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FDO RFC 20 - Enhanced Capabilities Support

This page contains an change request (RFC) for the FDO Open Source project. More FDO RFCs can be found on the RFCs page.

Status

RFC Template Version	(1.0)
Submission Date	April 30, 2008
Last Modified	Thomas Knoell Timestamp
Author	Thomas Knoell
RFC Status	Draft
Implementation Status	Pending
Proposed Milestone	3.4.0.0
Assigned PSC guide(s)	Greg Boone
Voting History	(vote date)
+1
+0
-0
-1

Motivation

This RFC addresses the following objectives:

Propose enhancements to the FDO capability interfaces enabling applications to eliminate current provider specific handling

FDO is supposed to provide applications with a set of interfaces allowing the general implementation of tasks without taking into account whether or not the data store is a RDBMS database (Oracle, MySQL, SQL Server) or a file (SHP, SDF). Therefore, an application should not need to implement provider specific processing. However, it was reported that the current capability set is not sufficient enough to achieve this. Hence, some provider specific handling is still required. The following lists an extract of the reported use cases:

An application may tessellate arc segments if a provider cannot handle them. To determine whether or not this is the case, the application can use a geometric capability that returns the list of supported geometry shapes a provider can process. If this information does not include arcs, the tessellation must be executed. The exception to this rule is an Oracle data store. BY default, Oracle supports storing arcs unless the current coordinate system is a Lat/Long system. In this case arcs are not supported. FDO does currently not provide any support to handle this case. Therefore, an application would have to implement provider specific processing for it.
Some providers support only an autogenerated identity (primary key) property while other providers can support either. FDO currently does not indicate on a provider level whether or not this feature is supported.

Both cases can be solved by introducing new capabilities which will allow for the removal of the provider specific processing.

Introduce the support for data store level capabilities (currently, only provider level capabilities are available)

There are cases where a provider may indicate the support of a capability when in fact the data store the provider connects to is not capable of actually supporting it. For example, a provider may indicate that it supports locking or writing to a data store although the data store my not be able to handle locking or has a read-only flag being set and hence writing to it would not be possible. Another example would be the ODBC provider. Currently, the capabilities indicate to support almost all of the functionality. However, since the provider can connect to a variaty of different data sources (from Excel and Access to various RDBMS'), the capabilities may be different dependening on the source the provider connects to.

As a result, it is not sufficient enough to just report provider level capabilities; it is also needed to report data store level capabilities. Data store level capabilities would be available only once a provider has a fully formed connection established to a data store.

Change the capability interface set to allow the addition of new capabilities without changing the interface set

The objective here is to simplify the FDO capability interfaces in that the addition of a new capability does not require FDO, providers (if they don't immediately support the new capability) and applications (if they don't use the new capability) to be rebuild.

Proposals

This section outlines the proposed changes to the FDO capability interfaces related to the three objectives to be addressed.

FDO Capability Interface Simplification

Currently, there is a FDO interface for each capability FDO supports. Any change to the capability set requires a new interface to be added to FDO to retrieve the value. As a result, FDO is no longer binary compatible once this is done and requires a re-build of FDO and any provider wanting to use the updated FDO version.

The aim of this requirement is to address this issue. The idea is to have a set of simplified capability interfaces that allows the addition of a new capability without requiring to rebuild FDO, any provider that does not immediately support the new capability or any application if the capability is not used. Doing this would not only reduce the number of capability interfaces but also the number of capability classes in FDO.

