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Silberschatz, Korth and Sudarshan14.2Database System Concepts - 6thEdition
Chapter 14: Transactions
Transaction Concept
Transaction State
Concurrent Executions
Serializability
Recoverability
Implementation of Isolation Transaction Definition in SQL
Testing for Serializability.
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Transaction Concept
A transactionis 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 systemcrashes
Concurrent execution of multiple transactions
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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 lostleading 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 Durabili ty 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 thedatabase by the transaction must persist even if there are software orhardware failures.
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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, itwill see an inconsistent database (the sum A + Bwill be less than itshould 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 significantbenefits, as we will see later.
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ACID Properties
Atomicity. Either all operations of the transaction are properly reflectedin 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 executingtransactions. Intermediate transaction results must be hidden from otherconcurrently executed transactions.
That is, for every pair of transactions Tiand Tj, it appears to Tithateither Tj, finished execution before Tistarted, or Tjstarted executionafter Tifinished.
Durability. After a transaction completes successfully, the changes ithas 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 databasesystem must ensure:
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Transaction State
Activethe initial state; the transaction stays in this state while it is
executing Partially committedafter the final statement has been executed.
Failed-- after the discovery that normal execution can no longerproceed.
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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Concurrent Executions
Multiple transactions are allowed to run concurrently in the system.
Advantages are: increased processor and disk util ization , leading to better
transaction throughput
E.g. one transaction can be using the CPU while another isreading from or writing to the disk
reduced average response timefor transactions: shorttransactions need not wait behind long ones.
Concurrency contro l schemes mechanisms to achieve isolation
that is, to control the interaction among the concurrenttransactions in order to prevent them from destroying the
consistency of the databaseWill study in Chapter 16, after studying notion of correctness
of concurrent executions.
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Schedules
Schedule a sequences 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 eachindividual transaction.
A transaction that successfully completes its execution will have acommit instructions as the last statement
by default transaction assumed to execute commit instruction as itslast step
A transaction that fails to successfully complete its execution will have
an abort instruction as the last statement
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Schedule 1
Let T1transfer $50 fromA to B, and T2transfer 10% of thebalance fromA to B.
A serial schedule in which T1is followed by T2:
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Schedule 2
A serial schedule where T2is followed by T1
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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 isequivalent 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 readand writeinstructions
We assume that transactions may perform arbitrarycomputations on data in local buffers in between readsand writes.
Our simplified schedules consist of only readand writeinstructions.
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Conflicting Instructions
Instructions liand ljof transactions Tiand Tjrespectively, conflict
if and only if there exists some item Qaccessed by both liand lj,and at least one of these instructions wrote Q.
1. li= read(Q), lj= read(Q). liand ljdont conflict.2. li= read(Q), lj= write(Q). They conflict.3. li= write(Q), lj= read(Q). They conflict4. li= write(Q), lj= write(Q). They conflict
Intuitively, a conflict between liand ljforces a (logical) temporalorder between them.
If liand ljare consecutive in a schedule and they do notconflict, their results would remain the same even if they hadbeen interchanged in the schedule.
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Conflict Serializabil ity
If a schedule Scan be transformed into a schedule Sby a series of
swaps of non-conflicting instructions, we say that Sand Sareconf lict equivalent.
We say that a schedule Sis confl ict serializableif it is conflictequivalent to a serial schedule
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Conflict Serializability (Cont.)
Schedule 3 can be transformed into Schedule 6, a serial
schedule where T2follows T1, by series of swaps of non-conflicting instructions. Therefore Schedule 3 is conflictserializable.
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 toobtain either the serial schedule < T3, T4>, or the serialschedule < T4, T3>.
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View Serializability
Let Sand Sbe two schedules with the same set of transactions. S
and Sare view equivalentif the following three conditions are met,for each data item Q,
1. If in schedule S, transaction Tireads the initial value of Q, then inschedule Salso transaction Ti must read the initial value of Q.
