MS RFC: Layer Plug-in Architecture
Jani Averbach
javerbach at EXTENDTHEREACH.COM
Mon Aug 22 14:35:57 EDT 2005
Hello,
Here is my formal proposal for Layer Plugin/VTable architecture for
MapServer. There are few questions and unclear things in the proposal,
hence I have provided it as ''draft''. If the MapServer Steering Committee
feels that the proposal is clear enough, I am fine if the Status is changed
to "proposed". The proposal should be valid restructured text.
If you have any comments, recommendations or issues about this proposal,
please let me know. Thank you.
BR, Jani
==============================================
MS RFC: Layer Plug-in Architecture
==============================================
:Date: 2005/08/22
:Author: Jani Averbach
:Contact: javerbach at extendthereach.com
:Last Edited: $Date: 2005-08-22 11:45:31 -0600 (Mon, 22 Aug 2005) $
:Status: draft
**Description:** Change current layer infrastructure from switch based
static solution to the virtual table based dynamic solution. The main
objectives for this proposal are cleaning and simplifying code and
provide a robust architecture to implement dynamic plug-in API for
external layer providers.
Abstract Solution
-----------------
Implement a virtual table structure for ``layerObj``. This structure will
contain function pointers for all layer specific operations. This
structure will be populated when the layer is created or when it is
opened. After that all layer operations will happen through this
vtable which is cached in the ``layerObj`` struct.
Technical Solution
------------------
All file names and numbers are against released MapServer 4.6.0 source
code.
1. Add new field to the ``layerObj``. It will be pointer to the vtable,
which will contains function pointers for this layer.
2. Add the virtual table architecture
2.1. Standard functions which are found currently from ``maplayer.c``
2.1.1. InitItemInfo
::
int (*fpLayerInitItemInfo)(layerObj *layer);
2.1.2. FreeItemInfo
::
void (*fpLayerFreeItemInfo)(layerObj *layer);
2.1.3. Open
::
int (*fpLayerOpen)(layerObj *layer);
Currently there are two layers which accept more than
the generic layer arg for LayerOpen function:
::
int msOGRLayerOpen(layerObj *layer,
const char *pszOverrideConnection);
int msWFSLayerOpen(layerObj *lp,
const char *pszGMLFilename,
rectObj *defaultBBOX);
However, these are called from ``msLayerOpen`` with
``NULL`` args, so I think that proposed interface for
this virtual table function should be fine.
2.1.4. IsOpen
::
int (*fpLayerIsOpen)(layerObj *layer);
2.1.5. WhichShapes
::
int (*fpLayerWhichShapes)(layerObj *layer,
rectObj rect);
2.1.6. NextShape
::
int (*fpLayerNextShape)(layerObj *layer, shapeObj *shape);
2.1.7. GetShape
::
int (*fpLayerGetShape)(layerObj *layer, shapeObj *shape,
int tile, long record);
2.1.8. LayerClose
::
int (*fpLayerClose)(layerObj *layer);
2.1.9. LayerGetItems
::
int (*fpLayerGetItems)(layerObj *layer);
2.1.10. GetExtent
::
int (*fpLayerGetExtent)(layerObj *layer,
rectObj *extent);
2.1.11. GetAutoStyle
::
int (*fpLayerGetAutoStyle)(mapObj *map, layerObj *layer,
classObj *c, int tile,
long record);
2.2. New functions and/or fields for vtable
2.2.1. CloseConnection
This function is used to actually close the
connection, in case that layer implements some kind of
connection pooling by its own. If layer doesn't use
any connection pooling, this function should be
implemented as no-op. Caller should first call layer's
"close" function, and finally at the very end
``CloseConnection``.
The signature of function is
::
int (*fpLayerCloseConnection)(layerObj *layer);
And the main place where this function will be called,
is ``mapfile.c: 4822``
::
void msCloseConnections(mapObj *map)
This function is needed because e.g. ``POSTGIS`` is
implementing this usage pattern at the moment
``maplayer.c:599``
::
void msLayerClose(layerObj *layer)
...
/*
* Due to connection sharing, we need to close the
results
* and free the cursor, but not close the connection.
*/
msPOSTGISLayerResultClose(layer);
2.2.2. GetConnectionTypeName
This function is used to get MAP-file name for this
layer/connectiontype. It is used when map file is
written back (``mapfile.c: 2856``), actual strings will
be: ``OGR``, ``POSTGIS`` and so on.
The signature for this function is
::
const char * (*fpGetConnectionTypeName)();
Another solution could be to add one extra field
(``const char * connectionTypeName``) to the ``layerObj``,
which is set by layer's initialization function.
2.2.3. SetTimeFilter
This function is used to create a time filter for
layer. At the moment we have three special cases
(``maplayer.c: 1635``):
1. ``POSTGIS`` with it's own function
2. Layers with backticks delimited expressions
3. Layers without backticks
The idea is provide a generic helper function,
::
int makeTimeFilter(layerObj *lp,
const char *timestring,
const char *timefield,
const bool bBackTicks)
And the actual layer's ``SetTimeFilter`` could use the
above, or implement something totally different as
``POSTGIS`` is doing at the moment.
