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Transaction Concept A transactionis a unitof program

execution that accesses and possiblyupdates various data items.

E.g. transaction to transfer $50 fromaccount A to account B:1. read(A)

2. A := A 503. write(A)4. read(B)5. B := B + 506. write(B)

Two main issues to deal with:

Failures of various kinds, such as hardwarefailures and system crashes

Concurrent execution of multipletransactions

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ACID Properties

Atomicity. Either all operations of the transaction areproperly reflected in the database or none are.

Consistency. Execution of a transaction in isolationpreserves the consistency of the database.

Isolation. Although multiple transactions may executeconcurrently, each transaction must be unaware of otherconcurrently executing transactions. Intermediatetransaction results must be hidden from other concurrentlyexecuted transactions. That is, for every pair of transactions Tiand Tj, it appears to Tithat

either Tj, finished execution before Ti started, or Tj startedexecution after Ti finished.

Durability. After a transaction completes successfully, thechanges it has made to the database persist, even if thereare 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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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 transactionare not reflected in the database

Consistency requirement in above example: the sum of A and B is unchanged by the execution of the transaction 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

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Example of Fund Transfer(Cont.) 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 thetransaction must persist even if there are software orhardware failures.

Isolation requirement if between steps 3 and 6, anothertransaction T2 is allowed to access the partially updateddatabase, it will see an inconsistent database (the sum A + Bwill be less than it should be). T1 T2

1. read(A)

2. A := A 503. write(A)

read(A), read(B), print(A+B)

4. read(B)

5. B := B + 50

6. write(B)

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Transaction State Active A transaction under execution. Partially committed A transaction

executing the last statement Failed -- after the discovery that normal

execution can no longer proceed.

Aborted after the transaction has beenrolled back and the database restored toits state prior to the start of thetransaction. Two options after it has beenaborted: 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.)

Ex

ecuting

Successful

Execution

Fault

Fault

Roll Back

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Implementation ofAtomicity and Durability

The recovery-management component of a databasesystem implements the support for atomicity and durability.

E.g. the shadow-database scheme:

all updates are made on a shadow copyof the database

db_pointer is made to point to the updated shadow copy

after the transaction reaches partial commit andall updated pages have been flushed to disk.

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Implementation of Atomicity andDurability (Cont.)

db_pointer always points to the currentconsistent copy of the database. In case transaction fails, old consistent copy pointed to

by db_pointer can be used, and the shadow copy canbe deleted.

The shadow-database scheme: Assumes that only one transaction is active at a time. Assumes disks do not fail Useful for text editors, but

extremely inefficient for large databases (why?)

Variant called shadow paging reduces copying of data, butis still not practical for large databases

Does not handle concurrent transactions

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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 formsof schedule equivalence give rise to the notions of:1.conflict serializability2.view serializability

Simplified view of transactions We ignore operations other than read and write

instructions

We assume that transactions may perform arbitrarycomputations on data in local buffers in between readsand writes.

Our simplified schedules consist of only read and writeinstructions.

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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 ljdont conflict. Since

the same value of Q is read by Ti and Tj

2. li = read(Q), lj = write(Q). They conflict. Ifli comes

befor e lj , then Ti does not read the value of Q that is written

by Tj in instruction lj.

3. li = write(Q), lj = read(Q). They conflict

4. li = write(Q), lj = write(Q). They conflict

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Conflict Serializability

If a scheduleS

can be transformedinto a schedule Sby a series ofswaps of non-conflictinginstructions, 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 series ofswaps of non-conflicting instructions.

Therefore Schedule 3 is conflict serializable.

Schedule 3 Schedule 6

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View Serializability Let S and Sbe two schedules with the same set of

transactions. S and Sare view equivalentif thefollowing three conditions are met, for each data item Q,

1. If in schedule S, transaction Tireads the initial value ofQ, then in

schedule Salso transaction Ti must read the initial value ofQ.

2. If in schedule S transaction Tiexecutes read(Q), and that value

was produced by transaction Tj (if any), then in schedule Salso

transaction Ti

must read the value ofQ that was produced by the

same 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 onreads and writes alone.

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View Serializability

(Cont.) A schedule S is 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 notconflict serializable.

What serial schedule is above equivalent to?

Every view serializable schedule that is not conflictserializable has blind writes.