Ch 15 Transaction

8/7/2019 Ch 15 Transaction

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Chapter 15:

TransactionsLoc Hoang

CS 157B


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Definition Atransaction is a discrete unit of

work that must be completely

processed or not processed at all. Example: Transferring funds from

a checking account to a savingaccount.


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Desirable Properties To ensure data data integrity. The

database system must maintain

the ACID properties.

Atomicity

Consistency

Isolation

Durability


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ACID continue Atomicity. Either all operations of

the transaction are reflected

properly in the database, or noneare.

Consistency. Execution of a

transaction in isolation (that is, withno other transaction executingconcurrently) preserves theconsistency of the database.


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ACID continue... Isolation. Even though multiple

transactions may execute

concurrently, the system guaranteesthat, for every pair of transactions Ti,and Tj, it appears to Ti, that either Tjfinished execution before Ti started,

or that Tj started execution after Tifinished. Thus, each transaction isunaware of the transactionsexecuting concurrently in the

system.


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ACID continue... Durability. After a transaction

completes successfully, the

changes it has made to thedatabase persist, even if there aresystem failures.


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Transaction States Because failures occurs, transaction are

broken up into states to handle various

situation.


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Transaction States Active, the initial state; the

transaction stays in this state until

while it is still executing.

A transition is terminated only if ithas either been committed oraborted.


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Transaction States Partially committed, After the

final statement has been executed

At this point failure is still possiblesince changes may have been onlydone in main memory, a hardwarefailure could still occur.


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Transaction States The DBMS needs to write out

enough information to disk so that,

in case of a failure, the systemcould re-create the updatesperformed by the transaction oncethe system is brought back up.

After it has written out all thenecessary information, it iscommitted.


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Transaction States Committed- after successful

completion.

Once committed, the transactioncan no longer be undone byaborting it.

Its effect could be undone only bya compensating transaction.


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Transaction States Failed, after the discovery that

normal execution can no longer

proceed

Once a transaction can not becompleted, any changes that itmade must be undone rolling itback.


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Transaction States Aborted, after the transaction has

been rolled back the the database

has been restored to its state prior tothe start of the transaction.

The DBMS could eitherkill the

transaction orrestart the transaction. A transaction may only be restarted as a

result of some hardware or softwareerror, and a restarted transaction isconsidered a new transaction.


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Concurrent execution

DBMS usually allow multipletransaction to run at once.

The transaction could by runserially (one after another) or theycould be interleaved(switchingback and forth betweentransactions)


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Concurrent execution Pro:

Improved throughput and resource

utilizationCould take advantage of multi-processsing

Reduce waiting time.Avoid short transaction waiting on long transaction

Improve average response time.


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Concurrent execution

Con:

Creates many complications in

data consistency, includingcascading roll-backs.

Consistency could be compromiseeven if each individual transactionis correct.


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Schedules

DBMS needs to make sure that alltransaction schedules preserves

the consistency of the database.

A schedule is the chronologicalorder in which instructions areexecuted in a system.


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Schedule

A schedule for a set of transactionmust consist of all the instruction of

those transaction and mustpreserve the order in which theinstructions appear in eachindividual transaction.


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Schedule exampleT1

-----------------

read(A)

A:=A-50

write(A)

read(B)B:= B+ 50

write(B)

T2

-----------------

read(A)

temp: A * 0.1

A: A-temp

write (A)

read(B)

B:=B +temp

write(B)

Schedule 1.


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Serial Schedule

In schedule 1 the all the instructions ofT1 are grouped and run together. Thenall the instructions of T2 are groupedand run together. This type of schedulesare called serial.

Concurrent transactions do not have to

run serially as in the next examples.


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Interleaved ScheduleT1

-----------------

read(A)

A:=A-50

write(A)

read(B)

B:= B+ 50

write(B)

T2

-----------------

read(A)temp: A * 0.1

A: A-temp

write (A)

read(B)

B:=B +temp

write(B)

Schedule 2


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Conflict Equivalent

Schedule 1 and 2 produce the sameresult even though they have differentsequence. It could be shown thatSchedule 2 could be transform intoSchedule 1 with a sequence of swaps,so Schedule 1 and 2 conflict equivalent.

Not all schedules are conflict equivalent.Consider Schedule 3


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Inconsistent ScheduleT1

-----------------

read(A)

A:=A-50

write(A)

read(B)

B:= B+ 50

write(B)

T2

-----------------

read(A)

temp: A * 0.1

A: A-tempwrite (A)

read(B)

B:=B +temp

write(B)

Schedule 3.


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Serializability

Suppose that A and B were bankaccounts with initial amounts of $1,000and $2,000 respectively. Then afterSchedule 1 and Schedule 2 are run. Theresult is $850 forA and $2,150 for B.And the sum A+B is still $3000.

After Schedule 3 runs, account A is $950and B $2100. The sum A+B is now$3,050 which is incorrect, since $50 wasmagically created in the process.


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Conflict Serializability

Schedule 2 is consistent becauseis it conflict equivalent to Schedule

1. Since Schedule 1 is serial,Schedule two is said to be conflictserializable.

Instructions within a schedulecould be swapped if they do notconflict. Instructions do not conflictif they access different data item orif the access the same data item


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Conflict Serializability

Schedule 2 could be turn into Schedule 1with the by the following steps:

Swap write(A) of T2 with read(B) of T1

Swap read(B) of T1 with read(A) of T2

Swap write(B) of T1 with write(A) of T2

Swap write(B) of T1 with read(A) of T2

Ch 15 Transaction

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