Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)

  • To: Jon Blower <j.d.blower@xxxxxxxxxxxxx>
  • Subject: Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
  • From: Rich Signell <rsignell@xxxxxxxx>
  • Date: Tue, 4 Jan 2011 07:51:07 -0500
Jon,

I would say it depends on how easy it is to implement (1) vs (2).
We have many datasets with 20+ variables all with the same time base,
so implementing (2) would pay off big.     But if you implement  (1),
implementing (2) is hardly necessary, since the getcaps doc will be
trivial in size even when repeated for 100 variables (except in very
unusual cases where the interval specification of time rivals the
explicit specification of time in size).

I'm glad you are planning to implement this soon -- the huge getCaps
documents are currently crippling our use of ncWMS for our IOOS
Testbed Project!

Thanks,
-Rich

On Tue, Jan 4, 2011 at 5:54 AM, Jon Blower <j.d.blower@xxxxxxxxxxxxx> wrote:
> Hi all,
>
> I can certainly see that there is a problem that needs to be addressed here 
> (explicit lists of all individual TIME values cause the Capabilities document 
> to blow up).  There are actually two approaches to this, which could be used 
> individually or in combination:
>
> 1) Use the syntax start/stop/period, potentially multiple times, to define 
> the TIME values instead of listing them explicitly.
>
> 2) Use Layer inheritance properties to define the time dimension once only, 
> if the same time axis is shared by all the children of a parent layer:
>
> <Layer>
>  <Title>My Model Output</Title>
>  <!-- Non-displayable parent Layer -->
>  <Dimension name="time"> ... values ... </Dimension>
>  <Layer>
>    <Title>sea_water_temperature</Title>
>    <Name>TMP</Name>
>    <!-- Inherits time axis from parent -->
>  </Layer>
>  <!-- More child layers -->
>  <!-- Children can override the time axis if theirs is different for some 
> reason -->
> </Layer>
>
> The most concise possible Capabilities doc would be achieved by combining 
> both approaches.
>
> I feel that we should ensure that only those time values that are actually 
> present should appear in the Capabilities doc - I think things get a bit 
> confusing if the Capabilities doc advertises "missing" times (what would the 
> returned image from a missing time look like?).  I also agree with Bob Simons 
> that the use of "nearest" values is dangerous, even though it's in the spec 
> (sorry Kyle) - the client can always perform the nearest-neighbour 
> calculation if this is required, given the server's advertised capabilities. 
>  (Feel free to disagree with me of course!)
>
> I can see two potential problems:
>
> 1. Solution 1 above is a bit tricky to implement in the general case, 
> avoiding the corner cases.  (Solution 2 would actually be pretty easy to 
> implement.)
>
> 2. As Ethan and Roy have pointed out, third-party client support for 
> multidimensional WMS is, er, generally not great.  It's hard enough to find a 
> client that supports TIME at all, never mind all the possible syntaxes.  I'm 
> torn on this - in one respect it's not our problem, but we don't want to cut 
> out portions of the user base.
>
> So, after all this, I propose a solution:
>
> 1. Implement one or both measures above, ensuring that the Capabilities 
> document is accurate.  This may involve being conservative.  The default 
> Capabilities doc would be much smaller.
>
> 2. Allow clients to specify a URL parameter to GetCapabilities that triggers 
> the generation of a Capabilities document that *does* list all the time 
> values explicitly, allowing compatibility with some GIS clients.  (Clients 
> usually require a URL to the Cap doc, which could include this non-standard 
> URL parameter.  Or the parameter could be considered part of the "base URL").
>
> Does anyone have any thoughts on this before I start an implementation?  It's 
> tempting to implement the "layer inheritance" solution first since it's 
> easiest; I think this would be effective in TDS, where each Cap doc usually 
> represents a single model run, which will usually have a single time axis, 
> shared among all variables.
>
> Happy New Year!
> Jon
>
> --
> Dr Jon Blower
> Technical Director, Reading e-Science Centre
> Environmental Systems Science Centre
> University of Reading, UK
> Tel: +44 (0)118 378 5213
> http://www.resc.reading.ac.uk
>
>
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-- 
Dr. Richard P. Signell   (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598



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