Points of Transactions
Purposes
- users need the ability to define a unit of work
- concurrent access to data by >1 user or program
Unit of work
Properties
• Atomicity
• Consistency
• Isolation
• Durability
Concurrent access
The Lost Update problem
The Uncommitted Data problem
Lock
- lock manager
- level of lock
Deadlock
– T1 locks data item X, then wants Y
– T2 locks data item Y, then wants X
– each waits to get a data item which the other transaction is
already holding
– could wait forever if not dealt with