Obviously, the new interfaces must be able to replace the current ones. Looking at the existing capability interfaces, they can be categorized as outlined next:

Boolean: These functions return a flag indicating whether or not a capability is supported. The category includes routines such as SupportsLocking(), SupportsCompositeId() or SupportsAutoIdGeneration().
Enumerates: These functions return an identifier from an enumeration. The category includes routines such as GetThreadCapability().
Array of enumerated values: These functions return an array of values from an enumeration and the number of elements in that array. The category includes routines such as GetSpatialContextTypes() or GetLockTypes().
Collection: These functions return a collection of complex objects. The category includes routines such as GetFunctions().
Bit-mask values: These functions return a bit-mask value of enumerated values. The category includes routines such as GetDimensionality().
Capability based numbers: These functions take a capability identifier and return the corresponding capability value. The category includes routines such as GetMaximumDataValueLength() or GetNameSizeLimit().
Numbers: These functions return number values of different types. The category includes func-tions such as GetMaximumDecimalPrecision() or GetMaximumDecimalScale().
Strings: These functions return strings. The category includes functions such as GetReservedCharactersForName().

A capability interface class – FdoICommand, FdoIConnection, FdoIExpression, FdoIFilter, FdoIGeometry, FdoIRaster, FdoISchema, FdoITopology – may have any number of functions of those different categories with the Boolean category containing the most functions.

The simplification of the FDO capability interfaces focuses on those categories as it creates interfaces depending on the expected return type. As a result, a capability interface will be processing a number of capabilities that have the same return type and hence requires the caller to identify the capability for which the value should be returned. In addition, the functions must also be able to indicate to the caller if a provided capability identifier is unknown. Therefore, the signature definition of those new interfaces must have at least two parameters:

A parameter to indicate the capability for which the value is to be retrieved.
A parameter with which the interface can indicate to the caller that the provided capability identifier is unknown.

Depending on the category, there may be the need for additional parameters – for example, the function returning an array of enumerated values also has to return the number of entries in that array.

The new capability interfaces will be defined in a new class - !FdoICapability - that resides in the directory Fdo\Unmanaged\Inc\Fdo\Connections\Capabilities. The following outlines the class definition:

#include <Fdo/Connections/Capabilities/CapabilityType.h>

class FdoICapability : public FdoIDisposable
{

  public:

   FDO_API virtual bool      GetBooleanCapability (FdoInt32 capability,                   bool &isUnknown) = 0;
   FDO_API virtual FdoInt32  GetInt32Capability   (FdoInt32 capability,                   bool &isUnknown) = 0;
   FDO_API virtual FdoInt64  GetInt64Capability   (FdoInt32 capability,                   bool &isUnknown) = 0;
   FDO_API virtual FdoString *GetStringCapability (FdoInt32 capability,                   bool &isUnknown) = 0;
   FDO_API virtual FdoInt32  *GetArrayCapability  (FdoInt32 capability, FdoInt32 &length, bool &isUnknown) = 0;
   FDO_API virtual void      *GetObjectCapability (FdoInt32 capability,                   bool &isUnknown) = 0;

}  //  FdoICapability

With the introduction of the class !FdoICapability, all other capability classes – FdoICommand, FdoIConnection, FdoIExpression, FdoIFilter, FdoIGeometry, FdoIRaster, FdoISchema, FdoITopology – including their interfaces become obsolete. There is no immediate plan to depricate those classes and functions but the intend is to do it eventually. Therefore, any application that uses the FDO capability interfaces should use the new interfaces once they become available to minimize later code changes.

The addition of the class !FdoICapability as a replacement of all other FDO capability classes requires a change to the class !FdoIConnection that provides access to the capabilities. That class needs a new interface to get the capabilities:

FDO_API virtual FdoICapability *GetCapability(bool datastoreLevel = false) = 0;

A corresponding change will also be made in the FDO managed code base:

...
BEGIN_NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES
...
public __gc __interface ICapability;
...
END_NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES

...

NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::ICapability *GetCapability(System::Boolean datastoreLevel);

...

The class FdoICapability has generalized interfaces for all return type categories the current set of capability interfaces return. It is feasable to add capability interfaces for all data types that FDO supports in preparation for additional capabilities that may be added at a later stage using those data types. However, the ones currently listed should cover most, if not all of the cases and hence the addition of interfaces for different FDO types is not planned.

