Lecture10-TransactionalProcessing

7/23/2019 Lecture10-TransactionalProcessing

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CMPT 431

Dr. Alexandra Fedorova

Lecture X: Transactions


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2CMPT 431 A. Fedorova

Transactions

A transaction is a collection o actions lo!icall"#elon!in! to!et$er

To t$e outside %orld& a transaction 'usta((ear as a single indivisible operation


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)se o Transactions *n

Distri#uted +"ste's Correct concurrent operations

, -xa'(le: u(datin! t$e #an account

/10 ead t$e current #alance in t$e account

/20 Co'(ute t$e ne% #alance

/30 )(date t$e data#ase to record t$e ne% #alance, n concurrent access& o(erations done #" 'ulti(le t$reads 'a"

interleave

, T$is leads to incorrect o(eration

, Transactions ensure (ro(er o(eration

Masking failures

, *n a re(licated #an data#ase , t$e account is u(dated at t%o sites, ne site is u(dated& t$e ot$er one cras$es #eore t$e u(date is

co'(lete

, T$e s"ste's state is inconsistent

, Transactions ensure (ro(er recover"


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AC*D Pro(erties o Transactions

Atomicity, transaction is indivisi#le , itco'(letes entirel" or not at all& des(ite ailures

Consistency ,s"ste' rules 5invariants6 are

'aintained des(ite cras$es or concurrent access

Isolation, transactions a((ear indivisi#le toeac$ ot$er des(ite concurrent access, * 'ulti(le t$reads execute transactions t$e e7ect is

t$e sa'e as i transactions %ere executed se8uentiall"in so'e order

Durability , e7ects o co''itted transactionssurvive su#se8uent ailures


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Concurrenc" Control

*'(le'ented usin! locs 5or ot$ers"nc$roni9ation (ri'itives6

A nave a((roac$: one !lo#al loc , notransactions can (roceed si'ultaneousl". Badperformance

A #etter a((roac$: associate a loc %it$ eac$data ite' 5or !rou( o ite's6

Ac8uire locs on ite's used in transactions

Turns out how "ou ac8uire locs is ver"i'(ortant


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Concurrenc" Control

Transaction 1

loc5x6

u(date 5x6

unloc 5x6

abort

Transaction 2

loc5x6

read5x6

unloc 5x6commit

sawinconsiste

nt data!



+trict T%o;P$ase Locin!

P$ase 1: A transaction can ac8uire locs& #utcannot release locs

P$ase 2: A transaction releases locs at the

end, %$en it a#orts or co''its



Deadloc =$en %e ac8uire 'ore t$an one loc at once %e are (rone to

deadlocks

Tec$ni8ues a!ainst deadlocs:

, Prevention

, Avoidance

, Detection

Prevention:loc orderin!. Do%nside:'a" li'itconcurrenc". Locs are $eld lon!er t$an necessar"

Avoidance: i a transaction $as %aited or a loc or toolon!& a#ort t$e transaction. Do%nside: transactions are

a#orted unnecessaril" etection:%ait;or !ra($ 5=F@6 , %$o %aits or %$o'. *

t$ere is a c"cle& a#ort a transaction in a c"cle. Do%nside:constructin! =F@s is ex(ensive in distri#uted s"ste's.



Maintainin! AC*D Pro(erties

To 'aintain AC*D (ro(erties& transaction(rocessin! s"ste's 'ust i'(le'ent:, Concurrenc" control 5*solation& Consistenc"6

, ailure "ecover# $Atomicit#% urabilit#&



T"(es o Failures

Transaction abort , to resolve deadloc or asre8uested #" t$e client

Cras', loss o s"ste' 'e'or" state. Dis 5or

ot$er non;volatile stora!e6 is e(t intact

isk failure

Catastrop'ic failure , 'e'or"& dis and#acu( co(ies all disa((ear

(e will discusst'ese in detail



A#ort ecover"

Acco'(lis$ed usin! transactional log

Lo! is used to >re'e'#er? t$e state o t$es"ste' in case recover" is needed

o% lo! is used de(ends on u(date se'antics:

, *n;(lace u(dates 5ri!$t a%a"6

, Deerred u(dates 5at t$e end o transaction6



Transactions =it$ *n;Place

)(dates )(date: record an undo record 5e.!.& t$e old

value o t$e ite' #ein! u(dated6 in an undolog& and u(date t$e data#ase

ead: si'(l" read t$e data ro' t$e data#ase

Co''it: Bus$ data#ase c$an!es to dis& thendiscard undo records

A#ort: )se t$e undo records in t$e lo! to #acout t$e u(dates



Transactions %it$ Deerred

)(dates )(date: ecord a redo record 5e.!.& t$e ne%

value o t$e ite' #ein! u(dated6 in a redo lo!

ead: co'#ine t$e redo lo! and t$e data#ase

to deter'ine t$e desired data

Co''it: )(date t$e data#ase #" a((l"in! t$eredo lo! in order& Bus$ t$e lo! to dis& thenre(ort successul co''it

Here commit needs not ush thedatabase to disk !ust the log

A#ort: do not$in!



Cras$ ecover"

A cras$ 'a" leave t$e data#aseinconsistent

,T$e data#ase 'a" contain data ro'unco''itted or a#orted transactions

,T$e data#ase 'a" lac u(dates ro'co''itted transactions

Ater t$e cras$ %e %ould lie

, e'ove data ro' unco''itted or a#ortedtransactions

, e;a((l" u(dates ro' co''itted transactions



ecover" =it$ )ndo Lo!!in!

All co''itted transactions %ould $ave #eenBus$ed to dis& so no need to redo t$e'

)se undo records to re'ove data ro'

unco''itted or a#orted transactions

=$at i an u(date %as %ritten to data#asebefore t$e undo record %as %ritten to lo!

"rite#ahead log rule$A undo record must be

fushed to disk beore the corresponding update isrefected in the database



ecover" %it$ edo Lo!!in!

co''it? lo! record 'ade it

to dis %edo rule$ We must fush the commit record to disk

beore telling the client that transaction is committed



Peror'ance Considerations:

Dis Access -ac$ transaction necessaril" involves dis access

5ex(ensive6

To reduce (eror'ance costs& lo! is e(t on a

se(arate dis t$an data#ase Lo! is %ritten se8uentiall" under nor'al o(eration

+e8uential %rites are ast

T$at is %$" redo lo!!in! is #etter or (eror'ance&since "ou dont $ave to Bus$ t$e data#ase to dison co''it

Data#ase is u(dated as"nc$ronousl"& (a!es areeventuallyBus$ed to dis& so its not a (eror'ance#ottlenec


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Peror'ance Considerations: Lo!

+i9e Lo! %ill ee( on !ro%in! orever

To (revent t$is& %e use c$ec(oints

* t$e data $as #een Bus$ed to t$e data#asedis& discard corres(ondin! co''it records

For eac$ transaction& ee( a lo! se8uencenu'#er 5L+


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+u''ar"

Transactions are used or concurrent o(erationsand or 'asin! ailures

AC*D (ro(erties:

, Ato'icit", Consistenc"

, *solation

, Dura#ilit"

To 'aintain t$ese (ro(erties& data#asesi'(le'ent:, Concurrenc" control 5t%o;($ase locin!& deadloc

resolution6

, Failure recover" 5lo!!in!& redoEundo6

Lecture10-TransactionalProcessing

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