Coordination ofDistributed Transactions.ppt

8/9/2019 Coordination ofDistributed Transactions.ppt

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Coordination of

Distributed Transactions

Distributed Concurrency Control

19 Distributed Transaction eco!ery


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Distributed Transaction Recovery

2 Basic Two-Phase Commit Algorithm (2PC) 2"1 2#C Durin$ %ormal &'eration 2"2 estart and Termination #rotocol

2"3 Inde'endent eco!ery 3 Transaction-Tree T(o-#)ase Commit * &'timi+ations5 ,essons ,earned

To marry is to halve your rights and to double your duties.(Arthur Schopenhauer)


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Fundamental Problem of Distributed Commit

Problem: Transaction o'erates on multi'le ser!ers resource managers . lobal commit needs unanimous local commits of all participants (agents) Distributed system may fail 'artially

ser!er cras)es net(or failures. and creates t)e 'otential dan$er of inconsistent decisions

Approach:

Distributed )ands)a e 'rotocolno(n as two-phase commit (2PC) (it) a coordinator ta in$ res'onsibility for unanimousoutcome eco!ery considerations for in-doubt transactions


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Idea: Getting Married over the Web

Will you ... ? Will you .. ?

Yes! Yes!

arried!


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PC During !ormal "#eration First phase (voting): coordinator sends prepare messa$es to 'artici'ants

and (aits for yes or no !otes econd phase (decision) coordinator sends commit or rollbac messa$es to 'artici'ants and (aits for ac s Participants (rite prepared lo$ entries in !otin$ ')ase

and become in"doubt (uncertain) 'otential bloc!ing dan$er breac) of local autonomy #artici'ants (rite commit or rollbac lo$ entry in decision ')ase Coordinator (rites begin lo$ entry Coordinator (rites commit or rollbac lo$ entry

and can no( $i!e return code to t)e client s commit re uest Coordinator (rites end (done# $orgotten) lo$ entry to facilitate garbage collection

*n messa$es 2n42 forced lo$ (rites 1 unforced lo$ (rite

(it) n 'artici'ants and 1 coordinator


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$tatechart for %asic PC

committed aborted

committed1 aborted1

initial

collectin$

for$otten

initial1

're'ared1

're'are1/ yes1 're'are1/ sorry1

commit1/ ac 1

abort1/ ac 1

commit1 / ac 1 abort1 / ac 1

committed2 aborted2

initial2

're'ared2 're'are2/ yes2

're'are2/ sorry2

commit2/ ac 2

abort2/ ac 2


/ 're'are1 're'are2

yes1 : yes2/ commit1commit2

sorry1 ; sorry2

/ abort1abort2

ac 1 :ac 2 ac 1

: ac 2

coordinator partici-pant "

partici-pant 2


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Restart and Termination ProtocolFailure model: 'rocess failures= transient ser!er cras)es

net(or failures= messa$e losses messa$e du'lications assum'tion t)at t)ere are no malicious commission failures >y+antine a$reement no assum'tions about net(or failure )andlin$ can use data$rams or sessions for communication

#estart protocol a$ter $ailure (F transitions): coordinator restarts in last remembered state and resends messa$es 'artici'ant restarts in last remembered state and resends messa$e or (aits for messa$e from coordinator

Termination protocol upon timeout (T transitions): coordinator resends messa$es

and may decide to abort t)e transaction in first ')ase 'artici'ant can unilaterally abort in first ')ase and

(ait for or may contact coordinator in second ')ase


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$tatechart for %asic PC &ith Restart'Termination

committed aborted

committed1 aborted1

initial

collectin$

for$otten

initial1

're'ared1

're'are1/ yes1 're'are1/ sorry1

commit1/ ac 1

abort1/ ac 1


committed2 aborted2

initial2

're'ared2 're'are2/ yes2

're'are2/ sorry2

T1;?1

T1;?1

T2;?2

T2;?2

commit2/ ac 2

abort2/ ac 2


/ 're'are1 're'are2

yes1 : yes2/ commit1commit2

sorry1 ; sorry2

/ abort1abort2

ac 1 :ac 2 ac 1

: ac 2C-'endin$ @-'endin$

T;?

