[OSM-dev] XML Schema

[Brett Henderson]

Frederik Ramm wrote:
> Does
>
> http://wiki.openstreetmap.org/index.php/OSM_Protocol_Version_0.5/DTD
>
> not work?
>
> You're right about the change files though, I don't know if there's 
> anything for that. Then again, while it is very easy to validate your 
> application's output against a DTD, it is quite difficult to validate 
> the input side - whether or not your application will really be able to 
> process any document allowed by the DTD!
>   
Are you referring to the change files currently used by osmosis and 
intended to be used for bulk uploads in 0.6?  If so, the intent was that 
each create/modify/delete element has an almost identical structure to 
the root osm element in a normal osm file.  I've never written a DTD 
file before, I must be odd because I've only ever used Xml Schema ;-)  
In the Xml Schema case you can define a complex type representing a set 
of way/node/relation elements whose definition can be re-used in the 
definition of osm/create/modify/delete element types (The relationship 
between Complex Types and Elements has parallels to the relationship 
between OO Classes and Objects).  I don't know if DTD has such a concept 
of re-use.

I've been meaning to create a schema for a long time but never got 
around to it, it didn't seem like a very high priority.  If people hate 
Schema that much then I guess a DTD should be created for change files 
instead.  One nice thing about creating a schema is that it becomes 
easier to use code generation tools for xml serialisation and 
deserialisation.

With either DTD or Schema it should be possible to define an open ended 
structure allowing for new element types and attributes to be added 
without breaking existing tools.  The definition just defines a 
structure for those elements we know about at a point in time.

Hmm, I don't think I've added anything useful to this discussion.  I'll 
be quiet now :-)