The simplification of the capability interfaces also affects code using it. This is especially the case for the new interfaces that return an array of enumerated values or a collection. Because of the generalization, the interfaces have to return a neutral rather than a specific data type as it is the case with the original interfaces. This requires the caller to cast the returned object to the expected type. The following shows an example of the old and new style request for a capability that returns an array of enumerated values.

Old Style Call

FdoPtr<FdoISchemaCapabilities> schemaCapabilities =
                                  connection->GetSchemaCapabilities();
FdoInt32 listCount;
FdoDataType *supportedDataTyes =
       schemaCapabilities->GetSupportedAutoGeneratedTypes(listCount);


New Style Call

FdoPtr<FdoICapability> capability = connection->GetCapability();
bool     isUnknown = false;
FdoInt32 listCount = 0;

FdoDataType *supportedDataTyes =
            (FdoDataType *)capability->GetArrayCapability(
                                       FdoCapabilityType_DataTypes,
                                       listCount,
                                       isUnknown);

The change to the unmanaged code requires a corresponding change to the managed code. The class ICapability will be added at Fdo\Managed\Src\OSGeo\FDO\Connections\Capabilities (file mgICapability.h). The following outlines this new class:

public __gc __interface ICapability : public System::IDisposable
{

  public:

    System::Boolean GetBooleanCapability (System::Int32 capability, System::Boolean *isUnknown);
    System::Int32   GetInt32Capability   (System::Int32 capability, System::Boolean *isUnknown);
    System::Int64   GetInt64Capability   (System::Int32 capability, System::Boolean *isUnknown);
    System::String  *GetStringCapability (System::Int32 capability, System::Boolean *isUnknown);
    System::Int32[] GetArrayCapability   (System::Int32 capability, System::Boolean *isUnknown);
    System::Object  GetObjectCapability  (System::Int32 capability, System::Boolean *isUnknown);

}

As part of the proposed change to the FDO capability interface set, a new enumeration will be added. This new enumeration - FdoCapabilityType - contains a list of all currently supported capabilities including the ones added as a result of this RFC. The following outlines this enumeration:

enum FdoCapabilityType
{
  // Command Capabilities

  FdoCapabilityType_CommandList,
  FdoCapabilityType_SupportsParameters,
  FdoCapabilityType_SupportsTimeout,
  FdoCapabilityType_SupportsSelectExpressions,
  FdoCapabilityType_SupportsSelectFunctions,
  FdoCapabilityType_SupportsSelectDistinct(),
  FdoCapabilityType_SupportsSelectOrdering(),
  FdoCapabilityType_SupportsSelectGrouping(),

  // Connection Capabilities

  FdoCapabilityType_ThreadCapability,
  FdoCapabilityType_SpatialContextTypes,
  FdoCapabilityType_LockTypes,
  FdoCapabilityType_SupportsLocking,
  FdoCapabilityType_SupportsTimeout,
  FdoCapabilityType_SupportsTransactions,
  FdoCapabilityType_SupportsLongTransactions,
  FdoCapabilityType_SupportsSQL,
  FdoCapabilityType_SupportsConfiguration,
  FdoCapabilityType_SupportsMultipleSpatialContexts,
  FdoCapabilityType_SupportsCSysWKTFromCSysName,
  FdoCapabilityType_SupportsWrite,
  FdoCapabilityType_SupportsMultiUserWrite,
  FdoCapabilityType_SupportsFlush,

  // Expression Capabilities

  FdoCapabilityType_ExpressionTypes,


  // Filter Capabilities

  FdoCapabilityType_ConditionTypes,
  FdoCapabilityType_SpatialOperations,
  FdoCapabilityType_DistanceOperations,
  FdoCapabilityType_SupportsGeodesicDistance,
  FdoCapabilityType_SupportsNonLiteralGeometricOperations,

