Database System Concepts, 6th Ed.
©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
Chapter 14: Transactions
©Silberschatz, Korth and Sudarshan14.2Database System Concepts - 6th Edition
Chapter 14: Transactions
 Transaction Concept
 Transaction State
 Concurrent Executions
 Serializability
 Recoverability
 Implementation of Isolation
 Transaction Definition in SQL
 Testing for Serializability
©Silberschatz, Korth and Sudarshan14.3Database System Concepts - 6th Edition
Transaction Concept
 A transaction is a unit of program execution that accesses and
possibly updates various data items.
 E.g. transaction to transfer $50 from account A to account B:
1. read(A)
2. A := A – 50
3. write(A)
4. read(B)
5. B := B + 50
6. write(B)
 Two main issues to deal with:
 Failures of various kinds, such as hardware failures and system
crashes
 Concurrent execution of multiple transactions
©Silberschatz, Korth and Sudarshan14.4Database System Concepts - 6th Edition
Example of Fund Transfer
 Transaction to transfer $50 from account A to account B:
1. read(A)
2. A := A – 50
3. write(A)
4. read(B)
5. B := B + 50
6. write(B)
 Atomicity requirement
 if the transaction fails after step 3 and before step 6, money will be “lost”
leading to an inconsistent database state
 Failure could be due to software or hardware
 the system should ensure that updates of a partially executed transaction
are not reflected in the database
 Durability requirement — once the user has been notified that the transaction
has completed (i.e., the transfer of the $50 has taken place), the updates to the
database by the transaction must persist even if there are software or
hardware failures.
©Silberschatz, Korth and Sudarshan14.5Database System Concepts - 6th Edition
Example of Fund Transfer (Cont.)
 Transaction to transfer $50 from account A to account B:
1. read(A)
2. A := A – 50
3. write(A)
4. read(B)
5. B := B + 50
6. write(B)
 Consistency requirement in above example:
 the sum of A and B is unchanged by the execution of the transaction
 In general, consistency requirements include
 Explicitly specified integrity constraints such as primary keys and foreign
keys
 Implicit integrity constraints
– e.g. sum of balances of all accounts, minus sum of loan amounts
must equal value of cash-in-hand
 A transaction must see a consistent database.
 During transaction execution the database may be temporarily inconsistent.
 When the transaction completes successfully the database must be
consistent
 Erroneous transaction logic can lead to inconsistency
©Silberschatz, Korth and Sudarshan14.6Database System Concepts - 6th Edition
Example of Fund Transfer (Cont.)
 Isolation requirement — if between steps 3 and 6, another
transaction T2 is allowed to access the partially updated database, it
will see an inconsistent database (the sum A + B will be less than it
should be).
T1 T2
1. read(A)
2. A := A – 50
3. write(A)
read(A), read(B), print(A+B)
4. read(B)
5. B := B + 50
6. write(B
 Isolation can be ensured trivially by running transactions serially
 that is, one after the other.
 However, executing multiple transactions concurrently has significant
benefits, as we will see later.
©Silberschatz, Korth and Sudarshan14.7Database System Concepts - 6th Edition
ACID Properties
 Atomicity. Either all operations of the transaction are properly reflected
in the database or none are.
 Consistency. Execution of a transaction in isolation preserves the
consistency of the database.
 Isolation. Although multiple transactions may execute concurrently,
each transaction must be unaware of other concurrently executing
transactions. Intermediate transaction results must be hidden from other
concurrently executed transactions.
 That is, for every pair of transactions Ti and Tj, it appears to Ti that
either Tj, finished execution before Ti started, or Tj started execution
after Ti finished.
 Durability. After a transaction completes successfully, the changes it
has made to the database persist, even if there are system failures.
A transaction is a unit of program execution that accesses and possibly
updates various data items.
Alternatively: A transaction is a sequence of operations that form a
logical unit of a program
To preserve the integrity of data the database system must ensure:
©Silberschatz, Korth and Sudarshan14.8Database System Concepts - 6th Edition
Transaction State
 Active – the initial state; the transaction stays in this state while it is
executing
 Partially committed – after the final statement has been executed.
 Failed – after the discovery that normal execution can no longer
proceed.
