nosql and column store · why choose rdbms over nosql : 1/3 1.if new relational systems can do...

CompSci 516DatabaseSystems

Lecture20NoSQLand

ColumnStore

Instructor:Sudeepa Roy

DukeCS,Fall2018 CompSci516:DatabaseSystems 1

ReadingMaterialNOSQL:• “ScalableSQLandNoSQLDataStores”RickCattell,SIGMODRecord,December2010(Vol.39,No.4)• seewebpagehttp://cattell.net/datastores/ forupdatesandmorepointers• MongoDBmanual:https://docs.mongodb.com/manual/

ColumnStore:• D.Abadi,P.Boncz,S.Harizopoulos,S.Idreos andS.Madden.TheDesignandImplementationof

ModernColumn-OrientedDatabaseSystems.FoundationsandTrendsinDatabases,vol.5,no.3,pp.197–280,2012.

• SeeVLDB2009tutorial:http://nms.csail.mit.edu/~stavros/pubs/tutorial2009-column_stores.pdf

Optional:• “Dynamo:Amazon’sHighlyAvailableKey-valueStore”ByGiuseppeDeCandia et.al.SOSP

2007

• “Bigtable:ADistributedStorageSystemforStructuredData”FayChanget.al.OSDI2006


NoSQL


Seetheoptional/additionalslidesonMongoDBonthecoursewebsiteMaybeusefulforHW3

Sofar-- RDBMS

• RelationalDataModel• RelationalDatabaseSystems(RDBMS)• RDBMSshave– acompletepre-definedfixedschema– aSQLinterface– andACIDtransactions


Today• NoSQL:”new”databasesystems– nottypicallyRDBMS– relaxonsomerequirements,gainefficiencyandscalability

• Newsystemschoosetouse/notuseseveralconceptswelearntsofar– e.g.“System---”doesnotuselocksbutusesmulti-versionCC(MVCC)or,

– “System---”usesasynchronousreplication• therefore,itisimportanttounderstandthebasics(Lectures1-18)eveniftheyarenotusedinsomenewsystems!


Warnings!

• MaterialfromCattell’spaper(2010-11)–someinfowillbeoutdated– seewebpagehttp://cattell.net/datastores/ forupdatesandmorepointers

• WewillfocusonthebasicideasofNoSQLsystems

• Optional readingslidesattheendonMongoDB– maybeusefulforHW3– therearealsocomparisontablesintheCattell’spaperifyouareinterested


OLAPvs.OLTP

• OLTP(OnLine TransactionProcessing)– Recalltransactions!– Multipleconcurrentread-writerequests– Commercialapplications(banking,onlineshopping)– Datachangesfrequently– ACIDproperties,concurrencycontrol,recovery

• OLAP(OnLine AnalyticalProcessing)– Manyaggregate/group-byqueries– multidimensionaldata– Datamostlystatic– WillstudyOLAPCubesoon


NewSystems• WewillexamineanumberofSQLandso- called“NoSQL”systemsor“datastores”

• DesignedtoscalesimpleOLTP-styleapplicationloads– todoupdatesaswellasreads– incontrasttotraditionalDBMSsanddatawarehouses– toprovidegoodhorizontalscalability(?)forsimpleread/writedatabaseoperationsdistributedovermanyservers

• OriginallymotivatedbyWeb2.0applications– thesesystemsaredesignedtoscaletothousandsormillionsofusers


NewSystemsvs.RDMS• Whenyoustudyanewsystem,compareitwithRDBMS-sonits– datamodel– consistencymechanisms– storagemechanisms– durabilityguarantees– availability– querysupport

• Thesesystemstypicallysacrificesomeofthesedimensions– e.g.database-widetransactionconsistency,inordertoachieveothers,e.g.higheravailabilityandscalability


NoSQL

• Manyofthenewsystemsarereferredtoas“NoSQL”datastores

• NoSQLstandsfor“NotOnlySQL”or“NotRelational”– notentirelyagreedupon

• Next:sixkeyfeaturesofNoSQLsystems


NoSQL:SixKeyFeatures

1. theabilitytohorizontallyscale“simpleoperations”throughputovermanyservers

2. theabilitytoreplicateandtodistribute(partition)dataovermanyservers

3. asimplecalllevelinterfaceorprotocol(incontrasttoSQLbinding)

4. aweakerconcurrencymodelthantheACIDtransactionsofmostrelational(SQL)databasesystems

5. efficientuseofdistributedindexesandRAMfordatastorage6. theabilitytodynamicallyaddnewattributestodatarecords


ImportantExamplesofNewSystems

• Threesystemsprovideda“proofofconcept”andinspiredmanyotherdatastores

1. Memcached2. Amazon’sDynamo3. Google’sBigTable


1.Memcached:mainfeatures

• popularopensourcecache

• supportsdistributedhashing(later)

• demonstratedthatin-memoryindexes canbehighlyscalable,distributing andreplicatingobjectsovermultiplenodes