  // Geometry Capabilities

  FdoCapabilityType_GeometryTypes,
  FdoCapabilityType_GeometryComponentTypes,
  FdoCapabilityType_Dimensionalities,
  FdoCapabilityTYpe_CSSupportsArcs,

// Raster Capabilities

  FdoCapabilityType_SupportsRaster,
  FdoCapabilityType_SupportsStitching,
  FdoCapabilityType_SupportsSubsampling,
  FdoCapabilityType_SupportsDataModel,

  // Schema Capabilities

  FdoCapabilityType_ClassTypes,
  FdoCapabilityType_DataTypes,
  FdoCapabilityType_SupportedAutoGeneratedTypes,
  FdoCapabilityType_SupportedIdentityPropertyTypes,
  FdoCapabilityType_MaximumDataValueLength_String,
  FdoCapabilityType_MaximumDataValueLength_BLOB,
  FdoCapabilityType_MaximumDataValueLength_CLOB,
  FdoCapabilityType_MaximumDataValueLength_Decimal,
  FdoCapabilityType_MaximumDataValueLength_Boolean,
  FdoCapabilityType_MaximumDataValueLength_Byte,
  FdoCapabilityType_MaximumDataValueLength_DateTime,
  FdoCapabilityType_MaximumDataValueLength_Double,
  FdoCapabilityType_MaximumDataValueLength_Int16,
  FdoCapabilityType_MaximumDataValueLength_Int32,
  FdoCapabilityType_MaximumDataValueLength_Int64,
  FdoCapabilityType_MaximumDataValueLength_Single,
  FdoCapabilityType_MaximumDecimalPrecision,
  FdoCapabilityType_MaximumDecimalScale,
  FdoCapabilityType_NameSizeLimit_Datastore,
  FdoCapabilityType_NameSizeLimit_Schema,
  FdoCapabilityType_NameSizeLimit_Class,
  FdoCapabilityType_NameSizeLimit_Property,
  FdoCapabilityType_NameSizeLimit_Description,
  FdoCapabilityType_ReservedCharactersForName,
  FdoCapabilityType_SupportsAssociationProperties,
  FdoCapabilityType_SupportsAutoIdGeneration,
  FdoCapabilityType_SupportsCalculatedProperties,
  FdoCapabilityType_SupportsCompositeId,
  FdoCapabilityType_SupportsCompositeUniqueValueConstraints,
  FdoCapabilityType_SupportsDataStoreScopeUniqueIdGeneration,
  FdoCapabilityType_SupportsDefaultValue,
  FdoCapabilityType_SupportsExclusiveValueRangeConstraints,
  FdoCapabilityType_SupportsInclusiveValueRangeConstraints,
  FdoCapabilityType_SupportsInheritance,
  FdoCapabilityType_SupportsMultipleSchemas,
  FdoCapabilityType_SupportsNetworkModel,
  FdoCapabilityType_SupportsNullValueConstraints,
  FdoCapabilityType_SupportsObjectProperties,
  FdoCapabilityType_SupportsSchemaModification,
  FdoCapabilityType_SupportsSchemaOverrides,
  FdoCapabilityType_SupportsUniqueValueConstraints,
  FdoCapabilityType_SupportsValueConstraintsList,
  FdoCapabilityType_SupportsWritableIdentityProperties,

  // Topology Capabilities

  FdoCapabilityType_SupportsTopology,
  FdoCapabilityType_SupportsTopologicalHierarchy,
  FdoCapabilityType_BreaksCurveCrossingsAutomatically,
  FdoCapabilityType_ActivatesTopologyByArea,
  FdoCapabilityType_ConstrainsFeatureMovements,