T;? T;?

/ commit1commit2

/ abort1abort2

coordinator partici-pant "

partici-pant 2


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Correctness of %asic PC

Theorem " ( a$et%):2#C $uarantees t)at if one 'rocess is in a final state t)eneit)er all 'rocesses are in t)eir committed stateor all 'rocesses are in t)eir aborted state"

Proo$ methodolog%:

Consider t)e set of 'ossible com'utation 'at)sstartin$ in $lobal state initial initial """ initial.and reason about in!ariants for states on com'utation 'at)s"

Theorem 2 (&iveness):?or a finite number of failures t)e 2#C 'rotocol(ill e!entually reac) a final $lobal state(it)in a finite number of state transitions"


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Reinfection of Partici#ants

#roblem= 2#C assumes t)at re$ular (or of 'artici'ants is finis)ed ()en t)e commit 'rotocol starts Deferred o'erations t)at are tri$$ered at commit time !iolate t)is assum'tion #re'ared 'artici'ant may become rein$ected

Solution= #artici'ant t)at recei!es re uest for deferred o'eration

after )a!in$ 're'ared itself needs to become 're'ared a$ain before re'lyin$ to t)e re uestor of t)e o'eration"


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Inde#endent Recovery

Theorem ':Ander t)e single-$ailure assumption it is 'ossible to desi$n

a distributed commit 'rotocol so t)at it can$uarantee independent recover% for a failed 'rocess"

Proo$ s!etch:

@!oid $lobal states ()ic) )a!e bot) local commit and abortstates in t)eir set of 'ossible $lobal successor states"Coordinator adds (illin$-to-commit state in bet(een collectin$and commit states and ma es T transition from collectin$ to abort"#artici'ant ma es ? transition from 're'ared to abort"

ndependent recovery* ability of a failed and restarted 'rocessto terminate )is 'art of t)e 'rotocol (it)out communicatin$to ot)er 'rocesses"


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2 >asic T(o-#)ase Commit @l$orit)m ' Transaction-Tree Two-Phase Commit * &'timi+ations

5 ,essons ,earned


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Transaction(Tree )ierarchical PC

Durin$ transaction eBecution t)e transaction formsa process tree rooted at transaction initiator (it)

bilateral communication lin s accordin$ to re uest-re'ly interactions

@t commit time t)e 'rocess tree can be $lattened 'ro!ided t)at all 'artici'ants are no(n to t)e coordinator

and t)at establis)in$ ne( communication lin s is ineB'ensi!e re-used as it (as usin$ a hierarchical form of 2#C (it) intermediate nodes bein$ bot) 'artici'ants and sub-coordinators restructured to c)oose a coordinator different from t)e initiator

considerin$ reliability and s'eed of 'rocesses and t)e communication to'olo$y and 'rotocol coordinator trans$er


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Flattened vs* )ierarchical Commit Tree

?lattened 2#C=

communication durin$transaction eBecution

communication durin$commit 'rotocol

initiator 'rocess 0.

'rocess 3 'rocess 1

'rocess 2 'rocess * 'rocess 5

initiator 'rocess 0.

'rocess 3 'rocess 1

'rocess 2 'rocess * 'rocess 5

ierarc)ical 2#C=


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Illustration of )ierarchical PC nitiator &rocess '

're'are

yes

&rocess &rocess + &rocess , &rocess -

're'are 're'are

're'are 're'are

yes yesyesyes

commit

ac

commit

commitcommit commit

ac ac ac ac


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2 >asic T(o-#)ase Commit @l$orit)m 3 Transaction-Tree T(o-#)ase Commit ptimi*ations

*"1 #resumed-@bort and #resumed-Commit *"2 ead-only Subtrees *"3 Coordinator Transfer *"* educed >loc in$

5 ,essons ,earned


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PC %ottlenec+s and "#timi,ations

oals= reduce t)e number of messa$es and forced lo$ (rites for )i$)er t)rou$)'ut s)orten t)e critical 'at) until local loc s can be released for faster res'onse time reduce t)e 'robability of bloc in$

#ossible o'timi+ations= fe(er messa$es and forced lo$ (rites by 'resum'tion in t)e case of missin$ information eliminatin$ read-only subtrees as early as 'ossible dynamic. coordinator transfer 'rotocols (it) reduced bloc in$ at eB'ense of more messa$es.


