Controlling Concurrent Access to Data Objects

Data objects are a special type of shared resources.
When jobs are scheduled preemptively, their accesses to data object may be interleaved.
To ensure data integrity it is common to require that reads and writes be serializable.
A sequence of reads and writes by a set of jobs is serializable if the effect produced by the sequence on all the data objects shared by the jobs is the same as the effect produced by a serial sequence.
Two phase locking protocol is used to access the data objects. It is :
- A way to ensure serializability
- A job never requests any lock once it releases some lock