}  //  enum FdoCapabilityType

The new enumeration will reside in the directory Fdo\Unmanaged\Inc\Fdo\Connections\Capabilities. A corresponding definition - CapabilityType - will be added to the managed code (Fdo\Managed\Src\OSGeo\FDO\Connections\Capabilities):

public __value enum CapabilityType
{
  // Command Capabilities

  CapabilityType_CommandList,
  CapabilityType_SupportsParameters,
  CapabilityType_SupportsTimeout,
  CapabilityType_SupportsSelectExpressions,
  CapabilityType_SupportsSelectFunctions,
  CapabilityType_SupportsSelectDistinct(),
  CapabilityType_SupportsSelectOrdering(),
  CapabilityType_SupportsSelectGrouping(),

  // Connection Capabilities

  CapabilityType_ThreadCapability,
  CapabilityType_SpatialContextTypes,
  CapabilityType_LockTypes,
  CapabilityType_SupportsLocking,
  CapabilityType_SupportsTimeout,
  CapabilityType_SupportsTransactions,
  CapabilityType_SupportsLongTransactions,
  CapabilityType_SupportsSQL,
  CapabilityType_SupportsConfiguration,
  CapabilityType_SupportsMultipleSpatialContexts,
  CapabilityType_SupportsCSysWKTFromCSysName,
  CapabilityType_SupportsWrite,
  CapabilityType_SupportsMultiUserWrite,
  CapabilityType_SupportsFlush,

  // Expression Capabilities

  CapabilityType_ExpressionTypes,


  // Filter Capabilities

  CapabilityType_ConditionTypes,
  CapabilityType_SpatialOperations,
  CapabilityType_DistanceOperations,
  CapabilityType_SupportsGeodesicDistance,
  CapabilityType_SupportsNonLiteralGeometricOperations,

  // Geometry Capabilities

  CapabilityType_GeometryTypes,
  CapabilityType_GeometryComponentTypes,
  CapabilityType_Dimensionalities,
  CapabilityTYpe_CSSupportsArcs,

// Raster Capabilities

  CapabilityType_SupportsRaster,
  CapabilityType_SupportsStitching,
  CapabilityType_SupportsSubsampling,
  CapabilityType_SupportsDataModel,

  // Schema Capabilities

  CapabilityType_ClassTypes,
  CapabilityType_DataTypes,
  CapabilityType_SupportedAutoGeneratedTypes,
  CapabilityType_SupportedIdentityPropertyTypes,
  CapabilityType_MaximumDataValueLength_String,
  CapabilityType_MaximumDataValueLength_BLOB,
  CapabilityType_MaximumDataValueLength_CLOB,
  CapabilityType_MaximumDataValueLength_Decimal,
  CapabilityType_MaximumDataValueLength_Boolean,
  CapabilityType_MaximumDataValueLength_Byte,
  CapabilityType_MaximumDataValueLength_DateTime,
  CapabilityType_MaximumDataValueLength_Double,
  CapabilityType_MaximumDataValueLength_Int16,
  CapabilityType_MaximumDataValueLength_Int32,
  CapabilityType_MaximumDataValueLength_Int64,
  CapabilityType_MaximumDataValueLength_Single,
  CapabilityType_MaximumDecimalPrecision,
  CapabilityType_MaximumDecimalScale,
  CapabilityType_NameSizeLimit_Datastore,
  CapabilityType_NameSizeLimit_Schema,
  CapabilityType_NameSizeLimit_Class,
  CapabilityType_NameSizeLimit_Property,
  CapabilityType_NameSizeLimit_Description,
  CapabilityType_ReservedCharactersForName,
  CapabilityType_SupportsAssociationProperties,
  CapabilityType_SupportsAutoIdGeneration,
  CapabilityType_SupportsCalculatedProperties,
  CapabilityType_SupportsCompositeId,
  CapabilityType_SupportsCompositeUniqueValueConstraints,
  CapabilityType_SupportsDataStoreScopeUniqueIdGeneration,
  CapabilityType_SupportsDefaultValue,
  CapabilityType_SupportsExclusiveValueRangeConstraints,
  CapabilityType_SupportsInclusiveValueRangeConstraints,
  CapabilityType_SupportsInheritance,
  CapabilityType_SupportsMultipleGeometries,
  CapabilityType_SupportsMultipleSchemas,
  CapabilityType_SupportsNetworkModel,
  CapabilityType_SupportsNullValueConstraints,
  CapabilityType_SupportsObjectProperties,
  CapabilityType_SupportsSchemaModification,
  CapabilityType_SupportsSchemaOverrides,
  CapabilityType_SupportsUniqueValueConstraints,
  CapabilityType_SupportsValueConstraintsList,
  CapabilityType_SupportsWritableIdentityProperties,

