Introduction to Database Systems CSE 444 Lecture 26: Distributed TransactionsAnnouncements Wrap%u p'l ecture'on'Friday' Short'review'+'example'problems'on'the'board' Project'4'due'this'Friday' Don’t'forget'to'terminate'your'jobs!!!' Course'evaluaDons'at'the'end'of'this'lecture' Today:'Distributed'transacDons' Because'you'loved'transacDons'so'much'the'first'Dme'2Partitioned data TID$eid$name$city$age$salary$t1'53666'Jones'Madras'28'35k't2'53688'Smith'Chicago'38'32k't3'53650'Smith'Chicago'29'48k't4'53831'Madayan'Bombay'41'20k't5'53832'Guldu'Bombay'32'20k'eid$name$city$age$salary$53666'Jones'Madras'28'35k'53688'Smith'Chicago'38'32k'53650'Smith'Chicago'29'48k'53831'Madayan'Bombay'41'20k'53832'Guldu'Bombay'32'20k'Horizontal'Fragment'VerDcal'Fragment'Employee'3Distributed Data UPDATE' 'Employee'SET ' 'salary=salary+2k'WHERE 'age>30'4Distributed Catalog 5 How'do'we'idenDfy'a'relaDon?' Naming'issues:'' local%name%+%birth%site%=%global&rela)on&name& +replica_id%=%global&replica&name& Centralized'catalog' Vulnerable'to'single%site'failure' Compromizes'site'autonomy' R*'approach:' Local'catalog'describing'all'local'relaDons' Birth'site'also'keeps'track'of'replicas'and'fragments' Could'be'cached'at'other'sites'Remember Transactions? 6 ACID' Distributed'Concurrency'Control' How'can'locks'for'objects'be'managed?' How'can'deadlocks'be'detected?' Distributed'Recovery' Atomicity'and'Durability'need'to'be'enforced'across'sites' In'a'distributed'secng,'a'Xact'spawns'subtransacDons'Distributed Lock management 7 Centralized' one'site'deals'with'lock'and'unlock'requests' Primary'Copy' One'copy'of'an'object'is'designated'as'primary,'and'requests'are'handled'at'that'site' Fully'Distributed' Manage'requests'locally'Deadlock detection 8 Local'and'global'waits%for'graphs' 3'algorithms:'1. Construct'global'waits%for'graph'periodically'at'a'centralized'site'2. Construct'waits%for'graphs'hierarchically'3. Abort'long'waiDng'transacDons' Phantom'Deadlocks!'T1' T2'At'site'A'T1' T2'At'site'B'T1' T2'Global'waits%for'graph'Distributed Recovery 9 Either'all'subtransacDons'must'commit'or'none'of'them' Regular'logging'+'commit'pro tocol' The'transacDo n'manager'at'the'originaDng'site'is'the'coordinator' The'transacDo n'managers'at'the'subtransacDons’'sites'are'the'subordinates'2 Phase Commit: Motivation 10 subordinate'1'subordinate'2'subordinate'3'1.'User'decides'to'commit' 2.'commit'3.'commit'4.'coordinator'crash!'But'I'already'aborted''coor dinator'2 Phase Commit 11 Use'2'phases:'a'voDng'phase'and'a'terminaDon'phase' Principle:' When'a'process'makes'a'decision,'it'votes'yes/no'or'commit/abort' A'subordinate'acknowledges'messages'(acks)' Force%write'log'record'before'sending' Log'records'include'Xact'and'coordinator'ids' Coordinator'logs'ids'of'subordinates'2 Phase Commit: Phase 1 12 coor dinator'subordinate'1'subordinate'2'subordinate'3'1.'User'decides'to'commit' 2.'prepare'2.'prepare'2.'prepare'3.'force%write'prepare'3.'force%write'prepare'3.'force%write'prepare'4.'yes'4.'yes'4.'yes'2 Phase Commit: Phase 2 13 coor dinator'subordinate'1'subordinate'2'subordinate'3'1.'Force%write'commit' 2.'commit'2.'commit'2.'commit'3.'force%write'commit'3.'force%write'commit'3.'force%write'commit'4.'ack'4.'ack'4.'ack'5.'Commit'Xact'and'''forget'it'5.'Commit'Xact'and'''forget'it'5.'Commit'Xact'and'''forget'it'5.'Write'end'then'forget'Xact'Xact'is'now'commiled'2 Phase Commit: Phase 1 with abort 14 coor dinator'subordinate'1'subordinate'2'subordinate'3'1.'User'decides'to'commit' 2.'prepare'2.'prepare'2.'prepare'3.'force%write'prepare'3.'force%write'abort'3.'force%write'abort'4.'yes'4.'no'4.'no'5.'Abort'Xact'and'''forget'it'5.'Abort'Xact'and'''forget'it'2 Phase Commit: Phase 2 15 coor dinator'subordinate'1'subordinate'2'subordinate'3'1.'Force%write'abort' 2.'abort'3.'force%write'abort'4.'ack'5.'Abort'Xact'and'''forget'it'5.'Write'end'then'forget'Xact'Restart after failure 16 How'do'we'know'if'we'are'coordinator'or'subordinate,'and'what'do'we'do?' We'see'a'commit'or'abort'record' We'are'coordinator:'send'to'subordinates'unDl'we'get'an'ack' We'see'a'prepare'record' We'are'subordinate:'contact'coordinator'to'determine'status' We'see'no'prepare,'commit'or'abort' We'can'unilaterally'abort'Any'issues?'Refinement: 2PC with presumed abort 17 ObservaDons:' Coordinator'waits'for'acks'to'‘forget’'Xact' no'info rmaDon'='abo rt' A'reader'does'not'care'for'commit/abort'outcome' Refinements:' If'abort'is'decided,'remove'Xact'from'Xact'table'immediately' If'I'get'an'abort'msg ,' no'need'to'ack' The'abort'log'record'of'the'coordinator'does'not'need'the'subordinate'list' Abort'records'don’t'need'to'be'force%wrilen' A'reader'Xact'votes'reader'instead'of'yes/no' Coordinator'does'not'need'to'communicate'further'with'readers'
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