[Openstreetmap] tile cacheing

[Simon Hewison]

Here's a thought:

if, as Steve said, the tiles are cached on the tile server for 48 hours, 
then that means that even parts of the world that haven't changed still 
end up with their cached tiles invalidated, and all the processing 
required to re-generate them (and harassing the landsat server) happens 
again.

It's clear within the database what parts of the planet have had new 
line segments or nodes on them since they were generated, so rather than 
just having a simple http cache expiry, when it does expire, it could go 
off to the database and look to see if there's any alteration to the 
bounding box covered by the tile since it was last generated. If so, it 
would keep the same image cached for a futher 48 hours, or whatever. In 
fact, if the database check is lightweight enough, the expiry time could 
be a several minutes, rather than hours/days.

I'm not sure that the current API supports this (has anything in this 
bounding box changed since time T) query, but it could be of huge benefit.

-- 
Simon Hewison