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If two transactions (both at RR isolation level) ask for the same item which is 2nd-level cached, and then they modify and store this item. Now, for reading that item, they did not run any SQL because it’s cached; so in this case, will they actually start a data base transaction? And when they commit their changes, will they run into lost update problem?
From a pessimistic point of view:
If the second level cache is configured to participate in the transaction, then only the one that first acquired the write lock would be able to modify the cached object, and then write the change to the database. When the second transaction wants to acquire the write lock, it would have to wait until the first transaction ends and releases it.
With optimistic locking, I guess a Concurrent Modification Exception (or similar name) should happen and the second transaction would retry the operation.