8/2/2019 Lj Article

1/6

TheSubversionProject:BuildingaBetterCVS==============================================

BenCollins-SussmanWritteninAugust2001PublishedinLinuxJournal,January2002

Abstract--------

Thisarticlediscussesthehistory,goals,featuresanddesignofSubversion(http://subversion.tigris.org),anopen-sourceprojectthataimstoproduceacompellingreplacementforCVS.

Introduction------------

Ifyouwor onany indofopen-sourceproject,you'veprobablywor edwithCVS.Youprobablyrememberthefirsttimeyoulearnedtodoananonymouschec outofasourcetreeoverthenet--oryourfirstcommit,orlearninghowtoloo atCVSdiffs.Andthenthefateful

daycame:youas

edyourfriendhowtorenameafile.

"Youcan't",wasthereply.

What?Whatdoyoumean?

"Well,youcandeletethefilefromtherepositoryandthenre-additunderanewname."

Yes,butthennobodywould nowithadbeenrenamed...

"Let'scalltheCVSadministrator.Shecanhand-edittherepository'sRCSfilesforusandpossiblyma ethingswor ."

What?

"Andbytheway,don'ttrytodeleteadirectoryeither."

Yourolledyoureyesandgroaned.Howcouldsuchsimpletas sbedifficult?

TheLegacyofCVS-----------------

Nodoubtaboutit,CVShasevolvedintothestandardSoftware

ConfigurationManagement(SCM)systemoftheopensourcecommunity.Andrightlyso!CVSitselfisFreesoftware,anditswonderful"nonloc

ing"developmentmodel--wherebydozensoffar-flungprogrammerscollaborate--fitstheopen-sourceworldverywell.Infact,onemightarguethatwithoutCVS,it'sdoubtfulwhethersitesli eFreshmeatorSourceforgewouldeverhaveflourishedastheydonow.CVSanditssemi-chaoticdevelopmentmodelhavebecomeanessentialpartofopensourceculture.

Sowhat'swrongwithCVS?

8/2/2019 Lj Article

2/6

BecauseitusestheRCSstorage-systemunderthehood,CVScanonlytrac filecontents,nottreestructures.Asaresult,theuserhasnowaytocopy,move,orrenameitemswithoutlosinghistory.Treerearrangementsarealwaysuglyserver-sidetwea s.

TheRCSbac -endcannotstorebinaryfilesefficiently,andbranchingandtaggingoperationscangrowtobeveryslow.CVSalsousesthenetwor inefficiently;manyusersareannoyedbylongwaits,becausefiledifferecesaresentinonlyonedirection(fromservertoclient,butnotfromclienttoserver),andbinaryfilesarealwaystransmittedintheirentirety.

Fromadeveloper'sstandpoint,theCVScodebaseistheresultoflayersuponlayersofhistorical"hac s".(RememberthatCVSbeganlifeasacollectionofshell-scriptstodriveRCS.)Thisma

esthecodedifficulttounderstand,maintain,orextend.Forexample:CVS'snetwor ingabilitywasessentially"stapledon".Itwasneverdesignedtobeanativeclient-serversystem.

RectifyingCVS'sproblemsisahugetas --andwe'veonlylistedjustafewofthemanycommoncomplaintshere.

EnterSubversion----------------

In1995,KarlFogelandJimBlandyfoundedCyclicSoftware,acompanyforcommerciallysupportingandimprovingCVS.Cyclicmadethefirstpublicreleaseofanetwor -enabledCVS(contributedbyCygnussoftware.)In1999,KarlFogelpublishedaboo aboutCVSandtheopen-sourcedevelopmentmodelitenables(cvsboo .red-bean.com).KarlandJimhadlongtal edaboutwritingareplacementforCVS;Jimhadevendraftedanew,theoreticalrepositorydesign.Finally,inFebruaryof2000,BrianBehlendorfofCollabNet(www.collab.net)offeredKarlafull-timejobtowriteaCVSreplacement.Karlgatheredateamtogetherandwor beganinMay.

Theteamsettledonafewsimplegoals:itwasdecidedthatSubversionwouldbedesignedasafunctionalreplacementforCVS.ItwoulddoeverythingthatCVSdoes--preservingthesamedevelopmentmodelwhilefixingtheflawsinCVS's(lac -of)design.ExistingCVSuserswouldbethetargetaudience:anyCVSusershouldbeabletostartusingSubversionwithlittleeffort.AnyotherSCM"bonusfeatures"weredecidedtobeofsecondaryimportance(atleastbeforea1.0release.)

Atthetimeofwriting,theoriginalteamhasbeencodingforalittleoverayear,andwehaveanumberofexcellentvolunteercontributors.(Subversion,li eCVS,isaopen-sourceproject!)

