I have some doubts on caching and would like to dicuss them with the Hibernate community.

1)
If we have a session cache level and supposing I have session instance per method, then where are the cached objects so they can be used between sessions?

2)
If the cache is never aware of changes made to the persistent store by another process, then we could be accessing old data. Even with a small expire time there could be the possibility a process can access old data. But if we put a small expire time for the cache, then what is the cache good for if the cached objects expire quickly.

3) The significant memory occupied by those objects in cache...

I don't think the session-level cache is optional from an architectural standpoint:

http://forum.hibernate.org/viewtopic.php?t=925665

In case of a Hibernate transaction that associates a database transaction with a ThreadLocal Session (like implemented by the Spring Framework), all data access methods that participate in a transaction will share the same Session. But generally, the Session is a means for consistency rather than long-term caching in the typical sense.



That's of course true for a JVM-level cache, and represents the typical tradeoff when using a cache. But for the Session-level cache, you can avoid staleness of data by setting a proper database transaction isolation level, provided that the scope of your transaction matches with the lifetime of your Session.

Juergen

It is not a matter of "is caching" good, but more a matter or what kind of caching should be used for what kinds of objects.

Optimistic locking is here to keep consistency and thus allow you to increase your expiration time.