[Mapserver-dev] OGC ServiceExceptions and invalid layers

[Paul Spencer]

All,

the purpose of this email is to solicit feedback on how to handle 
exceptions returned from OGC services like WMS and WFS.  Currently, 
MapServer WMS and WFS client code obtains the result of a WMS or WFS 
call and uses it as the source for a layer.  If the remote WMS/WFS 
returns an error as an XML document (ServiceExceptionReport), this 
causes the layer to be invalid and causes MapServer to fail to render 
the map completely.

With WMS, the only option appears to be requesting errors as INIMAGE, 
which is not always desirable.  With WFS, this is not an option (I 
think, please correct me if I am wrong).

This issue has come up (again) because we are beginning a series of 
projects involving WFS and filtering (newly added by Assefa).  The 
filter support in MapServer is somewhat picky about allowable filter 
types because of differences between the OGC filter model and 
MapServer's internal filtering model.  When attempting to apply 
dynamically generated filters, quite often we are experiencing service 
exceptions that relate to these differences.  We are trying to build the 
filters correctly and detect errors before the filter is applied, but 
there are always going to be cases that are either very difficult or 
impossible to catch.

I would like to propose that a different mechanism be put in place for 
handling layer errors such as this.  I am really only concerned about 
handling OGC service exceptions, but if this is applicable beyond the 
OGC stuff then that would be great (also why I am emailing the whole list).

The desired functionality is to have the various OGC clients to detect a 
ServiceException and push the information onto the error stack for 
presentation to the user, then disable the layer and allow the remaining 
layers to render normally.

Something like this is already done for WMS timeouts (when a layer takes 
too long to return a result).

Comments/questions/flames welcome.

Cheers,

Paul

-- 
  -----------------------------------------------------------------
|Paul Spencer                           spencer at dmsolutions.ca    |
|-----------------------------------------------------------------|
|Applications & Software Development                              |
|DM Solutions Group Inc                 http://www.dmsolutions.ca/|
  -----------------------------------------------------------------