MS RFC: Layer Plug-in Architecture

[Jani Averbach]

Hello, 

If you are interested in MapScript then please read on.

The biggest open problem with MS RFC-3 is what to do with layer
mutating (i.e. changing type of existing layers) from MapScripts.

I am going to amend the RFC-3 by following way:

1) The vtable will be initialized when any of the MapServer's layer
   functions is called for the layer first time. This makes it
   possible to build layers on-the-fly field by field with MapScripts.
   No extra initialization is needed for new layer.


2) To change the type of existing layer, following conditions have to
   be met: The to-be-changed layer instance has to be a copy, you can
   not change layer type without copying the layer instance first.
   Secondly, the change of connection type has to be made by function
   call, with connection type and optional library name as parameters.
   This will break existing scripts which modify directly connectiontype 
   field.

So far Sean and Frank have shown support for #2, I will include the second
option at the end of this email.  If you feel strongly against #2,
please speak up now so we can sort out issues before
voting/implementing this.  If the second point is too controversial,
we could implement it a backward compatible way, and tell that
modifying directly connectiontype is deprecated and will be removed in
the future.

Thanks,
Jani


Another solution for layer mutating:

Add an extra field to the vtable, which also tells connection type.

Then add to the layers interface functions a logic that if vtable's
and layer's connection type are different, then vtable will be
switched on-the-fly to reflect layer's connectiontype.

This solution will bring few consequences:
- Per every layer function call we have to do few extra check
- For custom layer functionality, we have to provide an encapsulated
  interface to change from one custom layer to another because for
  different custom layers, the connectiontype is same i.e. CUSTOM,
  and actual information which library and hence backend to use, is
  stored elsewhere in the layer.

-- 
Jani Averbach