MS RFC: Layer Plug-in Architecture
Jani Averbach
javerbach at EXTENDTHEREACH.COM
Tue Sep 6 16:59:21 EDT 2005
Hello,
Here is my updated proposal for the plug-in architecture.
One note: I kept the CloseConnection and CloseLayer because POSTGIS
needs both of them and it is already using new mappool API. So, if it
is possible to remove this extra logic, it would be nice if someone
who knows POSTGIS could comment. Otherwise, I think we should
implement both of CloseConnection and CloseLayer.
BR, Jani
==============================================
MS RFC: Layer Plug-in Architecture
==============================================
:Date: 2005/08/22
:Author: Jani Averbach
:Contact: javerbach at extendthereach.com
:Last Edited: $Date: 2005-09-06 14:36:04 -0600 (Tue, 06 Sep 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 back-end format specific 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 contain function pointers for this layer.
2. Add the virtual table architecture
The vtable will be initialized when the layerObj is created and
parsed, this will happen in the function (mapfile.c):
::
int loadLayer(layerObj *layer, mapObj *map)
The vtable will be populated with dummies when it is created and
these dummies will implement feasible default actions if there are
any (e.g. do nothing) or return error by default.
2.1. Standard functions which are found currently from ``maplayer.c``
2.1.1. InitItemInfo
::
int (*LayerInitItemInfo)(layerObj *layer);
2.1.2. FreeItemInfo
::
void (*LayerFreeItemInfo)(layerObj *layer);
2.1.3. Open
::
int (*LayerOpen)(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 (*LayerIsOpen)(layerObj *layer);
2.1.5. WhichShapes
::
int (*LayerWhichShapes)(layerObj *layer,
rectObj rect);
2.1.6. NextShape
::
int (*LayerNextShape)(layerObj *layer, shapeObj *shape);
2.1.7. GetShape
::
int (*LayerGetShape)(layerObj *layer, shapeObj *shape,
int tile, long record);
2.1.8. LayerClose
::
int (*LayerClose)(layerObj *layer);
2.1.9. LayerGetItems
::
int (*LayerGetItems)(layerObj *layer);
2.1.10. GetExtent
::
int (*LayerGetExtent)(layerObj *layer,
rectObj *extent);
2.1.11. GetAutoStyle
::
int (*LayerGetAutoStyle)(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 (*LayerCloseConnection)(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. 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 (*LayerSetTimeFilter)(layerObj *lp,
const char *timestring,
const char *timefield);
2.3. Extra functions to add to the vtable
2.3.1. FLTApplyFilterToLayer (``mapogcfilter.c: 1084``)
This is the main filter interface for layers. We will
provide two helper functions, one for "SQL" case and
the another for "non-SQL" case, and set all layers,
except ``POSTGIS``, ``ORACLESPATIAL`` and ``OGR`` to
call directly this "non-SQL" version of this helper
function (else-branch of if).
``ORACLESPATIAL``, ``POSTGIS`` and ``ORG`` could use
"SQL" version of the helper function (actual if-branch)
if the conditions for this are met, otherwise they will
use Non-SQL version of the function.
2.3.2. layerObj->items allocation
There will be vtable function for allocation ``items`` and
initialization for ``numitems``. If layer has some
special needs for these objects it can override default
function.
The signature for function will be:
::
int (*CreateItems)(layerObj *)
which does the allocation and set numitems to correct
value.
2.3.3. msCheckConnection (``mapfile.c: 4779``)
This API is deprecated. It is called only from
``msMYGISLayerOpen``. We will not provide this
functionality through vtable.
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.
* 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.
* 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
--------------
* MapScript. What is the impact of this change to the MapServer's
MapScript 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.
* Are there any special issues with the raster query layer support
which is handled via the layer API?
=============
Working Notes
=============
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--
Jani Averbach
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