paul.knibbs (1/17/2013)
Bit uncertain about the description of Isolation. Surely the whole point of record locking is that it DOES prevent the other transaction until the first one is finished? In your pizza example, there's somebody present who doesn't themselves eat any pizza, but whose job is to stop two people grabbing the same slice--and if the only way to stop that is to grab somebody's hand and stop them reaching for the slice until someone else has finished with it, well, that's just fine and dandy.
Depends on the lock, really. And the queries. Read up on Replication, Shared Locks, Exclusive Locks, etc.
Locking is an article in and of itself (possibly a series). I was just trying to cover the brief 50k foot view of the ACID concepts without going off on tangents.