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Subject: RE: [sca-bpel] Issue 2 - Does the spec allow a componentType side file? Some observations



Regarding your general comment that you believe that component types are should “only be used in rare cases where it is not possible to derive all necessary information through inspection of the implementation artifact”....


There is interesting difference between BPEL and C++ or pre-annotation Java is that BPEL uses XML for its syntax, and the schema for that XML explicitly allows extensions.  This means that it is always possible to specify all of the information necessary for the component type in the BPEL file.  So, given this, why have component types?  The answer is for specifying default bindings, targets and policies that a developer might not _want_ to specify within the BPEL file.  In fact, a different person might be in charge of developing the BPEL process from the person who is developing the “SCA view” of that process.   Or, if not a different person, perhaps a different tool.


Anish has also brought up the value of having the component type _override_ information that is in the implementation, and several of us, myself included, thought that this sounds like a valuable capability.


This gives component types a bit of a different flavor from one of just a “necessary evil”.


Now, regarding how you find the implementation that a component type corresponds to – yes, it is possible to do this by the name of the file of the component type.  Personally, I find this awkward.  I think it is bad design for an artifact (the component type) to be fundamentally tied to another artifact (the implementation) and yet for it not to record that fact.  Depending on the location of component type means that whenever you look at a component type, you need to know where it came from in order to make any sense of it.  If you were to copy it into a registry, for example, it would no longer make sense unless you added the information about where it came from.


In the early days of Java, I worked in a development environment which assumed that the generated .class files would be in the same directory as the .java files (which is the default).  In many cases this is just the wrong organization of files – but the developers of this environment thought that they knew better about what would be “easy”.  They were wrong.  I believe that we should not make the same mistake.


These are my reasons for wanting the approach represented by my motion at the end of the last meeting, although in the interest of compromise, I could imagine supporting a mixed approach, along the lines of what Alex has proposed.




From: Mike Edwards [mailto:mike_edwards@uk.ibm.com]
Sent: Friday, January 18, 2008 7:03 AM
Subject: Re: [sca-bpel] Issue 2 - Does the spec allow a componentType side file? Some observations



Watching the debate on this issue, I wonder if we are collectively losing sight of the big picture here.

Let me start by giving my viewpoint, since it will colour the rest of what I have to say.  I take the view that
componentType files are a necessary evil, only to be used in rare cases where it is not possible to
derive all the necessary information through introspection of the implementation artifact(s) (Note [1])

In general, developers do not thank us for creating ever more files for them to deal with and I think
that SCA in general should try to keep files beyond the implementation artifacts to a necessary minimum.

- Having said that, I argue that an SCA BPEL runtime should handle things FIRSTLY by introspecting the
BPEL process document.  I would encourage the idea that IF "pure" BPEL is not sufficient to unambiguously
define the componentType of the process, then it is valid and useful to add SCA metadata to the process
to remove the ambiguity.  This I view as a close equivalent to the SCA annotations for a Java class.

FOR the (hopefully rare) case where there is the need to provide additional componentType metadata
separately from the BPEL process itself, I argue that we should again keep this as simple as possible.

- The general design for componentType files (for any type of implementation) currently keeps those files
clear of any metadata that directly relates to the implementation artifact.  The componentType file that
relates to a given implementation artifact is handled by filename - it is the filename that is used to establish
the connection with the implementation artifact.  The location of the file relative to the implementation
artifact is then left up to the implementation type specification to decide.

In the debate on this issue, it is clear that there is a desire to move away from this design.  First, there is
introduction of a concept of a "label" within the componentType file which is used to indicate which
implementation artifact it relates to.  Secondly, there is a desire to free the location of the componentType
file, so that it may be in any place within the same contribution as the implementation artifact, but that
each runtime implementation may choose which location(s) it will scan when looking for componentType

- I  note that one effect of these desires is to place implementation specific metadata into the
componentType file - and I also note that there are other proposals in the BPEL group to put yet more
metadata specific to BPEL into the componentType files.

- Examining the "label" concept, it adds a level of extra work, since the label must be added to the
componentType file and it must point to the appropriate artifact.  However, it does permit the the filename
of the componentType file to be made independent of the name of the implementation artifact.  It also
(in principle) allows for multiple <componentType/> declarations in a single file.  I note that this labelling
could extend to ANY type of implementation artifact, although the details of the labelling would
necessarily vary  by implementation type.

- Allowing for variations in the location of the componentType files within a contribution certainly
affects portability.  A contribution which locates the componentType files in location A may be processed
correctly by runtime Z, but incorrectly by runtime Y.  Secondly, there is the question of the understanding
of the developers.  "Skills portability" is perhaps more important than artifact portability.  I suspect
that developers will find it annoying that the location skips around when they move from runtime Z
to runtime Y.  This may possibly be masked by tools, but people don't always use tools, especially
when things go wrong and they start debugging.

- One aspect I definitely DONT like is the idea that the runtime has to search around the contribution as
a whole in order to find these componentType files.  They should be in some "well defined" place to
limit the search space.

- Another concern is the confusion if BPEL treats its componentType files in a way very differently to
that for the other implementation types.  It would be best to see a coherent set of changes across all
implementation types, otherwise developers dealing with mixed assemblies will keep having to
switch from one way of doing things to another.

- The latest set of proposals give us "the worst of all worlds" by proposing to allow searching for
componentType files BOTH by filename and ALSO by label matching.  I don't think that this kind of
"flexibility" really helps anyone and introduces error cases (filename and label point to different
BPEL artifacts) - I make a plea for us to make up our minds and have one single consistent

OK, so to cut to the chase (sorry I made you read so much to get here) - what do I propose?

1) Keep the current mechanism based on location and filename
2) Fix the location of the componentType file relative to the BPEL process artifact


[1] I acknowledge that there will be cases such as C++ where even though it may be possible to derive
all the required information about the implementation artifact, this has to be done at compile & build time
and that the information so extracted has to be put somewhere by the tools used to do this.  The
"somewhere" is in fact a componentType file, which as a result is a mandatory element of the handling
of such implementation artifacts.

Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com


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