Concurrency Control

1. Transaction
A transaction is a unit of work performed within a database
Transaction is a logical unit that is
independently executed for
data retrieval or updates
Transactions are represented as Schedules
t0
t1
t2
t3
t4
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2. States of a Transaction
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3. SQL Transaction is helpful to execute one more statements as a set.
SQL Transaction
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4. States of a Transaction: COMMIT
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5. States of a Transaction: ROLLBACK
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6. Complete life cycle of a Transaction
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7. Properties of a Transaction
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8. ACID Properties
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9. ACID Properties: ATOMICITY
ATOMICITY
A transaction must be fully complete, saved (committed) or completely
undone (rolled back).
Begin
Commit
RollBack
ALL
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10. ACID Properties: CONSISTANCY
CONSISTENCY
The transaction must be fully compliant with the state of the
database as it was prior to the transaction
CONSISTENCY
INCONSISTENCY
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11. ACID Properties: ISOLATION
ISOLATION
Transaction data must not be available to other transactions
until the original transaction is committed or rolled back.
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12. ACID Properties: DURABILITY
DURABILITY
Transaction data changes must be available, even in the
event of database failure.
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13. To perform one task at same time by two objects
Concurrency
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14. Concurrent vs Parallel
CONCURRENCY  Sharing of Resources
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15. Concurrent vs Parallel
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16. CONCURRENCY
IS
A
PROBLEM
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17. Problems of Concurrency
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18. A=10
A= A+10 =20
A=20
A= A+10 =30
A=30
A=10
Problems of Concurrency: DIRTY READ
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19. Problems of Concurrency: UNEPEATED READ
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20. Problems of Concurrency: LOST UPDATE
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21. Problems of Concurrency: PHANTOM READ
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22. Problems of Concurrency
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23. Problems of Concurrency: LOST UPDATE
DEAD LOCK
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24. 12-04-2019 24by : Jyoti Lakhani

25. Concurrency control is the procedure in DBMS for
managing simultaneous operations
without conflicting with each another
Concurrency Control: WHAT IS?
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26. Concurrency Control: PROTOCOLS
Concurrency Control Protocols
• Locking Protocol
•Two Phase Locking Protocol
•Time Stamp Protocol
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27. Concurrency Control: LOCKING PROTOCOL
Lock Data Items
A lock is a data variable which is associated with a data item
Locks helps to –
synchronize access to the database items
…. by concurrent transactions.
Transactions proceed only once the lock request is granted
Transactions proceed only once the lock request is granted
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Concurrency Control: LOCKING PROTOCOL

29. Concurrency Control: LOCKING PROTOCOL
• Binary Locks
• Shared/ Exclusive Locking
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30. Concurrency Control: LOCKING PROTOCOL
Binary Locking
A Binary lock on a data item can either locked or unlocked states
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31. Concurrency Control: LOCKING PROTOCOL
Shared / Exclusive Locks
This type of locking mechanism separates the locks based on
their uses.
Shared Lock (S)
Read-only lock
data item can be shared
between transactions
you will never have
permission to update data
a data item can be read as well
as written
Exclusive Lock (X)
This is exclusive and can't be held
concurrently on the same data
item
Transactions may unlock the data
item after finishing the 'write'
operation
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32. Concurrency Control: LOCKING PROTOCOL
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33. Concurrency Control: Locking Protocol
Problem with Locking Protocol
Starvation
Deadlock
Starvation is the situation when a transaction needs to wait for an indefinite
period to acquire a lock.
Deadlock refers to a specific situation where two or more processes are
waiting for each other to release a resource or more than two processes
are waiting for the resource in a circular chain.
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34. Concurrency Control: Two Phase Locking Protocol
This locking protocol divides the execution phase of a
transaction into three different parts
• Growing Phase
• Locked Phase
• Shrinking Phase
Growing Phase: In this phase transaction may obtain locks but may not release any
locks.
Shrinking Phase: In this phase, a transaction may release locks but not obtain any new
lock
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35. Concurrency Control: Two Phase Locking Protocol
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36. Strict Two-Phase Locking Method
Concurrency Control: Two Phase Locking Protocol
Transaction Hold Locks until end of the Transaction
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37. Concurrency Control: Two Phase Locking Protocol
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38. Concurrency Control: Time Stamp based Protocol
Uses a timestamp to serialize the execution of concurrent
transactions
This protocol ensures that every conflicting read and write
operations are executed in timestamp order.
The protocol uses the System Time or Logical Count as a
Timestamp.
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39. Concurrency Control: Time Stamp based Protocol
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40. In Time Stamp Protocol ensures that any conflicting read and write operations are
executed in time stamp order
if not such an operation is rejected and transaction will be rolled back.
The rolled back transaction will be restarted with a new Time Stamp.
Concurrency Control: Time Stamp based Protocol
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41. Concurrency Control: Time Stamp based Protocol
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42. Advantages
1. Serializability
2. Ensures freedom from dead lock
Disadvantage
Starvation may occur due to continuously
getting aborted and restarting the
transaction.
Concurrency Control: Time Stamp based Protocol
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