1) The two-phase locking protocol requires transactions to acquire locks in two phases - the growing phase where locks can be acquired but not released, and the shrinking phase where locks can be released but not acquired. 2) The two-phase locking protocol ensures conflict serializability by ordering transactions based on their lock points. 3) However, it does not prevent deadlocks and can cause cascading rollbacks if transactions release locks before committing. Modifications like strict two-phase locking prevent this by requiring locks to be held until commit.

1. DATABASE MANAGEMENT
SYSTEM:
T WO PHASE LOCKING PROTOCOL
1 7 0 4 1 0 1 0 7 0 2 7
D H A N A N J AY S I N H J H A L A
S Y C E - I B AT C H B

2. INDEX
• Locks
• Locking protocols
• 2 phase locking protocol
–Phases
–Merits
–demerits

3. LOCKS
To ensure serializability, it is required that data items should be
accessed in mutual exclusive manner, i.e. if one transaction is accessing
a data item, no other transaction can modify that data item.
To implement this requirement locks are used.
A transaction is allowed to access a data item only if it is currently
holding a lock on that item.
There are 2 modes in which a data item may be locked:
Shared mode: if a transaction T1 has obtained a shared mode lock on
item Q, then T1 can read, but cannot write Q. it is denoted by S.
Exclusive: if a transaction T1 has obtained a exclusive mode lock on
item Q, then T1 can read & also write Q. it is denoted by X.

4. LOCKING PROTOCOLS
When a transaction requests a lock on a data item in a particular mode
and no other transaction has a lock on the same data item, the lock can
be granted.
Supposing a transaction T1 requests a lock on data item Q in a particular
mode M, the concurrency control manager grants the lock provided that
There is no other transaction holding a lock on Q in a mode that
conflicts with M
There is no other transaction that is waiting for a lock on Q, and that
made its lock request before T1.

5. 2 PHASE LOCKING PROTOCOL
• This protocol requires that each transaction issue lock and
unlock requests in two phases:
– Growing phase: in this phase, a transaction may obtain locks, but
may not release any lock.
– Shrinking phase: in this phase, a transaction may release locks, but
may not obtain any new locks.
– Initially, a transaction is in the growing phase. The transaction
acquires locks as needed. Once a transaction releases a lock, it enters
in the shrinking phase and it cannot issue more lock requests.

6. 2 PHASE LOCKING PROTOCOL
• E.g. Transaction given below is a two phase transaction.
• T3:
– lock X(B);
– read(B);
– B:= B-50;
– write(B);
– lock X(A);
– read(A);
– A:=A+50;
– write(A);
– unlock(B);
– unlock(A);

7. MERITS OF 2 PHASE LOCKING PROTOCOL
• The two phase locking protocol ensure conflict serializability.
• Consider any transaction, the point in the schedule where the
transaction has obtained it's final lock is called the lock point
of the transaction.
• Now, transaction can be ordered according to their lock
points. This ordering is a serializability ordering for the
transactions.

8. DEMERITS OF 2 PHASE LOCKING PROTOCOL
• The two phase locking protocol Does not ensure freedom from
deadlock:
– Consider the schedule shown in figure
– Transaction T3 and T4 are two phases but they are deadlocked.

9. • Cascading rollback may occur under two phase locking:
– Consider the schedule shown in figure

10. • As shown in the above figure transactions T5,T6 and T7 are
two phase, but failure of T5 after the read(A) instruction of T7
leads to cascading rollback of T6 and T7.
• Cascading rollback can be avoided by modification of two
phase locking:
– 1. Strict two phase locking protocol: This protocol requires that
locking should be two phase and all executive mode locks taken by a
transaction should be held until the transaction commits. This
requirement prevents any transaction from reading the data written
by any uncommitted transaction under exclusive mode until the
transaction commits.

11. • 2. The rigorous two phase locking protocol: This protocol
requires that all locks be held until the transaction commits.