SDS plugin
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The idea in this project is to enable JOSM users to record additional data
with OSM objects, but that additional data lives in a different repository.
The driver, in this instance, was that the data to be collected was
unsuitable for public release due to privacy issues (personal data like
household income etc.), but there may be other uses, for example if you want
to record stuff that is too volatile or detailed for OSM.

The separate data store has all information keyed against OSM object IDs,
i.e. it cannot record geometries - only additional tags.

The SDS plugin makes it possible to have JOSM query another data source for
additional data related to objects just downloaded from OSM. For example,
you download ways #15, #20, #25 from OSM, then the SDS plugin will query a
different server "do you have extra info pertaining to ways #15, #20, #25?"
and the server may or may not return extra info.

These extra tags are then brought into JOSM just like any other tags, and
they can be edited, styled, filtered, and validated normally.

On upload, the plugin will again separate the extra tags from normal OSM
tagging, and will upload extra tags to the separate server only. (This is
based on a tag name rule, i.e. tags that begin with a defined prefix go to
the separate server, and all else goes to OSM.)

Geofabrik has also written a server (in Ruby on Rails) that serves as the
counterpart for the SDS plugin. The server is slightly more HOT specific
than the JOSM plugin, and comes with a search function and web editing
interface for tags stored there.

For more documentation on the SDS plugin and server, see:
http://github.com/geofabrik/sds-server