| Date: | 2009/01/16 |
|---|---|
| Authors: | Alan Boudreault |
| Contact: | aboudreault at mapgears.com |
| Last Edited: | 2009/01/16 |
| Version: | MapServer 5.4 |
| Id: |
This RFC proposes the addition of XML/BXML Mapfiles support.
Presently, MapServer supports only normal mapfiles that are parsed by Flex. The current mapfiles are parsed very fast by MapServer but can not be parsed by any other software due to the parser complexity. So, a client interface to build mapfiles is difficult to make.
XML is seen as a solution that can provide a strict syntax to mapfiles to have a strong validation with XML Schema.
The main task is to write a XML Mapfile Schema to have a strict XML syntax that will provide a strong validation and to code a new parser for the XML encoding as an alternative to the current mapfiles format. It will use a similar C API as the traditionnal. The Cubewerx’s CWXML library (http://www.cubewerx.com/main/cwxml/) will be used for the new parser.
The main blocker for this feature so far has been the poor performance that we expect from parsing large XML documents at runtime. Fortunately, a Binary XML (BXML) discussion paper has been submitted to OGC and Cubewerx has released an open source library that implements it. The use of BXML addresses the potential performance issue and makes XML/Binary XML mapfiles a viable option.
There is a risk related to the maturity and stability of the CWXML library on which this implementation will be based: the library seems quite good on the surface but we will only know for sure once we have started working with it.
First, it is plan to write a schema and a parser that will simply allow us to do the same things that the traditionnal mapfile does. Each keyword will be almost the same, only the XML syntax will be different.
Secondly, the XML Parser needs to support XML mapfiles as well as BXML mapfiles. This part should be transparent with the CWXML library.
Most of the changes will be in the new file “mapfilexml.c” that contains the new parser. A few minor changes will be needed to determine which parser to use in the msLoadMap() method.
An example of XML mapfile layer definition (prototype):
<Layer>
<name>popplace</name>
<type>POINT</type>
<debug>5</debug>
<status>ON</status>
<Metadata>
<item name="DESCRIPTION">Cities</item>
<item name="TEST">TESTING</item>
<item name="RESULT_FIELDS">NAME</item>
</Metadata>
<data>popplace</data>
<labelItem>Name</labelItem>
<classItem>Capital</classItem>
<Class>
<name>Cities</name>
<expression>1</expression>
<template>ttt_query.html</template>
<symbol>2</symbol>
<size>8</size>
<Label type="TRUETYPE">
<colorAttribute>[COULEUR]</colorAttribute>
<font>sans</font>
<angle>0</angle>
<size>8</size>
<outlineColor red="255" green="255" blue="255"/>
</Label>
<color red="0" green="255" blue="0"/>
</Class>
<Class>
<name>Cities</name>
<expression>/2|3/</expression>
<template>ttt_query.html</template>
<tolerance>5</tolerance>
<Label type="TRUETYPE">
<colorAttribute>[COULEUR]</colorAttribute>
<font>[FONT]</font>
<angle>[ANGLE]</angle>
<size>8</size>
<outlineColor red="255" green="255" blue="255"/>
<position>AUTO</position>
<partials>FALSE</partials>
</Label>
<Style>
<symbol>7</symbol>
<size>6</size>
<colorAttribute>[COULEUR]</colorAttribute>
</Style>
</Class>
</Layer>
In the future, some enhancement could be good to added:
- Special tags for all WMS options (including styles)
- Reusable xml block with ID
TBD
No issues expected.