Diff-Based Tile Expiry Script
		    =============================

This script, "update.rb", and its (hopefully re-usable) library,
"expire.rb", can be used to automatically dirty tiles on a mod_tile +
mapnik tile server. 

Usage
-----

First, edit the "update.rb" script, as there are several system-
specific settings at the top. If you want to use a mirror you should
edit REMOTE_REPOS. LOCAL_REPOS is the directory to which .osc diffs
will be downloaded to. OSM2PGSQL_DIR is the directory where, surprise
surprise, the osm2pgsql binary lives.

Second, download and import a full planet dump, using osm2pgsql's slim
mode.

Setup the timestamp in $LOCAL_REPOS/timestamp.txt. The format is the
same as the server timestamp in $REMOTE_REPOS/timestamp.txt. You
should ensure that there is some overlap, usually one hour between the
end of the planet dump and the beginning of the hourly diffs, as the
planet dump can occasionally have referential integrity problems. For
example, if the planet dump is dated 2008-10-08 (meaning that dumping
started about 1am on that day) I would set the initial timestamp to be
2008-10-08 00:00Z.

Run the "update.rb" script. The script can handle several hours worth
of diffs in one invocation.

Method
------

The script collects all node locations which are in the diff and all
node locations referenced from ways within the diff. Each of these is
used to build a set of dirty tile codes at the maximum zoom level,
which is successively coarsened until the minimum zoom level is
reached.

Known Limitations
-----------------

1) Relations. The script just doesn't handle them. It should be fairly 
easy to extend, however.

2) Skipped tiles. If a way, consisting of two nodes, is modified, but
there are (meta)tiles between the two nodes, e.g:

+-------+-------+-------+
|       |     ==|=X     |
|       |  ===  |       |
|     X=|==     |       |
+-------+-------+-------+

where each box is a meta-tile. The middle box won't be expired.
However, I have not experienced this in practice, as most ways tend to
be shorter than a meta-tile. This can also happen in two dimensions,
where an area (e.g: park) might entirely enclose a meta-tile without
any nodes inside it.

3) Lack of intelligence. If a node or way appears in the diff file
then it will trigger expiry. However, by examination of the osm2pgsql
style file and mapnik style rules it should be possible to eliminate
many expiries of non-rendered properties (e.g: note tags).