The XML2 format
The XML2 format is used in Opensim to serialize objects or linksets (called “SceneObjectGroups” in the Opensim parlance). XML2 dumps of objects can be obtained using the ‘save xml2’ or ‘save prims xml2’ console commands, and are used internally to serialize objects in Opensim Archive (OAR) and Inventory Archive (IAR) files. For example, OARs are gzipped tar files containing a directory called ‘objects’ with one XML2 file for each linkset in the region.
XML2 files are standard XML files using an ASCII encoding and no whitespace. This means that there’s no extra indenting or formatting between tags to make the file more pleasant to the eye. You can use any XML editor (including Internet Explorer) to display an XML2 file with some additional prettyprinting and formatting.
XML2 is practically undocumented, but the fact that it’s undocumented is itself (informally) documented :-), in comments scattered through the Opensim wiki and in the Opensim-dev and Opensim-users lists. Essentially, what this means is that the format is subject to change, but not much change: attributes (i.e., new tags) may be added to the format without breaking it — the developers program the classes taking this possibility into account and storing default values when an attribute is expected but not present (for example, if you’re loading an old OAR file which does not have the ‘AllowedDrop’ attribute into a newer installation which does have this attribute).
In this article I document partially the structure of XML2 files using a variant of the BNF notation. I include the productions for most of the format, interspersing comments wherever I have some idea of the meaning of the tag contents. I’ve obtained this information by manually looking at the XML2 files when developing the simple OAR editor I called ZOE and in recent modifications to that editor. I’ve also taken a look at the source code for Opensim and at the code for libopenmetaverse. There may be errors and there are for sure omissions; any kind of comment, addition, correction or clarification would be greatly appreciated.
The whole SceneObjectGroup/SceneObjectPart model is about to be heavily refactored to be able to include very cool things like hyerarchical objects or meshes, and this will most probably require a new format (XML3?) to serialize Opensim objects. I am writing this article as a way to teach myself, and I’m making it public in case somebody else can find its contents useful. Please keep in mind at all times that this is not official documentation :-) — that’s one of the reasons why this appears as a blog article instead as a page in the Opensim wiki, the other being that I’d prefer to get some feedback before writing such a complicated wiki page, in case it’s considered useful to have it.