[Tilecache] Read Only Cache and Coverage...

[Andrew Hughes]

Hi All,

I'll try and be very concise here, it would be nice to have the live 
(external) WMS-C service serve only cached tiles, and an internal 
process to control what populates the cache. (This is for disk cache).


Here's why:

Limiting the cache to a simple "extent"/"bbox" means that when we get 
new coverage we can't increase the extent without modification to our OL 
clients. We want world-wide..... even if we don't have EU maps... yet!

Further more, having an internal process to populate the cache means the 
cache coverage can be quite sophisticated. For example, we might cache 
different resolutions for different area's and also complex polygons 
e.t.a. or maybe draw on polygons from our spatial database to execute 
the seed script.

Next, most "extents"... (ie a BBox)... is a very inefficient use of 
resources as the cache has a good proportion of useless tiles (ie sea or 
desert).

Performing "live caching" from client requests could be based on another 
approach... Where the tile must me certain conditions (ie coverage or 
resolution), this is normally a slow process that will degrade 'live' 
service. It is also most likely that the conditions that need to be met 
are defined and dependent on external spatial data. This is most likely 
not TC's responsibility.


With all this in mind, I do have two suggestions....

The parameter 'FORCE' is currently set to replace the tile cache if it 
appears in the request. 'FORCE' could have two options...
FORCE=true : create or replace the tile in cache
FORCE=false : return 'map out of coverage' image, when cache-miss.
A new py script could be used to add/remove this param from the request 
(so it would work out of the box) or mod_rewrite (not recommended).

OR

The tilecache.cfg can have a new parameter per layer that has a 
'FORCE_PASSWORD' which is not the most secure of solutions.. but can be 
easily implemented.



Cheers,

Andrew