2.Dynamo:mainfeatures

• pioneeredtheideaofeventualconsistencyasawaytoachievehigheravailabilityandscalability

• datafetchedarenotguaranteedtobeup-to-date

• butupdatesareguaranteedtobepropagatedtoallnodeseventually


3.BigTable :mainfeatures

• demonstratedthatpersistentrecordstoragecouldbescaledtothousandsofnodes

• “columnfamilies”

• https://cloud.google.com/bigtable/• https://static.googleusercontent.com/media/research.google.co

m/en//archive/bigtable-osdi06.pdf


BASE(notACIDJ)

• RecallACIDforRDBMSdesiredpropertiesoftransactions:– Atomicity,Consistency,Isolation,andDurability

• NOSQLsystemstypicallydonotprovideACID

• BasicallyAvailable• Softstate• Eventuallyconsistent


ACIDvs.BASE• TheideaisthatbygivingupACIDconstraints,onecanachievemuchhigherperformanceandscalability

• Thesystemsdifferinhowmuchtheygiveup– e.g.mostofthesystemscallthemselves“eventuallyconsistent”,meaningthatupdatesareeventuallypropagatedtoallnodes

– butmanyofthemprovidemechanismsforsomedegreeofconsistency,suchasmulti-versionconcurrencycontrol(MVCC)


“CAP”Theorem

• OftenEricBrewer’sCAPtheoremcitedforNoSQL

• A systemcanhaveonlytwooutofthreeofthefollowingproperties:• Consistency– doallclientsseethesamedata?

• Availability– isthesystemalwayson?

• Partition-tolerance– evenifcommunicationisunreliable,doesthesystemfunction?

• TheNoSQLsystemsgenerallygiveupconsistency– However,thetrade-offsarecomplex


TwofociforNoSQLsystems

1. “Simple”operations

2. HorizontalScalability


1.“Simple”Operations

• Readingorwritingasmallnumberofrelatedrecordsineachoperation– e.g.keylookups– readsandwritesofonerecordorasmallnumberofrecords

• Thisisincontrasttocomplexqueries,joins,orread-mostlyaccess

• Inspiredbyweb,wheremillionsofusersmaybothreadandwritedatainsimpleoperations– e.g.searchandupdatemulti-serverdatabasesofelectronic

mail,personalprofiles,webpostings,wikis,customerrecords,onlinedatingrecords,classifiedads,andmanyotherkindsofdata


2.HorizontalScalability

• Shared-NothingHorizontalScaling

• Theabilitytodistributeboththedataandtheloadofthesesimpleoperationsovermanyservers– withnoRAMordisksharedamongtheservers

• Not“vertical”scaling– whereadatabasesystemutilizesmanycoresand/orCPUsthatshareRAManddisks

• Someofthesystemswedescribeprovidebothverticalandhorizontalscalability


2.Horizontalvs.VerticalScaling

• Effectiveuseofmultiplecores(verticalscaling)isimportant– butthenumberofcoresthatcansharememoryislimited

• horizontalscalinggenerallyislessexpensive– canusecommodityservers

• Note:horizontalandverticalpartitioningarenotrelatedtohorizontalandverticalscaling (Lecture18)– exceptthattheyarebothusefulforhorizontalscaling


WhatisdifferentinNOSQLsystems

• WhenyoustudyanewNOSQLsystem,noticehowitdiffersfromRDBMSintermsof

1. ConcurrencyControl2. DataStorageMedium3. Replication4. Transactions


ChoicesinNOSQLsystems:1.ConcurrencyControl

a) Locks– somesystemsprovideone-user-at-a-timereadorupdatelocks– MongoDBprovideslockingatafieldlevel

b) MVCCc) None– donotprovideatomicity– multipleuserscaneditinparallel– noguaranteewhichversionyouwillread

d) ACID– pre-analyzetransactionstoavoidconflicts– nodeadlocksandnowaitsonlocks


ChoicesinNOSQLsystems:2.DataStorageMedium

a) StorageinRAM– snapshotsorreplicationtodisk– poorperformancewhenoverflowsRAM

b) Diskstorage– cachinginRAM


ChoicesinNOSQLsystems:3.Replication

• whethermirrorcopiesarealwaysinsynca) Synchronousb) Asynchronous– faster,butupdatesmaybelostinacrash

c) Both– localcopiessynchronously,remotecopies

asynchronously


ChoicesinNOSQLsystems:4.TransactionMechanisms

a) supportb) donotsupportc) inbetween– supportlocaltransactionsonlywithinasingle

objector“shard”– shard=ahorizontalpartitionofdataina

database


ComparisonfromCattell’spaper(2011)


DataModelTerminologyforNoSQL

• UnlikeSQL/RDBMS,theterminologyforNoSQLisofteninconsistent– wearefollowingnotationsinCattell’spaper

