Database Transactions
 

Ik heb zoveel videos gekeken over two phase locking maar snap het nog steeds niet :lisa:

http://imageshack.com/a/img540/5281/odkzMy.png


We use the following abbreviations:

S(I) for obtaining a shared lock on item I

X(I) for obtaining an exclusive lock on item I

US(I) for releasing a shared lock on item I

UX(I) for releasing an exclusive lock on item I

Note that a transaction is allowed to upgrade and downgrade a lock:

Upgrade: A transaction can acquire a shared lock S(I) and later an exclusive lock X(I) on the same item; this is called upgrade of the lock.

Downgrade: If a transaction holds both a shared lock S(I) and an exclusive lock X(I) on the same item I, then, in the unlocking phase, it can release the exclusive lock UX(I) while keeping the shared lock; this is called a downgrade of the lock.

Kan iemand mij aub uitleggen waar ik mee moeten beginnen en waar ik rekening mee moet houden?