Archiving Scientific Data

SusanB.DavidsonCIS700:AdvancedTopicsinDatabases

MW1:30-3

Towne309

http://www.cis.upenn.edu/~susan/cis700/homepage.html

• Datachangesovertime• Newdataisadded

• Mistakesarecorrected• Olddataisremoved

• Toenablereproducibilityandverifiability,itmustbepossibletoaccessthestateofadatabaseasofacertainpointintime.• Alsocrucialfordereferencingcitations

• Mayalsowanttoaskquestionsabouthowthedatabasehaschanged.

Why archive?

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• Manydatabasesperiodicallypublishnewversions• Keepcopyofeachversion

• Allowsdataasofacertaintimetobeaccessedquickly

• Maynotbespaceefficientsinceverylittlemaychangebetweenversions

• Doesn’tallowefficientqueriesoverthechangehistory

• Keepalogofchanges(“sequenceofdelta”)• Spaceefficient• Maybeexpensivetorecomputedataasofacertaintime

• Maybeexpensivetoquerychangehistory

How to archive?

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• Versioningandcitation:experienceswitheagle-i• ArchivingXMLdatasets• Conclusions

Outline

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• eagle-iisanRDFdatasetwhichcontainsinformationaboutresourcesfortranslationalresearch(e.g.software,celllines,labfacilities)

• Eachresourcehasanimmutableeagle-iid;thesubjectofeachresourcetripleisaneagle-iid

• Resourcesareclassifiedusinganontology,andthecitationdependsontheclassificationoftheresource.

• eagle-italkedaboutcitationbutdidn’tautomateit…

Our experience: eagle-i

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Citation architecture

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• Thelatestcopyofeagle-iisavailableonthewebsite,butitisnot“versioned”

• Wedidadailydownloadsincewedidn’tknowhowfrequentlyitchanged(notfrequently!)

• Needed“timequeries”tounderstandhowthedatasetchangedovertime• Whattripleswereadded/deletedintheperiod[t,t’]?

• WhatwastheobjectoftripleXattimet?

• WhenwastripleYfirstadded/deleted

eagle-i versioning manager

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Example: versioning 2 RDF triples

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• Whenshouldversioningbetriggered?• Atleastwhenausercitesaneagle-iresource

• Whatshouldbeversioned?• Atleastchangestotheresourcebeingcited.

Ø Ifaversionofaresourceisnotcited,itdoesnothavetobestored.

Ø However,time-basedquerieswillonlydetectchangeswithrespecttocitationsratherthanallchanges.

Versioning and citation

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• Versioningandcitation:experienceswitheagle-i• ArchivingXML• Conclusions

Outline

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• Keepcopyofeachnewversionofthedatabase• Allowsdataasofacertaintimetobeaccessedquickly

• Maynotbespaceefficientsinceverylittlemaychangebetweenversions

• Doesn’tallowefficientqueriesoverthechangehistory

• Keepalogofchanges(“sequenceofdelta”)• Spaceefficient• Maybeexpensivetorecomputedataasofacertaintime

• Maybeexpensivetoquerychangehistory

Recall: approaches to archiving

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• Ignoresthe“semanticcontinuityofkeys”byfocusingonminimaleditdistance

Problem with diff-based approaches

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• Focusonhierarchicalscientificdatasets• XML-based• Changesareprimarilyinsertions

• Changesidentifiedbasedonkeys• Versionmergingbasedonkeys• Inheritanceoftimestamps

• Timestampisstoredatachildelementonlywhenitisdifferentfromthetimestampofitsparentelement

Ø “Key-based+merging”approach

Proposed approach in paper

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Example: sequence of versions

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Adding keys

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Example of an archive

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Representing archive in XML

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• Akeyhasform(Q,{P1,…,Pk}),whereQ,Piarepathexpressions• Qidentifiesthetargetset

• Piarekeypaths,analogoustokeyattributesinrelations

• AnXMLdocumentsatisfiesakey(Q,{P1,…,Pk})if• FromanynodeidentifiedbyQ,everyPiexistsuniquely• Iftwonodesn1andn2identifiedbyQhavethesamevalueattheendofeachkeypathin{P1,…,Pk}thenn1andn2arethesamenode.

What is a key for XML?

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• SinceXMLishierarchical,wealsoneedtospecifykeysrelativetoacontextnode• (Q,(Q’,{P1,…,Pk}))

• Examples• (/,(db,{})).Thereisatmostonedbelementbelowtheroot.

• (/db,(dept,{name})).Everydeptnodewithinadbnodecanbeuniquelyidentifiedbythecontentsofitsnamesubelement.

• (/db/dept,(emp,{fn,ln})).Everyempnodewithinadeptnodealongthepath/db/deptcanbeuniquelyidentifiedbythecontentsofitsfnandlnsubelements.

• (/db/dept/emp,(sal,{})).Thereisatmostonesalsubelementundereachempnodealongthepath/db/dept/emp.

Relative keys

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• Assumptions:• Everykeydefinedforanodeisrelativetoitsparent,e.g.thekeyforempisrelativetoitsparentdeptnode

• Frontiernodesidentifyunkeyedportionsofthedocument

Archiver architecture

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• Recursivelymergenodesintheincomingversion(D)tonodesinthearchive(A)thathavethesamekeyvalue,startingfromtheroot.

• WhenanodeyinDismergedwithanodexfromA,thetimestampofxisaugmentedwithi(thenewversionnumber),andsubtreesarerecursivelymerged.

• NodesinDthatdonothavenodesinAaresimplyaddedwithiasthetimestamp

Nested merge

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Further compaction under frontier node

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• Whatisthedatabaseatt=1?

• WhendidJoeDoegetasalaryraise?

• Whatwerethechangestothedatabasebetweent=1andt=3?

Querying the archive

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• Versioningisimportantformanydifferentapplications

• Whiletechniquesaresimilarbetweendifferentrepresentations(e.g.files,relations,XML,RDF),differencesinassumptionscanbeusedtobuildmoreefficientsolutions.• Andtheoperations(e.g.queries)youwishtoperformareimportanttoo!

Conclusions

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