Subversion'sFeatures----------------------

Here'saquic run-downofsomeofthereasonsyoushouldbeexcitedaboutSubversion:

*Realcopiesandrenames.TheSubversionrepositorydoesn'tuseRCSfilesatall;instead,itimplementsa'virtual'versioned

8/2/2019 Lj Article

3/6

filesystemthattrac stree-structuresovertime(describedbelow).Files*and*directoriesareversioned.Atlast,therearerealclient-side`mv'and`cp'commandsthatbehavejustasyouthin .

*Atomiccommits.Acommiteithergoesintotherepositorycompletely,ornotall.

*Advancednetwor layer.TheSubversionnetwor serverisApache,andclientandserverspea

WebDAV(2)tooneanother.(Seethe'design'sectionbelow.)

*Fasternetwor access.Abinarydiffingalgorithmisusedtostoreandtransmitdeltasin*both*directions,regardlessofwhetherafileisoftextorbinarytype.

*Filesystem"properties".Eachfileordirectoryhasaninvisiblehashtableattached.Youcaninventandstoreanyarbitrary ey/valuepairsyouwish:owner,perms,icons,app-creator,mime-type,personalnotes,etc.Thisisageneral-purposefeatureforusers.Propertiesareversioned,justli efilecontents.Andsomepropertiesareauto-detected,li ethemime-typeofafile(nomorerememberingtousethe'- b'switch!)

*Extensibleandhac

able.Subversionhasnohistoricalbaggage;itwasdesignedandthenimplementedasacollectionofsharedClibrarieswithwell-definedAPIs.Thisma esSubversionextremelymaintainableandusablebyotherapplicationsandlanguages.

*Easymigration.TheSubversioncommand-lineclientisverysimilartoCVS;thedevelopmentmodelisthesame,soCVSusersshouldhavelittletroublema ingtheswitch.Developmentofa'cvs2svn'repositoryconverterisinprogress.

*It'sFree.SubversionisreleasedunderaApache/BSD-styleopen-sourcelicense.

Subversion'sDesign-------------------

Subversionhasamodulardesign;it'simplementedasacollectionofClibraries.Eachlayerhasawell-definedpurposeandinterface.Ingeneral,codeflowbeginsatthetopofthediagramandflows"downward"--eachlayerprovidesaninterfacetothelayeraboveit.

Let'sta eashorttouroftheselayers,startingatthebottom.

-->TheSubversionfilesystem.

TheSubversionFilesystemis*not*a ernel-levelfilesystemthatonewouldinstallinanoperatingsystem(li etheLinuxext2fs.)Instead,itreferstothedesignofSubversion'srepository.Therepositoryisbuiltontopofadatabase--currentlyBer eleyDB--andthusisacollectionof.dbfiles.However,alibraryaccessesthesefilesandexportsaCAPIthatsimulatesafilesystem--

8/2/2019 Lj Article

4/6

specifically,a"versioned"filesystem.

Thismeansthatwritingaprogramtoaccesstherepositoryisli ewritingagainstotherfilesystemAPIs:youcanopenfilesanddirectoriesforreadingandwritingasusual.Themaindifferenceisthatthisparticularfilesystemneverlosesdatawhenwrittento;oldversionsoffilesanddirectoriesarealwayssavedashistoricalartifacts.

WhereasCVS'sbac

end(RCS)storesrevisionnumbersonaper-filebasis,Subversionnumbersentiretrees.Eachatomic'commit'totherepositorycreatesacompletelynewfilesystemtree,andisindividuallylabeledwithasingle,globalrevisionnumber.Filesanddirectorieswhichhavechangedarerewritten(andolderversionsarebac edupandstoredasdifferencesagainstthelatestversion),whileunchangedentriesarepointedtoviaashared-storagemechanism.Thisishowtherepositoryisabletoversiontreestructures,notjustfilecontents.

Finally,itshouldbementionedthatusingadatabaseli eBer eleyDBimmediatelyprovidesothernicefeaturesthatSubversionneeds:dataintegrity,atomicwrites,recoverability,andhotbac ups.(Seewww.sleepycat.comformoreinformation.)

-->Thenetwor layer.

Subversionhasthemar ofApachealloverit.Atitsverycore,theclientusestheApachePortableRuntime(APR)library.(Infact,thismeansthatSubversionclientshouldcompileandrunanywhereApachehttpddoes--rightnow,thislistincludesallflavorsofUnix,Win32,BeOS,OS/2,MacOSX,andpossiblyNetware.)

However,SubversiondependsonmorethanjustAPR--theSubversion"server"isApachehttpditself.

WhywasApachechosen?Ultimately,thedecisionwasaboutnot

reinventingthewheel.Apacheisatime-tested,open-sourceserverprocessthatreadyforserioususe,yetisstillextensible.Itcansustainahighnetwor load.Itrunsonmanyplatformsandcanoperatethroughfirewalls.It'sabletouseanumberofdifferentauthenticationprotocols.Itcandonetwor pipeliningandcaching.ByusingApacheasaserver,Subversiongetsallthesefeaturesforfree.Whystartfromscratch?

