[Freegis-list] Re: Compressing GML

[Chris Holmes]

> I am curious if people out there have made any progress in the area of
> compression/decompression of GML for transport. ( I recall a while
> back someone complaining of GML’s verbosity, and others replying
> that’s ok, you can always gZip it
)

> Our case: my people are testing all sorts of permutations of clients
> seeking data from WMS and WFS, and are making a series of Excel charts
> documenting what happens in each case, varying the number of
> simultaneous users (they generate multiple user threads to attack the
> server), the server characteristics (RAM, O.S.), and the data size as
> retrieved. Among other things they are finding that retrieving a
> several megabyte GML as generated by MapServer’s WFS, is not possible
> under the generous 10 minute timeout limitation
with client and server
> on machines in the same laboratory and connected via 100 Mbit switch.
> However, when they fast-Zip a 100+ MB file (manually) they find that
> it is reduced to about 5MB. So it would seem to be a good idea for the
> server to do this, under certain circumstances.

> So, who is doing this? Is there code available?  How are we on the
> issue of GML compression? Or on the occasional use of GML-binary?
Yes, we wrestled with this in GeoServer for awhile.  Some said that it
should always just be the http server that you set up in front of your
WFS to deal with.  Like Daniel says, Apache is easily configured to do
this, and then you can use a connector to tomcat, or whatever servlet
container you like.  Browsers will transparently handle the requests. 
And then wfs clients could also be coded to do it at the level of http
transport.

In GeoServer we also support gzipped gml2 as an output format (use
'GML2-GZIP' for the outputformat param), and the gml production will go
through a gzipping output stream.  Some thought it might be useful, so
it's just an extra format.  No one's asked about this for awhile, but I
think it should work.  We no longer advertise it in the capabilities
document, because it breaks the CITE conformance tests, which is a bit
annoying to say the least.  I think the schema can support additional
output formats, but CITE only recognizes a certain subset.  Or at least
that's what I recall, it's been awhile.

Gabriel Roldan (cced) was the author of the capability, and he also
looked into bxml encodings for GeoServer.  If I recall correctly it
wasn't as big of a performance win as we were hoping.  He may be able
to tell you more.

But if MapServer is taking too long to generate the GML file, then I
don't think gzipping on the fly will help - I assume your networks are
fast and it's not the transport, but the making of the GML file.

Chris




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