The signature for layer's vtable function is
::
int (*fpLayerSetTimeFilter)(layerObj *lp,
const char *timestring,
const char *timefield);
2.3. Things to fix or extra functions to add to the vtable
I would like to see following items fixed, but I don't know
which way to go.
2.3.1. FLTApplyFilterToLayer (mapogcfilter.c: 1084)
Could we create two helper functions, one for "SQL"
case and the another for "non-SQL" case, and set all
layers, except ``POSTGIS`` and ``ORACLESPATIAL`` to call
directly this "non-SQL" version of this helper function
(else-branch of if). ``ORACLESPATIAL`` and ``POSTGIS`` could
use "SQL" version of the helper function (actual
if-branch) if the conditions for this are met. This way
we could remove all layer specific logic from this
function, and make this function's logic actually generic.
2.3.2. layerObj->items allocation
There is layer (SDE) specific code in ``msLayerWhichItems``
(``maplayer.c: 880``) when initial items allocation is
done.
::
/* TODO: it would be nice to move this into the SDE
* code itself, feels wrong here...
*/
if(layer->connectiontype == MS_SDE) {
...
One solution would be to provide a layer specific
function
::
int (*fpCreateItems)(layerObj *)
which does the allocation and set numitems to correct
value.
2.3.3. msCheckConnection (``mapfile.c: 4779``)
This api looks to be deprecated. It is called only from
``msMYGISLayerOpen``. Is it hard to fix GIS to use
mappool? If this isn't feasible thing to do, we could
remove all other cases and just leave GIS inside this
function.
2.4. Interface functions for internal layers
We have to add at least some new interface functions for
internal layers, e.g. ``msXXXInitializeLayerVirtualTable``,
where ``XXX`` is the layer's name.
3. Remove unwanted interfaces
Frank Warmerdam proposed [FW1]_ that we remove all layer specific
interface functions from ``map.h``.
I see each "built-in" module such as mapsde.c providing a
registration function such as "msSDEInitializeLayerVirtualTable"
so that none of the layer type specific definitions need to
appear in map.h any more.
.. [FW1]
| ``Frank Warmerdam on the mapserver-dev:``
| ``Message-Id: <smtpd.490f.4303f2ee.d8246.1 at mtaout-c.tc.umn.edu>``
| ``Date: Wed, 17 Aug 2005 22:31:09 -0400``
| ``Subject: Re: Mapserver Plug-in Infastructure: RFC and PATCH``
Files and objects affected
--------------------------
This proposal will affect at least following files and objects:
* ``map.h``
* ``layerObj`` will contain new fields. There will be a new
object ``vtableObj`` in the ``map.h``.
* ``maplayer.c``
* Various changes, layer specific ``switch``-statements will go
away, vtable handling and layers vtable initialization will be
added.
* ``mapfile.c``
* mapfile writing will change slightly, no more ``if-else``
structure for connectiontype strings.
* Removing/Cleaning of ``msCheckConnection``
* Vtable for ``msCloseConnection``
* ``mapogcfilter.c``
* Remove layer-logic from ``FLTApplyFilterToLayer``
* ``mapXXX.c``, where ``XXX`` is the name of layer.
* Add new initialization function
* Add all new interface functions
* Fix existing interface functions, if needed / add wrappers for
``msOGRLayerOpen`` and ``msWFSLayerOpen``.
Backwards compatibility issues
------------------------------
This is binary and source code level backward incompatible change. The
proposal will remove some previously public functions, and add new
field(s) to the ``layerObj`` struct. The proposal is ``MAP``-file
backward compatible.
Implementation Issues
---------------------
* Biggest problem is probably that the author has ignored or missed
something by oversight which will show up during implementation.
However, there is a prototype implementation of external plug-in
architecture which works at the moment and is based on ideas
presented in this proposal. So there is some real life experience
that this architecture change is feasible thing to do.
* To find best place to initialize layer's vtable. One possible place
could be ``msLayerOpen``. The problem with that is that if we need
before opening the layer something layer-specific information, it
won't be available.
* I also like to note that this proposal won't remove all layer
specific code from MapServer e.g. ``WMF``, ``WMS`` and ``GRATICULE``
are used as special cases from place to place.
Bug ID
------
unassigned
Voting history
--------------
None
Open questions
--------------
* ``SWIG``. What is the impact of this change to the MapServer's SWIG
interface?
* How do we like to expose layer's virtual table to the layers. We
have at least two different routes to go:
* expose it as a struct, layers will fill vtable pointers by
accessing directly struct's field.
* expose it as a complete opaque type, vtable pointers will be set
by accessing functions ``setLayerOpenFP``, ``setLayerCloseFP`` and
so on.
The advance of second option is that this way we could easily add
new functions to the struct if we refactor code more or found some
logic which is ignored by oversight in this proposal.
=============
Working Notes
=============
.. Local Variables:
mode: text
End:
--
Jani Averbach
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