuse?
Best regards,
-Ron
