Q about overlapping transactions...

I'm tying myself in knots designing a db system - I seem to remember reading about rollbacks that need to unwind chains of transactions and am confused as to what sort of dependency this could relate to. Suppose transaction T1 acquires a write-lock L1 in a sub-transaction ST1, over some data, makes changes then releases the lock, but with T1 as a whole yet unfinished.
Further suppose a second transaction T2 takes a lock on the same data, but then T1 is, for some reason, rolled back. My question: is T2 forced to roll back too? Are there different behaviours in different db systems?

My early understanding was that a committed transaction never need be undone - any data changes remain valid and immutable whatever happens in subsequent transactions or to the db (setting aside issue of crashing/unflushed data).

Perhaps my confusion is with the semantics of overlapping transactions...

Can anyone set me aright on this subject?

Thanks
Chris Q. Received on Tue Aug 28 2001 - 00:47:43 CEST