How does MapServer store vector data internaly?

[Tom Kazimiers]

Frank Warmerdam schrieb:
> Tom Kazimiers wrote:
>> Hello,
>>
>> can you tell me how the MapServer deals with vector data? I mean what 
>> happens after a Request?
>> E.g.: If file not already open, the file is opened, a quadtree is 
>> build and the request is performed on the quadtree data and no 
>> further file reading is required.
>
> Tom,
>
> This is a very broad question.  The definition of what MapServer does 
> is in
> the source.  But I'll try to give a wee bit of information.  I'll address
> this in the context of a WMS GetMap request with vector layers from 
> shapefiles.
>
> 1) The map file to be used is identified, and parse into a mapObj.
>
> 2) The request parameters are parsed and used to manipulate the mapObj 
> state.
>    (turning layers on/off, setting the region and resolution, etc)
>
> 3) Each "on" layer is rendered in turn in msDrawLayer().
>
> 4) For vector layers, the layer is "opened" which basically means the
> associated data files are opened.  In normal use vector file handles are
> not kept open from request to request (since each request is a new cgi
> invocation) and not even from layer to layer in the same map draw.  So
> called "connection caching" in a fastcgi situation is an exception to the
> rule.  For shapefiles this open results in opening the .shp, .shx, .dbf
> and .qix if there is one.
>
> 5) The draw function loops through asking for shapes from the shapefile
> code.  If there is a .qix file, mapserver will use it and the spatial
> extent to figure out the shape indices to load. (actually, I think it may
> do this in one complete pass and then sort the indices rather than doing
> it on an ongoing basis).
>
> 6) each shape is drawn.  Note that the in-memory representation (a 
> shapeObj)
> normally only lives long enough to be drawn and is freed before the next
> features is read.
>
> 7) the layer is closed, and all associated files are closed.
>
> NOTE: there is no in-memory quadtree built.  If a qix (spatial index 
> using
> quadtrees) exists on disk it is used, otherwise all features are 
> scanned and
> compared to the spatial and attribute filters.  I think there is some 
> short
> circuiting of the shape reading to first look at the shape extent before
> processing the rest of the shape.
>
> Particulars vary based on the file type, the kind of request made, and
> various other circumstances.  I'm also likely a bit off on some low level
> details.  But hopefully this is a helpful response to what I claim is 
> a vague
> question!
>
> Best regards,
Frank,

thank you for your quick response. This is what I wanted to read indeed.
I did also take a look at the source, found also msdraw.c but somehow 
missed orientation :)

This (vague) question came up by thinking about how I can change a tool 
I have written which is basically a GIS application for PDAs in way that 
I can for example identify single features of a shapefile (since 
currently I only draw them without making constraints like attributes 
available).

Do you think that if MapServer would be a whole GIS application on its 
own and no "server" and has only to deal with about 20 maps or so it 
would also do displaying like it is done now? Or could it be that in 
this case the opening and closing of files would be too slow compared to 
an in-memory quadtree for example?

Thanks again
Tom