[Tilesathome] Towards a more efficient lowzoom

[Martijn van Oosterhout]

Hi,

I've been fixing seatiles for a while and become quite annoyed at how
out of date lowzoom is, yet when you run it it takes ages because it
has to download so much. However, it seems to me that for any
particular lowzoom run, most tiles don't need changing at all. So I've
been thinking as to how to improve the situation.

Best case, one zoom-12 tile changes, you only actually need to
download 19 tiles to get all the zooms upto level 6 correct again.
This should only take a few moments. So I think we can use this idea
to make lowzoom work better. Two attacks:

1. Lowzoom is wildly inefficient

I've made some small changes to the lowzoom.pl so that if you tell it
which tiles have changed since the last run, it will only download and
generate the tiles necessary to get everything up to date again.

2. Lowzoom doesn't get run often enough.

I've written a script that, given database access, would determine
which zoom-6 tiles are out of date and if, execute the lowzoom script
with the necessary informtion to do the minimum amount of work to get
everything up to date.

One problem I have is that with a 4-5k turnover of zoom-12 tiles in a
day, a single machine could update all of lowzoom in a reasonably
short time so I dont know if we need to go to all the effort of making
a queue for it. I think 24 hour latency for lowzoom isn't too bad.

Thoughts?
-- 
Martijn van Oosterhout <kleptog at gmail.com> http://svana.org/kleptog/