SubversionusesWebDAVasitsnetwor protocol.DAV(DistributedAuthoringandVersioning)isawholediscussioninitself(seewww.webdav.org)--butinshort,it'sanextensiontoHTTPthatallowsreads/writesand"versioning"offilesovertheweb.TheSubversionprojectishopingtorideaslowlyrisingtideofsupportforthis

protocol:allofthelatestfile-browsersforWin32,MacOS,andGNOMEspea thisprotocolalready.Interoperabilitywill(hopefully)becomemoreandmoreofabonusovertime.

ForuserswhosimplywishtoaccessSubversionrepositoriesonlocaldis ,theclientcandothistoo;nonetwor isrequired.The"RepositoryAccess"layer(RA)isanabstractAPIimplementedbyboththeDAVandlocal-accessRAlibraries.Thisisaspecificbenefitofwritinga"librarized"versioncontrolsystem;it'sabigwinoverCVS,whichhastwoverydifferent,difficult-to-maintaincodepathsfor

8/2/2019 Lj Article

5/6

localvs.networ repository-access.Feelli ewritinganewnetworprotocolforSubversion?JustwriteanewlibrarythatimplementstheRAAPI!

-->Theclientlibraries.

Ontheclientside,theSubversion"wor

ingcopy"librarymaintainsadministrativeinformationwithinspecialSVN/subdirectories,similarinpurposetotheCVS/administrativedirectoriesfoundinCVSwor

ingcopies.

AglanceinsidethetypicalSVN/directoryturnsupabitmorethanusual,however.The`entries'filecontainsXMLwhichdescribesthecurrentstateofthewor ingcopydirectory(andwhichbasicallyservesthepurposesofCVS'sEntries,Root,andRepositoryfilescombined).Butotheritemspresent(andnotfoundinCVS/)includestoragelocationsfortheversioned"properties"(themetadatamentionedin'SubversionFeatures'above)andprivatecachesofpristineversionsofeachfile.Thislatterfeatureprovidestheabilitytoreportlocalmodifications--anddoreversions--*without*networ access.AuthenticationdataisalsostoredwithinSVN/,ratherthaninasingle.cvspass-li efile.

TheSubversion"client"libraryhasthebroadestresponsibility;itsjobistominglethefunctionalityofthewor ing-copylibrarywiththatoftherepository-accesslibrary,andthentoprovideahighest-levelAPItoanyapplicationthatwishestoperformgeneralversioncontrolactions.

Forexample:theCroutine`svn_client_chec out()'ta esaURLasanargument.ItpassesthisURLtotherepository-accesslibraryandopensanauthenticatedsessionwithaparticularrepository.Itthenas stherepositoryforacertaintree,andsendsthistreeintothewor ing-copylibrary,whichthenwritesafullwor ingcopytodis(SVN/directoriesandall.)

Theclientlibraryisdesignedtobeusedbyanyapplication.WhiletheSubversionsourcecodeincludesastandardcommand-lineclient,itshouldbeveryeasytowriteanynumberofGUIclientsontopoftheclientlibrary.Hopefully,theseGUIsshouldsomedayprovetobemuchbetterthanthecurrentcropofCVSGUIapplications(themajorityofwhicharenomorethanfragile"wrappers"aroundtheCVScommand-lineclient.)

Inaddition,properSWIGbindings(www.swig.org)shouldma etheSubversionAPIavailabletoanynumberoflanguages:java,perl,python,guile,andsoon.InordertoSubvertCVS,ithelpstobeubiquitous!

Subversion'sFuture-------------------

ThereleaseofSubversion1.0iscurrentlyplannedforearly2002.Afterthereleaseof1.0,Subversionisslatedforadditionssuchasi18nsupport,"intelligent"merging,better"changeset"manipulation,client-sideplugins,andimprovedfeaturesforserveradministration.(Alsoonthewishlistisaneclecticcollectionofideas,suchasdistributed,replicatingrepositories.)

8/2/2019 Lj Article

6/6

AfinalthoughtfromSubversion'sFAQ:

"Wearen't(yet)attemptingtobrea newgroundinSCMsystems,norareweattemptingtoimitateallthebestfeaturesofeverySCMsystemoutthere.We'retryingtoreplaceCVS."

If,inthreeyears,Subversioniswidelypresumedtobethe"standard"SCMsystemintheopen-sourcecommunity,thentheprojectwillhavesucceeded.Butthefutureisstillhazy:ultimately,Subversionwillhavetowinthispositiononitsowntechnicalmerits.

Patchesarewelcome.

ForMoreInformation--------------------

PleasevisittheSubversionprojectwebsiteathttp://subversion.tigris.org.Therearediscussionliststojoin,andthesourcecodeisavailableviaanonymousCVS--andsoonthroughSubversionitself.