 Aborted – after the transaction has been rolled back and the
database restored to its state prior to the start of the transaction.
Two options after it has been aborted:
 restart the transaction
 can be done only if no internal logical error
 kill the transaction
 Committed – after successful completion.
©Silberschatz, Korth and Sudarshan14.9Database System Concepts - 6th Edition
Transaction State Diagram
©Silberschatz, Korth and Sudarshan14.10Database System Concepts - 6th Edition
Concurrent Executions
 Multiple transactions are allowed to run concurrently in the system.
Advantages are:
 increased processor and disk utilization, leading to better
transaction throughput
 E.g. one transaction can be using the CPU while another is
reading from or writing to the disk
 reduced average response time for transactions: short
transactions need not wait behind long ones.
 Concurrency control schemes – mechanisms to achieve isolation
 that is, to control the interaction among the concurrent
transactions in order to prevent them from destroying the
consistency of the database
 Will study in Chapter 16 (Recovery System), after studying
notion of correctness of concurrent executions.
©Silberschatz, Korth and Sudarshan14.11Database System Concepts - 6th Edition
Schedules
 Schedule – a sequence of instructions that specify the chronological
order in which instructions of concurrent transactions are executed
 a schedule for a set of transactions must consist of all instructions
of those transactions
 must preserve the order in which the instructions appear in each
individual transaction.
 A transaction that successfully completes its execution will have a
commit instruction as the last statement
 by default, a transaction is assumed to execute the commit
instruction as its last step
 A transaction that fails to successfully complete its execution will have
an abort instruction as the last statement
©Silberschatz, Korth and Sudarshan14.12Database System Concepts - 6th Edition
Schedule 1
 Let T1 transfer $50 from A to B, and T2 transfer 10% of the
balance from A to B.
 A serial schedule in which T1 is followed by T2 :
©Silberschatz, Korth and Sudarshan14.13Database System Concepts - 6th Edition
Schedule 2
• A serial schedule where T2 is followed by T1
©Silberschatz, Korth and Sudarshan14.14Database System Concepts - 6th Edition
Schedule 3
 Let T1 and T2 be the transactions defined previously. The
following schedule is not a serial schedule, but it is
equivalent to the serial Schedule 1.
In Schedules 1, 2 and 3, the sum A + B is preserved.
©Silberschatz, Korth and Sudarshan14.15Database System Concepts - 6th Edition
Schedule 4
 The following concurrent schedule does not preserve the
value of (A + B ).
©Silberschatz, Korth and Sudarshan14.16Database System Concepts - 6th Edition
Serializability
 Basic Assumption – Each transaction preserves database
consistency.
 Thus serial execution of a set of transactions preserves
database consistency.
 A (possibly concurrent) schedule is serializable if it is
equivalent to a serial schedule.
 Different forms of schedule equivalence give rise to the notions
of:
1. conflict serializability
2. view serializability
©Silberschatz, Korth and Sudarshan14.17Database System Concepts - 6th Edition
Simplified view of transactions
 We ignore operations other than read and write instructions
 We assume that transactions may perform arbitrary
computations on data in local buffers in between reads and
writes.
 Our simplified schedules consist of only read and write
instructions.
©Silberschatz, Korth and Sudarshan14.18Database System Concepts - 6th Edition
Conflicting Instructions
 Instructions li and lj of transactions Ti and Tj respectively, conflict
if and only if there exists some item Q accessed by both li and lj,
and at least one of these instructions wrote Q.
1. li = read(Q), lj = read(Q). li and lj don’t conflict.
2. li = read(Q), lj = write(Q). They conflict.
3. li = write(Q), lj = read(Q). They conflict
4. li = write(Q), lj = write(Q). They conflict
 Intuitively, a conflict between li and lj forces a (logical) temporal
order between them.
 If li and lj are consecutive in a schedule and they do not
conflict, their results would remain the same even if they had
been interchanged in the schedule.
©Silberschatz, Korth and Sudarshan14.19Database System Concepts - 6th Edition
Conflict Serializability
 If a schedule S can be transformed into a schedule S’ by a series of
swaps of non-conflicting instructions, we say that S and S’ are
conflict equivalent.
