Where to store the cache.

Hello,


In Patterns of Enterprise Application Architecture, Martin Fowler
suggests that the cache (Identity Map) should be put in a
session-specific place to avoid dealing with concurrent modifications
(unless teh data is read only).
If you are in a servlet environemnt, I could place the cache in the
HttpSession but then my data layer (I'm using a Data Mapper) would
need to know about the servlet layer; this would also affect
scalability as teh whole cache would be serialized each time teh
session is serialized.. And what if I switch to am EJB environment?
Fowler does not address this issue in the book.

What's your thinking?

Thanks,

-Vincent.