  // Topology Capabilities

  CapabilityType_SupportsTopology,
  CapabilityType_SupportsTopologicalHierarchy,
  CapabilityType_BreaksCurveCrossingsAutomatically,
  CapabilityType_ActivatesTopologyByArea,
  CapabilityType_ConstrainsFeatureMovements,

}  //  CapabilityType

Like the command enumeration, the capability enumeration can be extended by the providers to add their own capabilities if this is required.

Issues

The introduction of the simplified FDO capability interfaces will result in some usability issues not present in the current implementation because it uses different functions for each of the supported capabilities. This means that the user does not have to know much about the capability he/she is about to use. Instead, the user gets hold of the capability class, finds the corresponding interface and executes it. By doing so, the user is also ensured that a valid value is returned that can be used for further processing in an application.

With the simplified version of the FDO capability interfaces, this is no longer the case. Because a given interface now serves many capabilities, the user requires some knowledge about the capability before being able to get the value and faces additional challenges as result of a capability interface call:

The user has to identify the capability for which the value is to be returned.
The user has to know the capability's return type in order to select the correct capability interface in the first place.
The user has to deal with error scenarios previously unknown as for a capability interface the provided capability identifier may be unknown.

Each of the issues are addressed in more detail next.

Capability Identification

To keep the interface generic and reach the objective outlined for this enhancement, the data type for the capability identifier has to be generic as well. Two options to achieve this were discussed in this context:

The first option would define the parameter as a string. To identify the capability the user would enter the capability name.
The second option would define the parameter as a numeric type. To identify the capability the user would enter the numeric representation of the capability.

Both options in common is the question how a user would know what to provide as the capability identifier. For the first option, the user would have to know a valid name, for the second the numeric representation. It would be possible to define the valid identifiers in FDO. However, because of the objective to be reached with this enhancement this definition would act merely as a kind of documentation than concrete help. For example, if a new capability is to be added, the list may not be updated as otherwise FDO would require a rebuild which would trigger additional rebuilds of providers and applications - something that is to be avoided. Therefore, the list would only represent a snapshot of available capabilities. The same would apply for the documentation of the capabilities in documents (like the User Guide) or help files.

Despite this, the proposed solution adds an enumeration that contains all the capabilities FDO supports (including the ones proposed in the RFC). As indicated before it is a snapshot of the then available capability set only and by no means should it be looked at as a complete list. It provides some rudamentary support to the users to identify a capability.

Interface Selection

To select the correct FDO capability interface to retrieve the value of a capability, the user has to know the capability's return type. Like with the capability identifier, this is information that requires to be documented somewhere where the user can easily access it. The current intent is to add this information to the description of the enumerations FdoCapabilityTYpe and CapabilityType. An alternative option would have been to split the current enumeration into different enumeration depending on the return value. However, given the usability of those enumerations (see previous section) it is not seen as a necessity to do so.

Error Handling

Unlike with the current FDO capability interfaces, with the simplified version, a capability interface may have to indicate to a caller that a provided capability identifier is unknown. A common scenario may be that a new capability is added and not all providers would support it immediately. If the corresponding FDO capability interface was to be invoked with the new capability identifier for a provider that does not yet support it, the interface must be able to indicate this to the caller. As defined in the proposed solution, this aspect has been addressed by using a function parameter (isUnknown) to inform the caller whether or not the provided capability is unknown. If so, the caller has to ignore any value the function may return.

