it is a most common mechanism that is used to control concurrency in two phases
for achiving the serializability.the two phases are Growing and Shrinking.
1)a transaction acqires locks on data items it will need to complete the
transaction.this is called growing process.a transaction may obtain lock but may
not release any lock.
2)one lock is released no other lock may be acquired.this is called shrinking
process.a transaction may release locks but may not obtain any newlocks.