> My initial feedback was more about how to advertise what functions are
> available -
allowing only these functions is possibly the higher priority.
> so geoserver can report them as available etc.
>There is one thing to check in the WFS specification; I am under the
impression that server needs to report back a filter capabilities
(including a function list). These capabilities may be for all feature
types? Or hopefully on a feature type by feature type basis ...
depending on what the specification says we may need to set up a change
request for the OGC.
Ahh! Herein lies a quandry. WFS doesnt allow per feature functions to be
advertised, and most interesting features have implicit or explicit operations
that would be nice to advertise per feature as functions. Ideally, those
functions are only allowed on those features too.
IMHO WFS does not fully implement the ISO General Feature model, in its current
form.
That said, just knowing the function name isnt very useful... you really need
to know the range of the values of the arguments. This brings us back to the
more general issue of advertising a query model for a service, and I think the
functions are part of this. Simon Cox and I submitted a change request to WFS
based on my prompting - that allowed a per-feature type query model to be
advertised by reference, so we didnt have to prematurely define the ultimate
query model definition language just yet. Unfortunately there just isn't
enough experience with WFS out there for people to fully grasp the implications
- they still think in terms of eyeballing all the data on the disk to see how
to formulate a sensible filter.
IMHO any SDI will need to catalog query models ("what questions can you ask")
and link services to these, rather than cataloging services and then hoping to
magically find a way of understanding what they do. This takes the metadata
out-of-band, and we can implement an SDI with current WFS quite happily.
Rob
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