Version 4 (modified by ronnielouie, 13 years ago) (diff)


FDO RFC 23 - DescribeSchema Enhancement and add new EnumerateFeatureClasses command

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


RFC Template Version(1.0)
Submission DateJuly 8, 2008
Last Modified(Ronnie Louie) Timestamp?
Author(Ronnie Louie)
RFC Statusdraft
Implementation Statuspending
Proposed Milestone3.4.0.0
Assigned PSC guide(s)(when determined)
Voting History(vote date)


This RFC is for adding new APIs for retrieving a specific subset of available information that would otherwise be obtained from executing a full DescribeSchema command.


Applications using FDO to interact with underlying datastores are often interested in retrieving a list of the available feature classes, and the set of properties available in each of those feature classes from the available FDO provider. This information is used to configure a connection to a provider, and to obtain supported geometries and properties from the requested datastore.

Currently this information is retrieved from executing a DescribeSchema command, which returns a full database schema, containing detailed information about every feature class available from that provider. For a typical real-world database deployment, containing hundreds of feature classes, it can take a very long time to retrieve all the data for a full schema.

For commonly required tasks such as requesting a list of feature class names, or a list of properties for a specific class or classes, it could be accomplished more efficiently by retrieving only the requested information.

Proposed Solution

To retrieve the feature class names without having to incur the cost of a DescribeSchema, a new command, EnumerateFeatureClasses, will be added to the API for obtaining a list of feature class names for a given schema. If schema is not specified, the list will consist of all feature classes in the feature source.

/// \brief
/// The FdoIEnumerateFeatureClasses interface defines the EnumerateFeatureClasses command,
/// which retrieves the list of available feature class names. 
/// The Execute operation returns an FdoStringCollection object.
class FdoIEnumerateFeatureClasses : public FdoICommand
    friend class FdoIConnection;

    /// \brief
    /// Gets the name of the schema for the enumeration. This function is optional;
    /// if not specified, execution of the command will enumerate the classes in all schemas.
    /// \return
    /// Returns the schema name
    FDO_API virtual FdoString* GetSchemaName() = 0;

    /// \brief
    /// Sets the name of the schema for the enumeration. This function is optional; if not
    /// specified execution of the command will enumerate the classes in all schemas.
    /// \param value 
    /// Input the schema name
    /// \return
    /// Returns nothing
    FDO_API virtual void SetSchemaName(FdoString* value) = 0;

    /// \brief
    /// Executes the EnumerateFeatureClasses command and returns a 
    /// FdoStringCollection. If the specified schema name does not exist,
    /// the Execute method throws an exception.
    /// \return
    /// Returns the string collection of the feature classes for the specified schema.
    FDO_API virtual FdoStringCollection* Execute() = 0;

To retrieve specific feature class definitions, the existing DescribeSchema API will be modified to add the following methods to instruct the DescibeSchema command to return a schema that contains only the requested classes.

    /// \brief
    /// Gets the names of the feature classes to retrieve. This is optional,
    /// if not specified execution of the command will describe all feature classes.
    /// \return
    /// Returns the collection of feature class names
    FDO_API virtual FdoStringCollection* GetFeatureClassNames() = 0;

    /// \brief
    /// Sets the name of the feature classes to retrieve. This is optional, if not
    /// specified execution of the command will describe all feature classes.
    /// \param value 
    /// Input the collection of feature class names
    /// \return
    /// Returns boolean value to indicate if the provider supports specifying class names for this command.
    FDO_API virtual bool SetFeatureClassNames(FdoStringCollection* value) = 0;

The proposed changes will need to be completed for all FDO providers. Generally speaking, response time for a full DescribeSchema command is not much of an issue for the file-based FDO providers when compared to the response time for the RDBMS-based FDO providers.

The performance gain from using the new EnumerateFeatureClasses command will be most notable for the RDBMS-based FDO providers. The EnumerateFeatureClasses command will throw a "Not Implemented" exception when executed against non-RDBMS FDO providers. Additional information will be added to the GetCapabilities response to allow the providers to indicate its support for the new command. These will default to “false” for all non-RDBMS FDO providers.

The DescribeSchema command for non-RDBMS FDO providers will return the schema(s) for all the feature classes regardless of the set of classes specified by the SetFeatureClassNames method. SetFeatureClassNames will return a boolean to provide feedback to the caller for indicating whether the command supports specifying classes as a criteria when executed against a particular provider.


Application code that currently uses DescribeSchema to retrieve only class names or class definitions should be updated to utilize the new APIs for improved performance. The application developer should be aware that not all occurrences of the original DescribeSchema can be replaced, such as when a complete schema is required for a particular operation.

Test Plan

Test existing and new APIs to validate correct functionality.