[Rebuild] Switching a tile server over to clean data

[Paul Norman]

I've been giving some thought to how it would be practical to switch a tile
server over from CC by-sa data to ODbL data.

A brief overview of what a tileserver does is it mixes data from a database
of OSM data with coastline data and produces tiles which are cached as
images.

First of all, I'm operating under the assumption the tile server host is
going to continue to make their tiles available as CC by-sa. I'm also
assuming that you can have a produced work that is either CC by-sa with the
underlying data ODbL or cc by-sa with the underlying data CC by-sa. The
second is what we have right now. 

I'm also assuming that the attribution of CC by-sa tiles with CC by-sa data
and CC by-sa tiles with ODbL data will differ.
	
Once the first ODbL planet is release the host will need to re-import their
database.

If they were to render tiles from this database you would be mixing CC by-sa
only data (the coastlines) with ODbL only data (the new planet file). As
these are incompatible licenses, they could not do this.

They would need to update their coastline files at the same time. This isn't
too serious - it looks like the coastline may be in a mostly usable state
post-rebuild.

The next problem is the cached tiles. They will now have a mix of cc by-sa
tiles with cc by-sa data and cc by-sa tiles with odbl data. Each tile is its
own work, so that's not a problem, but how are you going to add attribution
to a map where it differs for each tile? I don't see any practical way to do
this, so the old cached tiles would need to be deleted. The problem is you
will then want to pre-render the low zoom levels for speed reasons.

What I see as the most practical solution is to release the first new planet
as dual-licensed and release the next one as ODbL only. This would give data
consumers a week to import the new planet, render some new tiles and delete
old tiles, then change their attribution.

I realize that this extends the process by a week but when you're dealing
with as much data as OSM has there needs to be a transition period for
consumers of the data.

I'm not familiar enough with Nominatim to know if there are similar issues
with a cache that needs to be flushed and takes time to regenerate.

Jxapi servers should just need the time to load the new planet file, so no
issues there. They just will be behind for about a week.