• Allsystemsprovideawaytostorescalarvalues– e.g.numbersandstrings

• Someofthemalsoprovideawaytostoremorecomplexnestedorreferencevalues


DataModelTerminologyforNoSQL

• Thesystemsallstoresetsofattribute-valuepairs– butusefourdifferentdatastructures

1. Tuple2. Document3. ExtensibleRecord4. Object


1.Tuple

• Sameasbefore• A“tuple”isarowinarelationaltable– attributenamesarepre-definedinaschema– thevaluesmustbescalar– thevaluesarereferencedbyattributename– incontrasttoanarrayorlist,wheretheyarereferencedbyordinalposition


2.Document

• Allowsvaluestobenesteddocumentsorlistsaswellasscalarvalues– thinkaboutXMLorJSON

• Theattributenamesaredynamicallydefinedforeachdocumentatruntime

• Adocumentdiffersfromatupleinthattheattributesarenotdefinedinaglobalschema– anda widerrangeofvaluesarepermitted


3.ExtensibleRecord

• A hybrid betweenatupleandadocument• familiesofattributesaredefinedinaschema• butnewattributescanbeadded(withinanattributefamily)onaper-recordbasis

• Attributesmaybelist-valued


4.Object

• Analogoustoanobjectinprogramminglanguages– butwithouttheproceduralmethods

• Valuesmaybereferencesornestedobjects


ExampleNOSQLsystems

• Key-valueStores:– ProjectVoldemort,Riak,Redis,Scalaris,TokyoCabinet,Memcached/Membrain/Membase

• DocumentStores:– AmazonSimpleDB,CouchDB,MongoDB,Terrastore

• ExtensibleRecordStores:– Hbase,HyperTable,Cassandra,Yahoo’sPNUTS

• RelationalDatabases:– MySQLCluster,VoltDB,Clustrix,ScaleDB,ScaleBase,NimbusDB,GoogleMegastore(alayeronBigTable)


SQLvs.NOSQL


Argumentsforbothsidesstillacontroversialtopic

WhychooseRDBMSoverNoSQL:1/31. Ifnewrelationalsystemscandoeverything

thataNoSQLsystemcan,withanalogousperformanceandscalability(?),andwiththeconvenienceoftransactionsandSQL,NoSQLisnotneeded

2. RelationalDBMSshavetakenandretainedmajoritymarketshareoverothercompetitorsinthepast30years– (network,object,andXMLDBMSs)


WhychooseRDBMSoverNoSQL:2/33. SuccessfulrelationalDBMSshavebeenbuilt

tohandleotherspecificapplicationloads inthepast:– read-onlyorread-mostlydatawarehousing– OLTPonmulti-coremulti-diskCPUs– in-memorydatabases– distributeddatabases,and– nowhorizontallyscaleddatabases


WhychooseRDBMSoverNoSQL:3/3

4. Whileno“onesizefitsall”intheSQLproductsthemselves,thereisacommoninterfacewithSQL,transactions,andrelationalschemathatgiveadvantagesintraining,continuity,anddatainterchange


WhychooseNoSQLoverRDBMS:1/31. Wehaven’tyetseengoodbenchmarksshowing

thatRDBMSscanachievescaling comparablewithNoSQLsystemslikeGoogle’sBigTable

2. Ifyouonlyrequirealookupofobjectsbasedonasinglekey– thenakey-valuestoreisadequateandprobablyeasiertounderstand

thanarelationalDBMS– Likewiseforadocumentstoreonasimpleapplication:youonlypay

thelearningcurveforthelevelofcomplexityyourequire


WhychooseNoSQLoverRDBMS:2/3

3. Someapplicationsrequireaflexibleschema– allowingeachobjectinacollectiontohavedifferentattributes

– WhilesomeRDBMSsallowefficient“packing”oftupleswithmissingattributes,andsomeallowaddingnewattributesatruntime,thisisuncommon


WhychooseNoSQLoverRDBMS:3/3

4. ArelationalDBMSmakes“expensive”(multi- nodemulti-table)operations“tooeasy”– NoSQLsystemsmakethemimpossibleorobviouslyexpensiveforprogrammers

5. WhileRDBMSshavemaintainedmajoritymarketshareovertheyears,otherproductshaveestablishedsmallerbutnon-trivialmarketsinareaswherethereisaneedforparticularcapabilities– e.g.indexedobjectswithproductslikeBerkeleyDB,orgraph-following

operationswithobject-orientedDBMSs


ColumnStore


Rowvs.ColumnStore

• Rowstore– storeallattributesofatupletogether– storagelike“row-majororder”inamatrix

• Columnstore– storeallrowsforanattribute(column)together– storagelike“column-majororder”inamatrix

• e.g.– MonetDB,Vertica(earlier,C-store),SAP/SybaseIQ,GoogleBigtable (withcolumngroups)



Ack:SlidefromVLDB2009tutorialonColumnstore

nosql and column store · why choose rdbms over nosql : 1/3 1.if new relational systems can do...

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