2. If in schedule S transaction Tiexecutes read(Q), and that value
was produced by transaction Tj (if any), then in schedule Salsotransaction Timust read the value of Qthat was produced by thesame write(Q) operation of transaction Tj.
3. The transaction (if any) that performs the final write(Q) operationin schedule S must also perform the finalwrite(Q) operation inschedule S.
As can be seen, view equivalence is also based purely on reads andwritesalone.
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View Serializability (Cont.)
A schedule Sis view serializableif 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 conflictserializable.
What serial schedule is above equivalent to?
Every view serializable schedule that is not conflict serializable hasblind writes.
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Other Notions of Serializabil ity
The schedule below produces same outcome as the serial
schedule < T1,T5>, yet is not conflict equivalent or viewequivalent to it.
Determining such equivalence requires analysis of operationsother than read and write.
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Testing for Serializabil ity
Consider some schedule of a set of transactions T1, T2, ..., Tn
Precedence graph a directed graph where the verticesare the transactions (names).
We draw an arc from Tito Tjif the two transaction conflict,and Tiaccessed the data item on which the conflict aroseearlier.
We may label the arc by the item that was accessed. Example 1
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Test for Conflict Serializabil ity
A schedule is conflict serializable if and only
if its precedence graph is acyclic. Cycle-detection algorithms exist which take
order n2time, where n is the number ofvertices in the graph.
(Better algorithms take order n+ e
where eis the number of edges.) If precedence graph is acyclic, the
serializability order can be obtained by atopological sortingof the graph.
This is a linear order consistent with thepartial order of the graph.
For example, a serializability order forSchedule A would beT5T1T3T2T4
Are there others?
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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 theclass of NP-complete problems.
Thus existence of an efficient algorithm is extremelyunlikely. However practical algorithms that just check some sufficient
conditionsfor view serializability can still be used.
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Cascadeless Schedules
Cascadelessschedules cascading rollbacks cannot occur; for
each pair of transactions Tiand Tjsuch that Tj reads a data itempreviously written by Ti, the commit operation of Ti appears before theread 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 cascadeless
A policy in which only one transaction can execute at a time generatesserial schedules, but provides a poor degree of concurrency
Are serial schedules recoverable/cascadeless?
Testing a schedule for serializability afterit has executed is a little toolate!
Goal to develop concurrency control protocols that will assureserializability.
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Concurrency Control vs. Serializabil ity Tests
Concurrency-control protocols allow concurrent schedules, but ensurethat the schedules are conflict/view serializable, and are recoverableand cascadeless .
Concurrency control protocols generally do not examine theprecedence graph as it is being created
Instead a protocol imposes a discipline that avoids nonseralizableschedules.
We study such protocols in Chapter 16.
Different concurrency control protocols provide different tradeoffsbetween the amount of concurrency they allow and the amount of
overhead that they incur. Tests for serializability help us understand why a concurrency control
protocol is correct.
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Levels of Consistency in SQL-92
Serializable default
Repeatable readonly committed records to be read, repeatedreads of same record must return same value. However, atransaction may not be serializable it may find some recordsinserted by a transaction but not find others.
Read committedonly committed records can be read, butsuccessive reads of record may return different (but committed)values.
Read uncommittedeven 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 ofconsistency called snapshot isolation (not part of the SQL
standard)
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Transaction Definit ion 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 workcommits current transaction and begins a newone.
Rollback workcauses current transaction to abort.
In almost all database systems, by default, every SQL statementalso commits implicitly if it executes successfully
Implicit commit can be turned off by a database directive
E.g. in JDBC, connection.setAutoCommit(false);
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Fi 14 01
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Figure 14.01
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Fi 14 03
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Figure 14.03
Fi 14 04
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Figure 14.04
Fi 14 05
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Figure 14.05
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Fi 14 08
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Figure 14.08
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Figure 14 11
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Figure 14.11
Figure 14 12
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Figure 14.12
Figure 14 13
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Figure 14.13
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Figure 14 15
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Figure 14.15
Figure 14 16
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Figure 14.16
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