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PC &ith Presum#tion

Idea= 'artici'ants donEt need to force commit/abort lo$ entry can in uire coordinator if in doubt if coordinator )as already for$otten t)e transaction

it re'lies to t)e in uiry (it) a presumed result coordinator may e!en be able to for$et transaction(it)out ac s from 'artici'ants

#roblem= need to ensure t)at t)e coordinatorEs 'resumed result is consistent amon$ t(o cases of missin$ information= a. transaction )as not yet entered t)e decision ')ase early loser. b. transaction )as been decided and $arba$e collected for$otten trans".

or needs to be able to discriminate t)e t(o cases

bservation* $orcing the begin log entry is usually unnecessary/losing it is never a correctness problem


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Illustration of Presumed(-bort Protocol &rocess '

're'are

%oordinator &rocess &rocess +

force-(rite 're'ared

're'are

're'areforce-(rite

're'aredyes

yes


no

abort abort

(riterollbac

(riterollbac

abort

(riterollbac

Case 1= transaction abort

&rocess '

're'are



're'are

're'areforce-(rite

're'aredyes

yes


yes

commit commit

$orce -(ritecommit

$orce -(ritecommit

commit

$orce -(ritecommit

Case 2= transaction commit

$orce-wri tecommit

ac! ac! ac!

writeend


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Illustration of Presumed(Commit Protocol

Case 1= transaction abort Case 2= transaction commit

&rocess '

're'are



're'are

're'areforce-(rite

're'aredyes

yes


no

abort abort

force-(riterollbac force-(riterollbac abort

force-(riterollbac

force-(rite be$in

forc e-(rite be$in

force-(riterollbac

ac ac ac

(rite

end

(rite

end

&rocess '

're'are



're'are

're'areforce-(rite

're'aredyes

yes


yes

commit commit

force-.(ritecommit(ritecommit commit

(ritecommit

force-(rite be$in

forc e-(rite be$in

force-(ritecommit


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Further "#timi,ations and ./tensions

#resumed-commit (it) non-forced be$in lo$ entry=

identify list of transaction ids of 'otential loser transactions inter!al of ids minus transactions (it) commit lo$ entry. by means of non-forced 'eriodic lo$ entries can no( discriminate for$otten (inners from early losers but may be unable to $arba$e-collect some lo$ entries after failures

Co-eBistence of #@ and #C 'artici'ants in )etero$eneous federation'resumed-any 'rotocol.= coordinator force-(rites lo$ entries so t)at it can co'e (it)

bot) #@ and #C 'artici'ants coordinator needs to no( ty'e of 'artici'ant to be included in be$in lo$ entry. and sends and eB'ects messa$es accordin$ to

'artici'ant ty'e


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Read(only $ubtree "#timi,ation

ead-only leaf-node 'artici'ant !otes 6read-only7

and releases all its loc s ead-only intermediate node !otes 6read-only7 ()en it )as recei!ed all !otes and none of t)em is 6yes7 or 6no7 Coordinator eliminates read-only subtrees from decision ')ase

ead-only o'timi+ation more effecti!e for #@ t)an for #C because #C still needs a forced be$in lo$ entry"#@ can terminate read-only transaction (it) +ero lo$$in$ cost"

%aveat (it) re$ard to reinfected 'artici'ants=If t)ere is a non-+ero 'robability of some 'artici'ant )a!in$to 'erform deferred o'erations none of t)e read-only 'artici'antmust release its loc s"&t)er(ise t)e resultin$ )istory could be $lobally non-seriali+able"


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Initiator( vs* Coordinator(Rooted Process Tree

@

> C

F ?D

Durin$ transaction eBecution= Durin$ commit 'rotocol=

C

F ?@

>

D

initiator coordinator


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Illustration of 0ast(-gent "#timi,ation

