Submitted By
Ujjwal Matoliya
Assignment :-
concurrency control
Protocol
Database management
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Transaction
Concurrency control
Type of Concurrency control protocol
INDEX

It is a set of opration used to perform
a logical unit of work
A transaction generally represent change in databass
Transaction
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Example of a simple transaction.
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READ
WRITE
A=1000
B=2000
Transfer
R(A)= 1000 R(B)=2000
A=A-500 B=B+500
W(A)=500 W(B)=2500
commit
A=500
B=2500

Commit: After all instructions of a transaction are
successfully executed, the changes made by transaction are
made permanent in the database.
Write (A): Write operation Write(A) or W(A) writes the value back to
the database from buffer.
Read(A): Read operations Read(A) or R(A) reads the value of A from
the database and stores it in a buffer in main memory.
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The concurrency control is the process to maintain
the data where there are multiple resources or
users are accessing the data element and
performing the database operations. There are
several enterprise systems such as banking, ticket
booking, and traffic light systems that use a shared
database as part of the data store associated with
concurrent transactions.
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Concurrency control

Shard – Exclusive Locking
Shared Lock (S): also known as Read-only lock. As the name
suggests it can be shared between transactions because while
holding this lock the transaction does not have the permission to
update data on the data item. S-lock is requested using lock-S
instruction.
Exclusive Lock (X): Data item can be both read as well as
written. This is Exclusive and cannot be held simultaneously on
the same data item. X-lock is requested using lock-X instruction.
REQUEST->
<-GRANT
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S E
S YES NO
E NO NO

Ex = Transaction (T1)
Transaction (T2)
Shard Locking (s)
Exclusive Locking(E)
Read (R)
Write (W)
T1
S(A)
R(A)
U(A)
T1
E(A)
R(A)W
U(A)
T2
E(A)
R(A)W
U(A)
T2
S(A)
R(A)
U(A) 8

Problem With Shard – Exclusive Locking
I. May not sufficient to produce only
serializable schedule
II. May not free from irrecoverability
III. May not free from deadlock
IV. May not free from starvation A
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T1 T2
E(A)
R(A)W
U(A)
S(A)
R(A)
U(A)
E(A)
R(A)W
U(A)
Not serializable
T1 T2
E(A)
R(A)W
U(A)
S(A)
R(A)
COMMIT
*
irrecoverability Deadlock
T1 T2
E(A)
E(B)
E(B)*
E(A)*
starvation
T1 T2 T3
S(A)
E(A)*
S(A)
U(A)
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Two Phase Locking Protocol
Growing Phase: New locks on
data items may be acquired but
none can be released.
Shrinking Phase: Existing
locks may be released but
no new locks can be
acquired.
T1 T2
E(A)
R(A)W
S(A)*
S(B)
R(B)
U(A)
U(B)
serializable
Growing
Shrinking
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Problem With Two Phase Locking Protocol
I. May not free from irrecoverability
II. not free from deadlock
III. not free from starvation
IV. Not free from cascading rollback
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Irrecoverability
T1 T2
E(A)
R(A)W
U(A)
S(A)
R(A)
.
.
.commit
*
Deadlock
T1 T2
E(A)
E(B)
E(B)*
E(A)*
starvation
T1 T2 T3
S(A)
E(A)*
S(A)
U(A)
Cascading rollback
T1 T2 T3
E(A)
R(A)W
U(A)
S(A)
R(A)
.
.
S(A)
R(A)
.
.
*
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Strict 2-PL –
This requires that in addition to the lock being 2-Phase all
Exclusive(X) locks held by the transaction be released
until after the Transaction Commits.
Rigorous 2-PL –
This requires that in addition to the lock being 2-Phase all
Exclusive(X) and Shared(S) locks held by the transaction be
released until after the Transaction Commits.
Conservative 2-PL –
This protocol requires the transaction to lock all the items it
access before the Transaction begins execution by
predeclaring its read-set and write-set. **free from deadlock
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Timestamp Ordering Protocol
T1
S(A)
R(A)
T2
S(A)
R(A)
10:00 10:10
100 200
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The main idea for this protocol is to order the transactions based
on their Timestamps. A schedule in which the transactions
participate is then serializable and the only equivalent serial
schedule permitted has the transactions in the order of their
Timestamp Values.
Or
Unique value assign to every transactions.

W_TS(X) is the largest timestamp of any transaction that
executed write(X) successfully.
R_TS(X) is the largest timestamp of any transaction that
executed read(X) successfully.
T1
S(A)
R(A)
T2
S(A)
R(A)
RTS=200
10:00 10:10
100 200
10:00 10:10
100 200
T1
E(A)
W(A)
T2
E(A)
W(A)
WTS=200
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Transaction Ti issues a W (A) operation,
If R_TS(X) > TS(T) , then rollback Ti
if W_TS(X) > TS(T), then rollback Ti
Otherwise Execute W(A) operation of T and
set W_TS(A) = TS(Ti).
Transaction Ti issues a R (A) operation
1. )If W_TS(A) > TS(Ti) ,rollback Ti
2.) Otherwise execute the R(A) operation
If W_TS(X) <= TS(T),of T and set R_TS(X) to the larger of
TS(Ti) and current R_TS(A).
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T1
R(A)
W(A)
W(A)
T2
W(A)
R(A)
W(A)
T1
W(A)
R(A)
W(A)
T2
R(A)
W(A)
W(A)
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