OpenStreetMap
1) Why do the OSM mapnik tiles not have timestamps via the if-modified-since HTML header?
2) I saw the question on http://wiki.openstreetmap.org/wiki/Talk:Tile_usage_policy#.22If-Modified-Since.22_HTTP_header as well. Is that the right place to discuss the topic, and if not, where should I ask?
Discussion
Comment from amm on 5 November 2009 at 20:24
1) As far as I know, the reason for dropping the last-modified header and replacing it with an etag derived from the md5 of the actual tile, is that the last-modified will change more often than the actual content of the tile. As the tile expiry and (meta) tile rendering over estimates the number of tiles that need to be rerender after a change in the data, many tiles get rerendered to the same content. If the cache expiry were based on the modified time instead, it would reduce the caching efficiency.
2) The dev mailing list is probably the best place to discuss this.
Comment from Firefishy on 9 November 2009 at 10:18
If I recall last-modified was dropped because apache uses an AND (as per RFC) on subsequent requests supplied with etag and if-modified-since, which was causing much lower hit ratio.