Alternative options for the error handling were also discussed in this context. Those include:

Exception: The function would throw an exception if the provided capability identifier is unknown.
Predefined Default Value: Each FDO capability interface will have a predefined value being returned if the provided capability is unknown.
Custom Default Value: In this case the caller supplies the value to be returned if a provided capability identifier is unknown.

Those options were dismissed for various reasons. Whereas the first option was dismissed because of cost related issues, the two other options were dismissed because it appears impossible to define a value for each of the interfaces that could be used to reliably indicate that a capability indentifier is unknown. For example, the interface handling capabilities returning a boolean value, can return either true or false, both of which indicate valid responses for a capability. Based on this, a caller receiving such a value cannot distinguish between whether this represents a valid capability value or that the capability is unknown.

Data Store Level Capability Support

To support both, provider and data store level capabilities, the parameter list of the functions defined for the FdoIConnection interface that provide access to the capabilities will be changed. All functions will get an optional boolean parameter that defines the capability level to be requested. The following shows the proposed changes:

FDO_API virtual FdoICommandCapabilities    *GetCommandCapabilities    (bool datastoreLevel = false) = 0;
FDO_API virtual FdoIConnectionCapabilities *GetConnectionCapabilities (bool datastoreLevel = false) = 0;
FDO_API virtual FdoIExpressionCapabilities *GetExpressionCapabilities (bool datastoreLevel = false) = 0;
FDO_API virtual FdoIFilterCapabilities     *GetFilterCapabilities     (bool datastoreLevel = false) = 0;
FDO_API virtual FdoIGeometryCapabilities   *GetGeometryCapabilities   (bool datastoreLevel = false) = 0;
FDO_API virtual FdoIRasterCapabilities     *GetRasterCapabilities     (bool datastoreLevel = false) = 0;
FDO_API virtual FdoISchemaCapabilities     *GetSchemaCapabilities     (bool datastoreLevel = false) = 0;
FDO_API virtual FdoITopologyCapabilities   *GetTopologyCapabilities   (bool datastoreLevel = false) = 0;
FDO_API virtual FdoICapability             *GetCapability             (bool datastoreLevel = false) = 0;

By default the optional parameter is set to FALSE, hence not changing the current behavior. Data store level capabilities are available only after a fully connection to a data store has been established. If requested before, an exception will be issued.

The changes to the unmanaged code requires a corresponding change to the managed code. There, new interfaces will be defined to reflect the parameter change in the unmanaged code. The following outlines the necessary changes:

NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::ICommandCapabilities    *GetCommandCapabilities    (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::IConnectionCapabilities *GetConnectionCapabilities (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::IExpressionCapabilities *GetExpressionCapabilities (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::IFilterCapabilities     *GetFilterCapabilities     (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::IGeometryCapabilities   *GetGeometryCapabilities   (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::IRasterCapabilities     *GetRasterCapabilities     (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::ISchemaCapabilities     *GetSchemaCapabilities     (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::ITopologyCapabilities   *GetTopologyCapabilities   (System::Boolean datastoreLevel);
NAMESPACE_OSGEO_FDO_CONNECTIONS_CAPABILITIES::ICapability             *GetCapability             (System::Boolean datastoreLevel);

If data store level capabilities are chosen, then the results will reflect any limitations that the data store has related to the capability. For example, the command capability will return a smaller list of supported commands if the datastore is read-only. In this case, commands such as Update will not be returned. Other capabilities, like the threat related capabilities, will remain unchanged from the provider level values.

It is anticipated that the most impact of the data store level capabilities will be with the ODBC provider. At the same time, the ODBC provider also represents the biggest risk as it is not yet known how much data store specific limitations the ODBC provider can determine through the ODBC API.

