Cached Shadow Maps

A friend just asked me about how to design a shadow map system for many lights with shadows. A quite good explanation was given in the following post already in 2003:

http://www.gamedev.net/community/forums/viewreply.asp?ID=741199

Yann Lombard explains on how to pick a light source first that should cast a shadow. He is using distance, intensity, influence and other parameters to pick light sources.

He has a cache of shadow maps that can have different resolutions. His cache solution is pretty generic. I would build a more dedicated cache just for shadow maps.
After having picked the light sources that should cast shadows, I would only constantly update shadows in that cache that change. This depends on if there is an object with a dynamic flag in the shadow view frustum.
If you think about it how it happens when you approach a scene with lights that cast shadows:
1. the lights are picked that are close enough and appropriate to cast shadows -> shadow maps are updated
2. then while we move on, for the lights in 1. we only update shadow maps if there is an object in shadow view that is moving / dynamic; we start than with the next bunch of shadows while the shadows in 1 are still in view
3. and so on.