Database System Concepts, 7th Ed.
©Silberschatz, Korth and Sudarshan
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Module 12: Transactions
©Silberschatz, Korth and Sudarshan12.2Database System Concepts - 7th Edition
Outline
▪ Transaction Concept
▪ Transaction State
▪ Concurrent Executions
▪ Serializability
▪ Recoverability
▪ Implementation of Isolation
▪ Transaction Definition in SQL
▪ Testing for Serializability.
©Silberschatz, Korth and Sudarshan12.3Database System Concepts - 7th 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 Sudarshan12.4Database System Concepts - 7th 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 been 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.
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Example of Fund Transfer (Cont.)
▪ Consistency requirement in the 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., the sum of balances of all accounts, minus the sum of loan
amounts must equal the 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
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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.
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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 executed 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. To preserve the integrity of data the database
system must ensure:
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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.
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Transaction State (Cont.)
©Silberschatz, Korth and Sudarshan12.10Database System Concepts - 7th 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
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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
forming the transactions
• Must preserve the order in which the instructions appear in each
individual transaction.
▪ A transaction that successfully completes its execution will have the commit
instruction as the last statement
• By default 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 Sudarshan12.12Database System Concepts - 7th 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 :
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Schedule 2
▪ A serial schedule where T2 is followed by T1
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Schedule 3
▪ Let T1 and T2 be the transactions defined previously. The following
schedule is not a serial schedule, but it is equivalent to Schedule 1
▪ In Schedules 1, 2 and 3, the sum A + B is preserved.
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Schedule 4
▪ The following concurrent schedule does not preserve the value of (A + B ).
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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
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Simplified view of transactions
▪ We ignore operations other than the 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 the read and write instructions.
©Silberschatz, Korth and Sudarshan12.18Database System Concepts - 7th 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 Sudarshan12.19Database System Concepts - 7th 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
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Conflict Serializability (Cont.)
▪ Schedule 3 can be transformed into Schedule 6, a serial schedule where T2
follows T1, by a series of swaps of non-conflicting instructions. Therefore
Schedule 3 is conflict serializable.
Schedule 3 Schedule 6
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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 Sudarshan12.22Database System Concepts - 7th 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,
1. 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.
2. 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.
3. 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.
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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 that is view-serializable but not conflict serializable.
▪ What serial schedule is the above equivalent to?
▪ Every view serializable schedule that is not conflict serializable has blind
writes.
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Other Notions of Serializability
▪ The schedule below produces the 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.
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Testing for Serializability
▪ Consider some schedule of a set of transactions T1, T2, ..., Tn
▪ Precedence graph — a direct graph where the vertices are the
transactions (names).
▪ We draw an arc from Ti to Tj if the two transaction 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 of a precedence graph
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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 the 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:
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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, the existence of an efficient algorithm is extremely unlikely.
▪ However practical algorithms that just check some sufficient conditions
for view serializability can still be used.
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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 T8 should abort, T9 would have read (and possibly shown to the user) an
inconsistent database state. Hence, the database must ensure that
schedules are recoverable.
Need to address the effect of transaction failures on concurrently
running transactions.
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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
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Cascadeless Schedules
▪ Cascadeless schedules — cascading rollbacks cannot occur;
• 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
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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 cascade-less
▪ 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/cascade-less?
▪ Testing a schedule for serializability after it has been executed is a little too
late!
▪ Goal – to develop concurrency control protocols that will assure
serializability.
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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
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Levels of Consistency in SQL-92
▪ Serializable — default
▪ Repeatable read — only committed records to be read.
• Repeated reads of the same record must return the 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.
• Successive reads of a record may return different (but committed)
values.
▪ Read uncommitted — even uncommitted records may be read.
©Silberschatz, Korth and Sudarshan12.34Database System Concepts - 7th Edition
Transaction Definition in SQL
▪ In SQL, a transaction begins implicitly.
▪ A transaction in SQL ends by:
• Commit work commits the current transaction and begins a new one.
• Rollback work causes the 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 by a database directive
▪ E.g., in JDBC – connection.setAutoCommit(false);
▪ Isolation level can be set at the database level
▪ Isolation level can be changed at the start of transaction
▪ E.g. In SQL set transaction isolation level serializable
▪ E.g. in JDBC -- connection.setTransactionIsolation(
Connection.TRANSACTION_SERIALIZABLE)
©Silberschatz, Korth and Sudarshan12.35Database System Concepts - 7th Edition
End of Chapter 12