MS RFC 35: Standards Compliance Enforcement

Date:2007/10/16
Author:Frank Warmerdam, Daniel Morissette
Contact:warmerdam at pobox.com, dmorissette at mapgears.com
Last Edited:2007/12/07
Status:Withdrawn (2007/12/07)

Overview

This RFC introduces a mechanism to tell mapserver to enforce OGC standards compliance, or alternatively to be permissive. MapServer should continue to implement the standards as closely as possible, but it has been found that in some cases strict compliance reduces interoperability instead of increasing it.

One example of this is the the requirement for the STYLES parameter in WMS GetMap requests which has been enforced in MapServer 5.0.0. Enforcing this requirement effectively prevented a number of WMS client implementations from connecting to MapServer because they did not include the required STYLES parameter in their GetMap request. This RFC also details short term adjustments to handling of the STYLE= required parameter in MapServer 5.

OWS_COMPLIANCE METADATA

This will be achieved by introducing a new MAP level metadata item named “ows_compliance” with the possible values “pedantic” and “permissive”, with the default being “permissive” if not specified. Variations for specific protocols will also be supported in the usual fashion for OWS metadata (ie. wms_compliance, wfs_compliance, wcs_compliance, sos_compliance).

If this keyword value is “permissive”, then when practical and unambiguous MapServer may attempt to be forgiving of missing or otherwise non-compliant requests and input in the interest of increasing MapServer’s usability in real life interoperability scenarios. This may include things such as allowing protocol parameters to be omitted in requests that are required by the OGC specifications, allowing services to operate even if metadata is missing from the mapfile to fill required fields in capabilities.

If this keyword value is “pedantic”, then to the extent practical MapServer will attempt to generate an error if it’s input (WxS protocol requests, mapfile incomplete, etc) is not in full compliance with the requirements of the OGC specifications.

It is foreseen that users wishing to encourage good standards compliance behavior in clients, and to ensure they don’t accidentally put up web services with incomplete metadata will use the “pedantic” mode. It is anticipated that “pedantic” mode will also be used when MapServer is run through OGC CITE and similar test suites.

It is foreseen that “permissive” mode will be used by those wishing to ensure their service is accessible by the broadest set of possible clients, even if they are not strictly operating to the standards.

Developers should keep in mind that adding more exceptions in the permissive mode should not be taken lightly as this encourages misuse of the specs and bloats the code with unnecessary exceptions.

msOWSLookupMetadata()

In actual code, the compliance keyword might be tested using msOWSLookupMetadata() in a manner similar to this:

if (stylesfound == 0 && sldfound == 0)
{
  if( strcasecmp(msOWSLookupMetadata(&(map->metadata),"MO","compliance"),
                 "pedantic") == 0 ) {
    msSetError(MS_WMSERR, "Missing required parameter STYLES",
               "msWMSLoadGetMapParams()");
    return msWMSException(map, nVersion, "MissingParameterValue");
  } else {
    msDebug( "WMS request missing STYLES parameter, permissively ignoring." );
  }
}

MapServer 5.0.1

For MapServer 5.0.1 the only change anticipated to be made in the code is the above example, requiring the STYLES parameter only in pedantic mode.

If other standards enforcement issues are discovered to be unnecessarily interfering with use of MapServer in real life interoperability they may also be changed in 5.0.x to be based off this keyword.

MapServer 5.1

In MapServer 5.1, and the future, if other standards enforcement issues are discovered to be unnecessarily interfering with use of MapServer in real life interoperability scenarios they may also be changed to be based off this keyword.

Once again, developers should keep in mind that adding more exceptions in the permissive mode should not be taken lightly as this encourages misuse of the specs and bloats the code with unnecessary exceptions.

As part of implementation of this RFC the authors will review all “WARNING:” entries emitted in capabilities documents in places like mapwms.c to see if they ought to be turned into fatal errors when operating in “pedantic” mode.

Dokumentation

The ows_compliance keyword will be added to the MapServer 5 reference documentation.

Implementation

The described changes will be made in MapServer 5.0 and MapServer 5.1 by Frank Warmerdam and/or Daniel Morissette with the MapServer 5.0 changes in time for a 5.0.1 release. It is hoped that all OWS service developers will apply the permissive/pedantic tests in OWS services in the future as required and as issues come up.

MapScript

No changes are anticipated in mapscript as it already has metadata setting methods.

Backwards Compatibility Issues

Implementation of this feature should restore backward compatible default behavior in MapServer 5 with MapServer 4.x. No other compatibility issues are anticipated.

Voting History

There was a first vote around 2007/11/08 which was veto’d... and after further discussion the RFC has been withdrawn in favor of simply restoring the behavior of MapServer 4.10 with respect to the STYLES= parameter (i.e. make it optional again). See ticket #2427 about this.