[OSM-talk] Mapnik tileset coherency issues

Robert (Jamie) Munro rjmunro at arjam.net
Thu Oct 18 17:43:08 BST 2007


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Patrick Weber wrote:
> Are we just running into a stone wall here of the current server being
> overwhelmed ? Or can we manage to minimise these inconcistencies by some
> clever algorithm as proposed?

The clever algorithm is to merge the osmosis daily dump into the mapnik
PostGIS DB. Then for high zoom tiles (say zoom levels over 12), work out
the bounding boxes of all the ways that have changed, and purge from the
cache any tiles that intersect any of those bounding boxes.

For zoom levels below a certain number (perhaps 12), I'd just re-render
the whole lot once a week or something.

I'm not convinced by the need for a queue for the high zoom tiles - I
think we should either render straight away or not at all. I think the
probability of any individual high level tile being re-visited is quite low.

A queue would only be helpful if we can instruct OpenLayers to try again
after 30 seconds or however long we expect that tile to be rendered in
(it may be as simple as setting a meta-refresh header on the http of the
tile download). If we can't instruct OpenLayers or the browser to retry
after a delay, it seems to me that many tiles in the queue will never be
looked at again, and the server is better off spending it's time
re-rendering the lower zoom levels. Maybe the hybrid queue is worth
doing for zoom levels 13 and 14 or something.

If we only delete tiles that we know are no longer valid, hopefully we
won't need to re-render tiles often as they will already be cached.

It would be nice to render names in a separate language specific layer.
The default tiles could have the local language naming, and then
language specific tiles could render over the top, but be transparent
where they would make no difference. They would have to include the
pixels necessary to hide the local language name from the default tiles,
unless we made the default tiles name free, but I think that as most
places don't have different names in different languages, that would not
be worth doing.

Robert (Jamie) Munro
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