MapGuide RFC 175 - WFS GetFeature support for hit count

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

Status

RFC Template Version	(1.0)
Submission Date	13 Jul 2019
Last Modified	17 Jul 2019
Author	Jackie Ng
RFC Status	implemented
Implementation Status	implemented
Proposed Milestone	4.0
Assigned PSC guide(s)	(when determined)
Voting History	(vote date)
+1	Jackie,Gordon,Martin
+0
-0
-1
no vote

Overview

This RFC proposes to add support for resultType=hits in WFS 1.1.0 GetFeature requests

Motivation

With WFS 1.1.0 onwards, a WFS client may pass an optional resultType=hits parameter to request a raw total of matching features instead of the full set of feature data in the requested format.

This parameter is an optional part of the WFS 1.1.0 spec that MapGuide currently does not support. As a result, a WFS client currently has no way to request a feature total from a MapGuide WFS service without requesting the full feature data, which is an extremely inefficient way to count features.

Proposed Solution

Add support for resultType=hits for WFS 1.1.0 GetFeature requests.

To support this we'll add a new internal GetWfsFeatureTotal method which does the same feature query as the original GetFeature request, but raw spins the resulting feature reader on the server-side to obtain the feature total.

class MG_PLATFORMBASE_API MgFeatureService : public MgService
{
...
INTERNAL_API:
    ////////////////////////////////////////////////////////////////////////////////////////////////////////
    /// \brief
    /// Retrieves the total feature count for the given WFS query
    ///
    /// \note1
    ///
    /// <!-- Syntax in .Net, Java, and PHP -->
    /// \htmlinclude DotNetSyntaxTop.html
    /// virtual int GetWfsFeatureTotal(MgResourceIdentifier featureSourceId, string featureClass, string filter, int maxFeatures);
    /// \htmlinclude SyntaxBottom.html
    /// \htmlinclude JavaSyntaxTop.html
    /// virtual int GetWfsFeatureTotal(MgResourceIdentifier featureSourceId, string featureClass, string filter, int maxFeatures);
    /// \htmlinclude SyntaxBottom.html
    /// \htmlinclude PHPSyntaxTop.html
    /// virtual int GetWfsFeatureTotal(MgResourceIdentifier featureSourceId, string featureClass, string filter, int maxFeatures);
    /// \htmlinclude SyntaxBottom.html
    ///
    /// \param featureSourceId (MgResourceIdentifier)
    /// The resource identifier defining the
    /// location of the feature source in
    /// the repository.
    /// \param featureClass (String/string)
    /// The feature class containing the features to retrieve.
    /// \param filter (String/string)
    /// An XML string containing the definition for an OGC filter
    /// \param maxFeatures (int)
    /// If greater than 0 and less than the real computed total, this will be the value returned
    ///
    /// \remarks
    /// This method is primarily used to service the WFS GetFeatures operation in resultType=hits mode
    /// where a total is desired over requesting the whole set of feature data
    ///
    /// \return
    /// Returns an MgByteReader containing the requested feature information.
    ///
    /// \exception MgInvalidArgumentException
    ///
    /// \since 4.0
    virtual INT32 GetWfsFeatureTotal(MgResourceIdentifier* featureSourceId,
        CREFSTRING featureClass,
        CREFSTRING filter,
        INT32 maxFeatures) = 0;
};

A question one may ask is why raw spin a feature reader to obtain the feature total instead of something more efficient like a SelectAggregates with COUNT()? The problem lies in the maxFeatures parameter that can be specified in any WFS GetFeatures request. If this parameter is present when requesting a hit count, *it needs to be respected* and none of the optimal counting methods we have allow us to specify an upper limit. As a result, raw spinning a feature reader is the only practical solution.

A sample GetFeature response with resultType=hits would look like this:

<?xml version="1.0" encoding="utf-8" ?>
<wfs:FeatureCollection xmlns:gml="http://www.opengis.net/gml" xmlns:wfs="http://www.opengis.net/wfs" xmlns:ns55197509="http://fdo.osgeo.org/schemas/feature/ns55197509" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/gml http://schemas.opengis.net/gml/3.1.1/base/feature.xsd http://www.opengis.net/wfs http://schemas.opengis.net/wfs/1.1.0/wfs.xsd" numberOfFeatures="11" />

If JSON format is requested, the response would look like this:

{
    "numberOfFeatures": 11
}

This implementation exists in the ​wfs_hits sandbox. Upon adoption of this RFC, will be merged into trunk.

Implications

This parmameter is only available for WFS 1.1.0. For WFS 1.0.0, this parameter has no effect.

Another important note is that the feature total will only be included *if and only if* resultType=hits is included. It is not included in regular GetFeature responses as it would require 2 queries (1 to count, 1 for the original query) and for the sake of simplicity in implementation that it was carried out in this way.

Test Plan

Verify resultType=hits returns hit count only for WFS 1.1.0 requests

Funding / Resources

Community