[fdo-internals] RFC 20 for review

Haris Kurtagic haris at sl-king.com
Wed Jul 9 14:52:46 EDT 2008

Hi Tomass,

thanks for answers.

I am very uncomfortable with this RFC. Perhaps I don't understand it
well enough so I will ask many questions and try to understand
motivation for it.


In your previous answer you said that motivation is to not rebuild FDO
and providers because of one provider change. 

I think I understand your answer and motivation as it is written in RFC
but my concern was that doing that is not right thing. 


My main concern is that this RFC goes against one of the key values of
FDO: access different spatial data formats with one unified interface.

I think unified and clear access to different data stores is far more
important than "fast" adding of new capability for one particular

I see this RFC as a path to start to write applications which are tied
to particular providers.


I would like to understand use case of it and would try to ask with

If I see it correctly use case of this would be that we have application
build against let say FDO 3.4. Then we would like to add new capability
to e.g. SDF provider and then use that new capability in new build of
application with same "old" FDO 3.4 but with new version of provider .

Does this implies that we could have different flavors of FDO 3.4 or it
would be 3.4.1 but just no need to add capability to other providers? We
would say it is FDO 3.4.1 because one provider have new capability ?

I think FDO capabilities "belongs" to FDO not to providers. 


As of discussion about new capabilities enumerators not coded anywhere
or use of strings. I also feel like it is wrong.

How we would share those enumerations or strings between providers ? It
could end up that because some new capability is not supported in all
providers at same time that it will have different number in different
providers ? Having some list (or few of them) of codes for capabilities
sounds very odd too me.


As of error handling: same result ("isUnknown" to true ) would be for
non existing capability as well as for wrong type of it ?

Again sounds like we could end up with different type reported from
different providers for same capability.

When writing FDO client application It would be wrong if we would need
to check at different lists to find codes for same capability and to
check it against all providers.

You mentioned : "certain unique mechanism in place to avoid
duplication".  This is yet to decide ?


Sorry if I am little hard or long but this RFC seems very important to




From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Thomas
Sent: Wednesday, July 09, 2008 7:20 PM
To: FDO Internals Mail List
Subject: RE: [fdo-internals] RFC 20 for review


Hi Harris,


Thanks for the comments.


As for the motivation, currently there is no way to add a new capability
to FDO without the need to implement that new capability in all
providers that want to use the updated FDO code base. The new interface
concept provides the chance to actually do this as capabilities can be
added without changing FDO. As a result, only the provider that wants to
support the new capability needs to implement the support.


As for the error handling, this is documented in RFC 20. If a user calls
a function - for example GetBooleanCapability for a capability that
returns an Int32 - the function would set the flag "isUnknown" to true.
It is up to the caller to check this flag an react accordingly. For
example, the caller could either use a default value that is appropriate
in this case or throw an exception. But this is all up to the caller.







From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Haris
Sent: Tuesday, July 08, 2008 7:45 PM
To: FDO Internals Mail List
Subject: RE: [fdo-internals] RFC 20 for review


I think this is much more complicated way of exposing capabilities
rather than like it says in RFC simplifying.


I am not sure about motivation for it. For example If application is
build with newer version of FDO core I would think that older provider
will not be used anyhow .

I can't see reasons when application which is build for use with one
version of FDO libraries will use older providers. 

Also Isn't it going to be a problem when application is linked with one
version of FDO core libraries and provider is like build with another.
With current dll naming it is not possible I think ?


I couldn't find in RFC error handling for case when function of wrong
type of capability is executed for existing capability ( e.g. Call
GetBooleanCapability for Int32 ) ?




From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Thomas
Sent: Tuesday, July 08, 2008 11:38 PM
To: fdo-internals at lists.osgeo.org
Subject: [fdo-internals] RFC 20 for review




RFC 20 (http://trac.osgeo.org/fdo/wiki/FDORfc20) has been posted. It
proposes a simplification of the FDO capability interfaces. Please
review the RFC and let me know of any questions and suggestions you may
have. The review deadline is set for end of day July 18th.  It is
intended to request a vote on the RFC afterwards.






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