Resend to the mailing list. ---------- Forwarded message ---------- From: Even Rouault <[email protected]> Date: Thu, May 17, 2012 at 12:05 PM Subject: Re: [gdal-dev] Can OGR WFS driver take advantage of WFS Server streaming? To: Yingqi Tang <[email protected]>
Le jeudi 17 mai 2012 20:52:07, Yingqi Tang a écrit : > Even, > > Just to clarify, when you say: > > "...server allows partial downloading of arbitrary ranges of bytes within > the file...", what does it really mean? If I simply put a static GML file > in IIS and share it through HTTP, will it support partial downloading? Yes, static pages should support partial downloading. But WFS servers that generate response on the fly generally don't. > > Thanks, > Yingqi > > On Thu, May 17, 2012 at 4:19 AM, Even Rouault > > <[email protected]>wrote: > > Le jeudi 17 mai 2012 00:25:52, Yingqi Tang a écrit : > > > Thanks for confirming it, Even. I have two follow up questions though: > > > > > > 1. Does the GML driver (reading local big GML file) also load the whole > > > > GML > > > > > in memory too? > > > > No, it will read it by small chunks. If no matching XML schema is > > available (or that the parser of the GML driver cannot manage to > > understand it because > > it is too complex (when the GML data is not limited to the Simple Feature > > convention)), there will be an initial full scan of the file to determine > > the > > schema of features. > > > > > 2. Does the GML driver take url pointing to a GML? > > > > It can work with /vsicurl/http://something, provided that the server > > allows > > partial downloading of arbitrary ranges of bytes within the file, which > > is rarely the case with GetFeature() requests. > > > > In theory the WFS driver could be improved to use the capability of the > > GML driver to read chunk by chunk, but there is some work to make that > > happen. This would probably require a /vsicurl_streaming/ virtual file > > system where basically only Read() requests would be available, thus not > > requiring the server to have download range capabilities. > > > > > Thanks > > > > > > On Wed, May 16, 2012 at 3:20 PM, Even Rouault > > > > > > <[email protected]>wrote: > > > > Le mercredi 16 mai 2012 23:58:29, Yingqi Tang a écrit : > > > > > Hi all, > > > > > > > > > > I am communicating against a WFS server which supports data (GML) > > > > > streaming, which means that once I send a GetFeature quest for a > > > > > big GML (2.0GB+), server will take only seconds to start streaming > > > > > GML back to client while at the same time it's still preparing the > > > > > rest > > > > of > > > > > > > the big > > > > > > > > GML. > > > > > > > > > Now I wonder if GDAL/OGR WFS client can take advantage of that? > > > > > Will > > > > I > > > > > > > be able to access the first few features while the streaming is > > > > > still going on? It seems to me that it doesn't because it always > > > > > waiting > > > > the > > > > > > > whole > > > > > > > > big > > > > > > > > > GML response is loaded in memory. > > > > > > > > Yes indeed, the OGR WFS driver waits for the whole response to be > > > > downloaded > > > > before parsing it. It could be desirable to have on-the-fly parsing > > > > indeed, but > > > > we currently lack the capability of doing it with the way we use the > > > > Curl > > > > > > library. > > > > > > > > > Thanks
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