 We say that a schedule S is conflict serializable if it is conflict
equivalent to a serial schedule
©Silberschatz, Korth and Sudarshan14.20Database System Concepts - 6th Edition
Conflict Serializability (Cont.)
 Schedule 3 can be transformed into Schedule 6, a serial
schedule where T2 follows T1, by series of swaps of nonconflicting
instructions.
 Therefore Schedule 3 is conflict serializable.
Schedule 3 Schedule 6
©Silberschatz, Korth and Sudarshan14.21Database System Concepts - 6th Edition
Conflict Serializability (Cont.)
 Example of a schedule that is not conflict serializable:
 We are unable to swap instructions in the above schedule to
obtain either the serial schedule < T3, T4 >, or the serial
schedule < T4, T3 >.
©Silberschatz, Korth and Sudarshan14.22Database System Concepts - 6th Edition
View Serializability
 Let S and S’ be two schedules with the same set of transactions. S
and S’ are view equivalent if the following three conditions are met,
for each data item Q,
 If in schedule S, transaction Ti reads the initial value of Q, then in
schedule S’ also transaction Ti must read the initial value of Q.
 Initial value == the very first operation of read (Q) in any T
 If in schedule S transaction Ti executes read(Q), and that value
was produced by transaction Tj (if any), then in schedule S’ also
transaction Ti must read the value of Q that was produced by the
same write(Q) operation of transaction Tj .
 The transaction (if any) that performs the final write(Q) operation
in schedule S must also perform the final write(Q) operation in
schedule S’.
As can be seen, view equivalence is also based purely on reads and
writes alone.
©Silberschatz, Korth and Sudarshan14.23Database System Concepts - 6th Edition
View Serializability (Cont.)
 A schedule S is view serializable if it is view equivalent to a serial
schedule.
 Every conflict serializable schedule is also view serializable.
 Below is a schedule which is view-serializable but not conflict
serializable.
 What serial schedule is it equivalent to?
 Every view serializable schedule that is not conflict serializable has
blind writes.
©Silberschatz, Korth and Sudarshan14.24Database System Concepts - 6th Edition
Other Notions of Serializability
 The schedule below produces same outcome as the serial
schedule < T1, T5 >, yet is not conflict equivalent or view
equivalent to it.
 Determining such equivalence requires analysis of operations
other than read and write.
©Silberschatz, Korth and Sudarshan14.25Database System Concepts - 6th Edition
Testing for Serializability
 Consider some schedule of a set of transactions T1, T2, ..., Tn
 Precedence graph is a direct graph where
 vertices are the transactions (names);
 an arc from Ti to Tj if the two transactions conflict, and Ti
accessed the data item on which the conflict arose earlier.
 We may label the arc by the item that was accessed.
 Example 1
T5T1
A
B
©Silberschatz, Korth and Sudarshan14.26Database System Concepts - 6th Edition
Test for Conflict Serializability
 A schedule is conflict serializable if and only if
its precedence graph is acyclic.
 Cycle-detection algorithms exist which take
order n2 time, where n is the number of
vertices in the graph.
 Better algorithms take order n + e where
e is the number of edges.
 If precedence graph is acyclic, the serializability
order can be obtained by a topological sorting
of the graph.
 This is a linear order consistent with the
partial order of the graph.
 For example, a serializability order for
Schedule in Fig. (a) would be
Ti  Tj  Tk  Tm
 Or with Tj and Tk swapped
©Silberschatz, Korth and Sudarshan14.27Database System Concepts - 6th Edition
Test for View Serializability
 The precedence graph test for conflict serializability cannot be used
directly to test for view serializability.
 Extension to test for view serializability has cost exponential in the
size of the precedence graph.
 The problem of checking if a schedule is view serializable falls in the
class of NP-complete problems.
 Thus existence of an efficient algorithm is extremely unlikely.
 However practical algorithms that just check some sufficient
conditions for view serializability can still be used.
©Silberschatz, Korth and Sudarshan14.28Database System Concepts - 6th Edition
Recoverable Schedules
 Recoverable schedule — if a transaction Tj reads a data item
previously written by a transaction Ti , then the commit operation of Ti
appears before the commit operation of Tj.
