LocalizedResourcePurger ttl configurable

[rochoa]

The ttl is hard coded right now: https://github.com/CartoDB/Windshaft/blob/0.43.0/lib/windshaft/server.js#L382-L384

It should be started only for ttl > 0.

[rochoa]

I've been going a bit deep into the code and the problem with LocalizedResourcePurger is bigger than I originally thought.

LocalizedResourcePurger does not consider at all the possibility of long living renderer that does not uses some assets for a long period of time but at the same time it's not destroyed and at some point it can use the asset.

Imagine LocalizedResourcePurger is created with ttl set to 6, the following scenario is possible:

• T0: Mapnik renderer created with two assets and some cartocss like

#layer {
  marker-file: url(https://example.com/icon.png);
  [zoom>9] {
    marker-file: url(https://example.com/icon10.png);
  }
}

• T0-T6: Map keeps getting requests for levels in zoom 0..18.
• T6: LocalizedResourcePurger runs and doesn't delete anything as it has been used
• T7-T12: Map only gets requests in zoom 0..9.
• T12: LocalizedResourcePurger runs and delete file for resource https://example.com/icon10.png as it was not used during the last ttl period.
• T13: Map gets a request in zoom 10: BOOM!

Conclusion: without considering the in use renderers is not safe to delete cached resources.

[rochoa]

My proposal is to remove LocalizedResourcePurger and don't look behind! 💃

One extra benefit of removing it is we could start caching XML for mapnik (with some changes).

[rochoa]

Final decision: we are gonna remove LocalizedResourcePurger for now as it creates more issues than the problems it solves. We will build a better mechanism that will consider live renderers so their associated assets are not removed.