MS RFC 103: Layer Level Character Encoding Handling




Thomas Bonfort





MapServer 7.0

1. The Current Situation

MapServer has no knowledge of the character encoding of a datasource, which has multiple implications:

  • FILTERs and EXPRESSIONs inside a mapfile must be written in the mapfile in the same encoding as the datasource. While this is no hassle for fully UTF8 datasources and mapfiles, this gets harder to maintain when using non UTF8 datasources (as the mapfile needs to be saved in another encoding), and becomes unmanageable for datasources with mixed encodings (e.g. layer1 is UTF8 while layer2 is latin1).

  • Handling of the encoding headers has to be manually specified so that correct content-type headers and xml encoding tags get sent back to clients for OGC documents (getcapabilities, getfeature, etc…)

  • For rendering, we ass-hatedly specify the encoding at the LABEL level to transform label texts to UTF8, given that our renderers only supported that.

2. Layer Level Encoding

This RFC proposes to add an ENCODING keyword at the LAYER level. Layer datasources that have an ENCODING set, and different than UTF8, will have its attributes converted to UTF8 in the GetShape and GetNextShape vtable wrappers.

2.1 Implications

  • The core of MapServer can forget about handling encodings, and will treat all strings as UTF8.

  • All text documents ( e.g. capabilities, getfeature, query, etc.. ) will be emitted as UTF8.

  • There will be no need to specify encoding metadata entries in the mapfile in order to produce somewhat correct ogc documents.

2.2 Backwards Compatibility

This RFC has some notable backwards incompatibility issues for users who were treating non UTF8 or ASCII datasets:

  • Mapfiles themselves will be expected to be UTF8-encoded. Non UTF8-encoded mapfiles will need to be iconv’d to utf8.

  • All textual documents will be emitted as UTF8, with corresponding headers and/or preambles. Emitting in other encodings is unsupported.

  • LABEL level ENCODING will be removed. As a means of helping backwards compatibility, we might want to set the LAYER level encoding to that of the LABEL one when loading a mapfile.

It should be noted however that these incompatibilities can and should be seen as a way to fix MapServer’s flawed handling of these encoded datasources.

2.3 Performance Implications

None or barely noticeable:

  • For UTF8 and ASCII datasources: none expected

  • For other datasources: the overhead of the iconv calls should be marginal.

3. Implementation Details

3.1 Affected files

  • mapfile.c : Layer level handling of the ENCODING keyword, possibly treating LABEL level ENCODING for backwards compatibility

  • maplayer.c: iconv calls to convert the encoding of attributes in msLayerGetShape() and msLayerGetNextShape()

  • TBD: removal of encoding workarounds and modification of header/preamble printing to announce utf8.

3.2 Tracking Issue


4. Discussion

While a more general treatment of this issue might also include handling the encoding of the actual mapfile (to avoid forcing UTF8 as a mandatory mapfile encoding), this solution is not proposed, as:

  • this adds substantial complexity throughout the codebase

  • UTF8 is the widely recommended encoding to use nowadays

5. Voting History

+1 from ThomasB, MikeS, DanielM, SteveW, PericlesN, SteveL, TamasS and StephanM