Eliminate Provider Specific Processing in Applications

As outlined in the motivation section of this RFC, it has been found that the current set of capabilities is not sufficient enough for applications to avoid implementing provider specific processing. As a result, it is proposed to add the following new capabilities.

Arc Support

An application may tessellate arc segments if a provider cannot handle them. To determine whether or not this is the case, the application can use a geometric capability that returns the list of supported geometry shapes a provider can process. If this information does not include arcs, the tessellation must be executed. The exception to this rule is an Oracle data store. By default, Oracle supports storing arcs unless the current coordinate system is a Lat/Long system. In this case arcs are not supported. FDO does currently not provide any support to handle this case. Therefore, an application would have to implement provider specific processing for it.

This case will be addressed by a new capability. It allows the user to provide a coordinate system name and returns a boolean flag indicating whether or not arc segments can be stored. In order to do this the new FDO capability interface to return a boolean value must be overwritten with a function that accepts an additional parameter (the coordinate system name). The following outlines the changes to the class !FdoICapability.

#include <Fdo/Connections/Capabilities/CapabilityType.h>

class FdoICapability : public FdoIDisposable
{

  public:

   FDO_API virtual bool      GetBooleanCapability (FdoInt32 capability,                   bool &isUnknown) = 0;
   FDO_API virtual bool      GetBooleanCapability (FdoInt32 capability, FdoString *value, bool &isUnknown) = 0;

   :


}  //  FdoICapability

The change to the class FdoICapability requires a change to the corresponding class !ICapability in the FDO managed code base:

public __gc __interface ICapability : public System::IDisposable
{

  public:

    System::Boolean GetBooleanCapability (System::Int32 capability,                        System::Boolean *isUnknown);
    System::Boolean GetBooleanCapability (System::Int32 capability, System::String *value, System::Boolean *isUnknown);

    :

}

In addition to those changes the capability identifier !FdoCapabilityTYpe_CSSupportsArcs will be added to the enumeration FdoCapabilityType and the corresponding identifier !CapabilityTYpe_CSSupportsArcs to the enumeration CapabilityType.

Calculated Property Support

Currently, the FDO capability set does not include a capability with which a provider can indicate whether or not it supports calculated properties. This deficit will be addressed by a new capability. The capability returns a TRUE if calculated properties are supported, FALSE otherwise. To address the capability, the identifier !FdoCapabilityType_SupportsCalculatedProperties is added to the enumeration FdoCapabilityType and !CapabilityType_SupportsCalculatedProperties to the enumeration CapabilityType.

Writable Identity Property Support

Some providers support only an autogenerated identity (primary key) property while other providers can support either. FDO currently does not provide a capability with nwhich this can be indicated. This deficit will be addressed by a new capability. The capability will return TRUE if a provider does not require autogenerated identity properties only, FALSE otherwise. To address the capability, the identifier !FdoCapabilityType_SupportsWritableIdentityProperties will be added to the enumeration FdoCapabilityType and !CapabilityType_SupportsWritableIdentityPropertiesto the enumeration CapabilityType.

Multiple Geometry Support

Some providers do not allow different shapes to be stored in the same geometry property. For example, if the geometry in a SHP file is set to store points, multi-point shapes are not allowed. Another example would be if the geometry type are lines, point data cannot be stored.

Currently, a provider cannot indicate whether or not it can handle a combination of different geometry shapes. This issue will be addressed by a new capability. It will return TRUE if multiple geometry types are supported, FALSE otherwise. To address the capability, the identifier !FdoCapabilityType_SupportsMultipleGeometries will be added to the enumeration FdoCapabilityType and !CapabilityType_SupportsMultipleGeometriesto the enumeration CapabilityType.

Test Plan

Existing unit tests will be enhanced and new unit tests added to test all changes.

Funding/Resources

Autodesk to provide resource / funding to implement the feature for the affected providers.

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