&rocess '( nitiator)

&rocess &rocess +(%oordinator)


6yes / 're'are / coordinator transfer7


6yes / 're'are / coordinator transfer7

force-(rite commit6commit7

force-(rite commit6commit7

force-(rite commit6ac 7

6ac 7

(rite end


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ll f


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Illustration of Dynamic PC

@

> C

F ?D

1= @ 're'are > @ 're'are C2= > 're'are D > 're'are F3= C 're'are ? C 're'are *= D yes > F yes >

> 're'ared5= > yes @

@ 're'ared= @ yes C8= ? yes C yes C

C becomes coordinator

1 1

2 2 3 3

* *

5

8 8

P l &i h R d d %l i


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Protocols &ith Reduced %loc+ing T)ree-')ase commit 'rotocol 3#C.= a!oid $lobal state (it) bot) local abort and local commit

in t)e set of 'ossible $lobal successor states usin$ t)ree ')ases= !otin$ dissemination decision no bloc in$ under sin$le-failure assum'tion and still correct in arbitrary failure situations

Coo'erati!e termination= in-doubt 'artici'ants contact ot)er 'artici'ants rat)er t)an (aitin$ for coordinator needs 'artici'ant list included in 're'are messa$e.

euristic commit or abort= in-doubt 'artici'ants on critical ser!ers

ma e unilateral decision u'on eBtra-lon$ timeout based on ty'e of transaction e"$" abort for (it)dra(al.


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2 >asic T(o-#)ase Commit @l$orit)m 3 Transaction-Tree T(o-#)ase Commit * &'timi+ations

+ &essons &earned


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0essons 0earned 2#C is a fundamental !ersatile and standardi+ed buildin$ bloc

for transactional federations includin$ Internet a''lications

Susce'tibility to bloc in$ is in)erent and t)e 'rice for consistency Scalability is limited to fe( 'artici'ants async)ronously ueued transactions are an alternati!e 'ro!ide atomicity and t)us need 2#C but sacrifice isolation

&'timi+ations aim to reduce forced lo$$in$ and messa$es #@ and #C. s)orten time until loc release read-only !otes last-a$ent o'timi+ation dynamic 2#C.

reduce bloc in$ coo'erati!e termination 3#C. Coordinator transfer is crucial to a!oid de'endency on

'oorly administered initiator or unreliable ser!era''lication ser!er is often an a''ro'riate coordinator.


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- 0oo+ %ac+

&art * %oncurrency %ontrol 3 %otions of Correctness #G* CC @l$orit)ms5 Gulti!ersion CC %otions of Correctness &G

8 CC @l$orit)ms on &bHects< CC on elational D>9 CC on Searc) Structures10 Im'l" : #ra$matic Issues

&art * 0ecovery11 Transaction eco!ery12 Cras) eco!ery Correctness13 C @l$orit)ms #G1* C @l$orit)ms &G

15 S'ecial Issues of eco!ery1 Gedia eco!ery18 @''lication eco!ery

&art 1* %oordination o$ 2istributed Transactions1< Distributed CC19 Distributed Transaction eco!ery


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Ta+e(-&ay Messages

Transactions are an im'ortant (ay of 'ro!idin$ eBecution $uarantees

T)eir basic abstraction mec)anisms allo(for a )ost of a''lications to be ca'tured

T)eir fundamental al$orit)mic a''roac)es

center around a fe( conce'ts Transactional $uarantees )a!e eB'anded(ay beyond t)e database area


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"utloo+

Transactions at least atomicity. on mobile 'latforms e"$" follo(in$ an auction from a car in motion s)o''in$ o!er t)e cell ')one ()ile in a train

Transactions in!ol!in$ 'rocess eBecutions and(eb ser!ices e"$"

JS-Coordination includin$ a$reement coordination asin 2#C and business acti!ity coordination JS-Transaction and Ka!a Transaction @#I KT@. &@SIS >usiness Transactions #rotocol

Coordination ofDistributed Transactions.ppt

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