Hi Andrea, thanks for the feedback, comments inline.
On Wed, Apr 13, 2011 at 12:54 AM, Andrea Aime
<[email protected]>wrote:
> On Wed, Apr 13, 2011 at 3:55 AM, Justin Deoliveira <[email protected]>
> wrote:
> > Hi all,
> > As most know the next version of the wfs spec (2.0) brings paging into
> the
> > mix which just boils down to the parameters startIndex and count on a
> > GetFeature request. While it looks quite promising that wfs 2.0 will
> > becoming to geoserver in the next few weeks / months, we have a client
> > currently that requires paging functionality via WFS. So I have put
> together
> > a patch that implements paging for all wfs versions.
> > http://jira.codehaus.org/browse/GEOS-4485
> > Feedback welcome.
>
> Hi Justin,
> I did not review deeply, just had a quick walk over the patch sources, but
> it
> looks pretty much like what I was expecting (one tricky part that we might
> want to double check, maybe with some more tests, it's the wfs 1.0 feature
> counting).
>
Sure thing... can you be more specific about what cases to test? Most of the
test cases there are wfs 1.0 with a mix of startIndex and maxFeatures. I
tried to catch most of the boundary cases but probably missed some.
>
> So I'm wondering about count/maxFeatures. Are they basically going to be
> aliases with the same meaning?
> So that one can do
> ...&startIndex=10&maxFeatures=5&...
> or
> ...&startIndex=10&count=5&...
> and the result will be the same?
>
That is the general idea yeah. The patch as written should do that for the
GET case. For the POST case count is not yet handled... i was going to put
that off until wfs 2.0 starts.
>
> I'm also wondering about paging stability.
> The current setup will return incoherent results if someone deletes
> or updates a feature while we page. Not complaining, doing a full dump
> of the results just for the sake of paging in a stable way would be a
> massive
> overhead, just wondering if the problem had been considered.
>
Yeah I did consider this after reading the spec because technically this is
a requirement of spec on the server. But seems quite unrealistic to me since
as you say we would have to maintain some sort of cache or something on the
server. It would make the WFS GetFeature a stateful operation... so I
imagine we would need so start creating sessions to track the request or
something. All in all I think just having this be a "limitation" of the
implementation is probably going to suffice in 99% of cases.
Also it seems that WMS GetMap already supports paging... and this is more or
less following suite what it is doing.
>
> Also curious about sorting... I remember something like sorting by default
> on the feature id while paging, but my memories of it are foggy, we
> discussed
> this with Gabriel a loong time ago (when the paging machinery was added to
> GeoTools).
>
As I understand things how they are implemented now using startIndex
requires the underlying datastore to be able to do sorting. And when the
client does not specify an explicit attribute to sort on this means doing a
natural sort (feature id). If you look at
ContentFeatureSource.getReader(Query) you will see a check there.
Which more or less means that paging can only be used with jdbc datastores.
Which personally I think is fine since they are the only ones that can
really do it efficiently. Thoughts on that?
>
> Cheers
> Andrea
>
> --
> -------------------------------------------------------
> Ing. Andrea Aime
> GeoSolutions S.A.S.
> Tech lead
>
> Via Poggio alle Viti 1187
> 55054 Massarosa (LU)
> Italy
>
> phone: +39 0584 962313
> fax: +39 0584 962313
> mob: +39 333 8128928
>
> http://www.geo-solutions.it
> http://geo-solutions.blogspot.com/
> http://www.youtube.com/user/GeoSolutionsIT
> http://www.linkedin.com/in/andreaaime
> http://twitter.com/geowolf
>
> -------------------------------------------------------
>
--
Justin Deoliveira
OpenGeo - http://opengeo.org
Enterprise support for open source geospatial.
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