The document discusses the concepts of transaction schedules in databases, distinguishing between complete (serial) and non-serial (interleaved) schedules, and the importance of serializability for ensuring consistency. It highlights conflicts that arise from interleaved transaction executions, such as write-read, read-write, and write-write conflicts. Additionally, it elaborates on concurrency control techniques, including locking protocols, specifically two-phase locking, and timestamp ordering to manage these transactions effectively.

7. Complete schedule:
It is a schedule of transactions where each transaction is committed before terminating. The example is shown below where transactions T1 and T2 terminates after committing the transactions.
Example:
T1 T2
A=1000
Read(A)
A=A+100
Write(A) Read(A)
B=A-100
Write(B)
Commit
Read(B)
Write(B)

11. Serializability
• Serializability of schedules ensures that a non-serial
schedule is equivalent to a serial schedule. It helps in
maintaining the transactions to execute simultaneously without
interleaving one another.

12. Schedules in DBMS are of two types:
• Serial Schedule - A schedule in which only one transaction
is executed at a time, i.e., one transaction is executed
completely before starting another transaction.

13. Example
Transaction-1 Transaction-2
R(a)
W(a)
R(b)
W(b)
R(b)
W(b)
R(a)
W(a)

14. Non-serial Schedule -
• A schedule in which the transactions are interleaving or
interchanging. There are several transactions executing
simultaneously as they are being used in performing real-
world database operations. These transactions may be
working on the same piece of data. Hence, the serializability
of non-serial schedules is a major concern so that our
database is consistent before and after the execution of the

15. Example
Transaction-1 Transaction-2
R(a)
W(a)
R(b)
W(b)
R(b)
R(a)
W(b)
W(a)

16. Anomalies due to interleave execution of transaction:
Anomalies due to Interleaved execution • There are three main
situations when the actions of two transactions T1 and T2 conflict with
each other in the interleaved execution on the same data object.
Write-Read (WR) Conflict: Reading Uncommitted data.
Read-Write (RW) Conflict: Unrepeatable Reads
Write-Write (WW) Conflict: Overwriting Uncommitted Data

17. Reading uncommitted values(WR
conflicts):
1.If you try to the read the value which
is not written on to the data base(not
committed) will leads to write-read
conflict which is called dirty read
operation.

18. Un repeatable reading data operation(RW conflicts):
Reading the same object twice before committing the transaction might
yield an inconsistency
–Read-then-Write (RW) Conflicts (Write-After-Read)
Unrepeatable problem means we get different values in different
reads. For example in S1 say T2 read initially x=5 then T1 updated
x=1 so now T2 will read x=1 here T2 has read two different values
during consecutive reads This shouldn't have been allowed as T1 has
not committed

19. Overwriting uncommitted data(WW conflicts)
WW conflicts if one transaction could over write
the value of an object A
which has been already modified by other
transaction T1 while first transaction still in
progress .This kind of conflict refer to blind write
conflict.

20. Lock-Based Concurrency Control
• Locking is a concurrent control technique used to ensure
serializability. •
• There are two types of locks.
• A transaction needs to acquire a lock before performing a
transaction. •
• The read lock is known as shared lock and write lock is known as
exclusive lock. •
• A locking protocol is a set of rules that a transaction follows to attain
serializability

21. Strict Two-Phase Locking (Strict 2PL) Protocol
The Strict 2PL has the following two rules:
Rule 1: A transaction can read data only when it acquires a
shared lock and can write data only when it acquires an
exclusive lock on object.
Rule 2: A transaction should release the locks when it is
completed.

23. Concurrency Control Techniques are
1.Two-phase locking Protocol
2.Time stamp ordering Protocol
3.Multi version concurrency control
4.Validation concurrency control

24. Two Phase Locking
• Locking is an operation which secures: permission to read,
OR permission to write a data item. Two phase locking is a
process used to gain ownership of shared resources without
creating the possibility of deadlock. The 3 activities taking
place in the two phase update algorithm are:
(i) Lock Acquisition
(ii) Modification of Data
(iii) Release Lock

25. Cont..
• Two phase locking prevents deadlock from occurring in distributed
systems by releasing all the resources it has acquired, if it is not
possible to acquire all the resources required without waiting for
another process to finish using a lock. This means that no process is
ever in a state where it is holding some shared resources, and waiting
for another process to release a shared resource which it requires.
This means that deadlock cannot occur due to resource contention. A
transaction in the Two Phase Locking Protocol can assume one of the
2 phases:

26. Cont..
• Growing Phase: In this phase a transaction can only acquire
locks but cannot release any lock. The point when a
transaction acquires all the locks it needs is called the Lock
Point.
• Shrinking Phase: In this phase a transaction can only
release locks but cannot acquire any.

27. Time Stamp Ordering
• A timestamp is a tag that can be attached to any transaction or any
data item, which denotes a specific time on which the transaction
or the data item had been used in any way. A timestamp can be
implemented in 2 ways. One is to directly assign the current value
of the clock to the transaction or data item. The other is to attach
the value of a logical counter that keeps increment as new
timestamps are required. The timestamp of a data item can be of 2
types:
• (i) W-timestamp(X): This means the latest time when the data item
X has been written into.
• (ii) R-timestamp(X): This means the latest time when the data item
X has been read from. These 2 timestamps are updated each time
a successful read/write operation is performed on the data item X.