 The following schedule (Schedule 11) is not recoverable if T9 commits
immediately after the read
 If T8 should abort, T9 would have read (and possibly shown to the user)
an inconsistent database state. Hence, database must ensure that
schedules are recoverable.
Need to address the effect of transaction failures on concurrently
running transactions.
©Silberschatz, Korth and Sudarshan14.29Database System Concepts - 6th Edition
Cascading Rollbacks
 Cascading rollback – a single transaction failure leads to a
series of transaction rollbacks.
 Consider the following schedule where none of the transactions
has yet committed (so the schedule is recoverable)
If T10 fails, T11 and T12 must also be rolled back.
 Can lead to the undoing of a significant amount of work
©Silberschatz, Korth and Sudarshan14.30Database System Concepts - 6th Edition
Cascadeless Schedules
 Cascadeless schedules — cascading rollbacks cannot occur if
 for each pair of transactions Ti and Tj such that Tj reads a data
item previously written by Ti, the commit operation of Ti appears
before the read operation of Tj.
 Every cascadeless schedule is also recoverable.
 It is desirable to restrict the schedules to those that are cascadeless.
©Silberschatz, Korth and Sudarshan14.31Database System Concepts - 6th Edition
Concurrency Control
 A database must provide a mechanism that will ensure that all possible
schedules are
 either conflict or view serializable, and
 are recoverable and preferably cascadeless
 A policy in which only one transaction can execute at a time generates
serial schedules, but provides a poor degree of concurrency
 Are serial schedules recoverable/cascadeless?
 Testing a schedule for serializability after it has executed is a little too
late!
 Goal – to develop concurrency control protocols that will assure
serializability.
©Silberschatz, Korth and Sudarshan14.32Database System Concepts - 6th Edition
Concurrency Control (Cont.)
 Concurrency-control schemes tradeoff between
 the amount of concurrency they allow and
 the amount of overhead that they incur.
 Some schemes
 allow only conflict-serializable schedules to be generated,
 while others allow view-serializable schedules that are not
conflict-serializable.
 But are recoverable and cascadeless.
©Silberschatz, Korth and Sudarshan14.33Database System Concepts - 6th Edition
Concurrency Control vs. Serializability Tests
 Concurrency control protocols generally do not examine the
precedence graph as it is being created
 Instead a protocol imposes a discipline that avoids non-seralizable
schedules.
 We study such protocols in Chapter 16 (Recovery System).
 Tests for serializability help us understand why a concurrency control
protocol is correct.
©Silberschatz, Korth and Sudarshan14.34Database System Concepts - 6th Edition
Weak Levels of Consistency
 Some applications are willing to live with weak levels of consistency,
allowing schedules that are not serializable
 E.g. a read-only transaction that wants to get an approximate total
balance of all accounts
 E.g. database statistics computed for query optimization can be
approximate (why?)
 Such transactions need not be serializable with respect to other
transactions
 Tradeoff accuracy for performance
©Silberschatz, Korth and Sudarshan14.35Database System Concepts - 6th Edition
Levels of Consistency in SQL-92
 SQL levels of concurrency control:
 Serializable — default
 Repeatable read — only committed records to be read, repeated reads
of same record must return same value. However, a transaction may not
be serializable – it may find some records inserted by a transaction but
not find others.
 Read committed — only committed records can be read, but successive
reads of record may return different (but committed) values.
 Read uncommitted — even uncommitted records may be read.
 Lower degrees of consistency useful for gathering approximate
information about the database
 Warning: some database systems do not ensure serializable schedules by
default
 E.g. Oracle and PostgreSQL by default support a level of consistency
called snapshot isolation (stronger than repeatable read but not part of
the SQL standard)
©Silberschatz, Korth and Sudarshan14.36Database System Concepts - 6th Edition
Transaction Definition in SQL
 Data manipulation language must include a construct for
specifying the set of actions that comprise a transaction.
 In SQL, a transaction begins implicitly.
 A transaction in SQL ends by:
 Commit work commits current transaction and begins a new
one.
 Rollback work causes current transaction to abort.
 In almost all database systems, by default, every SQL statement
also commits implicitly if it executes successfully
 Implicit commit can be turned off
 E.g. in JDBC, connection.setAutoCommit(false);
Database System Concepts, 6th Ed.
©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
End of Chapter 14