Managing XML Versions and Replicas in a P2P Context

Transcription

1 Managing XML Versions and Replicas in a P2P Conex Deise de Brum Saccol1 1,2, Nina Edelweiss 2, Renaa de Maos Galane 2,4, Carlo Zaniolo 3 2 Insiuo de Informáica - Universidade Federal do Rio Grande do Sul (UFRGS) Av. Beno Gonçalves, 9500, Poro Alegre, RS, Brazil {deise, nina, galane}@inf.ufrgs.br 3 Compuer Science Deparmen Universiy of California (UCLA) 405 Hilgard Avenue, Los Angeles, CA, Unied Saes zaniolo@cs.ucla.edu Absrac Peer-o-Peer (P2P) sysems seek o provide sharing of compuaional resources, which may be duplicaed or versioned over several peers. Duplicae resources (i.e. replicas) are he key o beer query performance and availabiliy. On he oher hand, muliple versions can be used o suppor queries on he lineage of resources and he evoluion of hisory. However, radiional P2P sysems are no aware of replicas and versions, which cause complexiy a he logical level and inefficiency a he physical level. To solve hese problems, we propose an environmen for deecing, managing and querying replicas and versions of XML documens in a P2P conex. We also show ha he proposed environmen can also be used for plagiarism deecion, web page ranking, and sofware clone idenificaion. 1. Inroducion P2P sysems refer o a class of applicaions ha use disribued resources o perform asks in a decenralized conex. Each paricipan acs boh as clien and server, providing access o resources hrough direc and decenralized communicaion [1]. Their usabiliy is mainly dependen on echniques used o find and rerieve resuls. The resuls qualiy may be measured by merics such as he resul se size, query saisfacion, and processing ime [2]. However, searching for resources in P2P sysems mus deal wih wo imporan issues: he firs is he exisence of replicas and he second is he presence of muliple versions of a resource. Replicas (i.e. muliple represenaions) are imporan for performance opimizaion: when he user poses a query hen he resuls mus be reurned from peers ha bes saisfy performance and fas response ime requiremens. To ake advanage of resource replicaion i is necessary o deec hese replicas; oherwise, redundan resuls a a high processing cos are reurned o he user. The second problem arises from he evolving behavior of some resources, which is a fundamenal aspec in persisen informaion sysems. This feaure is even more eviden in XML domain, wih frequen srucure and conen changes. The evoluion aspec mus be managed o allow hisorical analysis for dynamic resources. The applicaions of he version concep are many and diverse, for insance he managemen of he co-auhoring sofware, as sudied in [3]. However, pas approaches focus on cenralized managemen and ruly P2P disribued managemen sill represens a difficul challenge. In P2P sysems, versioning echniques mus consider ha versions and replicas may be spread over several peers. In such conex, deecing duplicaes and versions is mainly useful for query opimizaion. To address his issue, our paper proposes DeVX, an environmen for he deecion, managemen and querying of XML replicas and versions. The main conribuions of his paper are: A brief environmen specificaion o deec, manage and query replicas and versions in a P2P environmen; A replica and linear version deecion mechanism based on hash funcions and documen similariy; A emporal XML model, based on diff algorihms and imesamps, for represening versioned resources and supporing basic emporal queries. The paper is organized as follows: Secion 2 presens relaed works. Secion 3 briefly describes he proposed environmen. Secion 4 discusses he replica and version manager module; a similariy funcion is presened for version deecion in conen and srucural evoluions. Query capabiliies are presened in Secion 5. Secion 6 highlighs oher applicaions ha may use our mechanism. Secion 7 presens conclusions and fuure work. 1 This work has been parially suppored by CNPq under gran No /2004-4, Capes under gran No. 1451/06-5, PERXML under gran No / and DIGITEX - CTInfo under gran No / This work has been parially suppored by CNPq under gran No / (Edial Universal) and Fapergs under gran No (Auxílio PROAPP).

2 2. Relaed Work There has been some recen works on emporal XML models [5][6], exensions o is query languages [7], emporal libraries [8] and version conrol [4]. However, version conrol sysems model files as ex line sequences, soring he las version and using reverse ediing scrips o rerieve previous versions [21]. These sysems do no preserve he logic srucure of he original file and do no suppor complex queries, and hus are inadequae o suppor XML versions. These gaps are addressed in some works, such as [9][10] and [11][12], respecively. Previous works focus on version managemen raher han version deecion (i.e. he creaion of a new version from an old one). However, version deecion is essenial in our moivaing applicaion, since he anonymiy/disribued naure of P2P environmens prevens users from idenifying resources from which he new version or replica is being creaed. Moreover, exisen replica deecion proposals focus on idenifying muliple represenaions of he same objec in he real world [13], which may have conen or srucure differences. However, our work considers a replica as an idenical copy of a XML file. To address his issue, we propose a deecion mechanism based on file similariy. There is some research on change deecion ha can be used as a basis for measuring similariy. Some approaches use diff algorihms o deec differences beween files [14][15]. Anoher possibiliy is o analyze heir ordered ree represenaions by calculaing he edi disance, i.e. he minimum cos o ransform one ree ino anoher ree using basic operaions [16][17]. Diff algorihms can be used o deec differences and, in a cerain way, a similariy value beween files. However, diff resuls are a dela scrip wih no semanic informaion regarding he similariy beween documens. Also, he ree edi disance resuls do no conain valuable informaion relaed o he similariy level ha could be used o deec resource versions. Our work focuses on his gap and proposes an environmen for deecing and managing replicas and versions of XML documens in a P2P conex. Many applicaions may use version deecion mechanisms. For plagiarism deecion, comparing file checksums is enough for deecing exac replicas, bu insufficien for parial copies [22][23]. By considering parial copies as versions, such plagiarism can be deeced. The web page ranking process can also ake advanage of he deecion mechanism by ranking new versions of exisen op-ranked pages [25]. A las, he sofware clone problem ha arises during he developmen of sysems may have a negaive impac on heir mainenance [24]. The proposed mechanism can help o deec such clones. 3. DeVX Environmen DeVX is an environmen for deecing and managing replicas and versions of XML documens in a P2P conex [26]. DeVX is based on a super peer archiecure [19]. Super peers are responsible for receiving he query and resending i o aggregaed peers and oher super peers. Peers mus (re)connec in super peers in order o share heir files. Shared XML files are relaed o a knowledge domain, used as a peer grouping crierion in super peers. An onology is used o represen he knowledge domain [18]. Super peers are managed by he adminisraive super peer, as depiced in Figure 1. Fig. 1. DeVX Environmen Files may be duplicaed or versioned over he super peer nework. To provide he funcionaliies for replica and version deecion, his work proposes he following modules. The peer manager is responsible for (re)connecing peers and periodically verifying modificaions in shared files. The onology manager mainains he onology reposiory and associaes onologies o super peers. The replica and version manager idenifies and manages documen replica and versions. The query processor is responsible for verifying he query domain and roaing queries o peers. Meadaa play a fundamenal role and are deailed in Secion 3.1. In his paper, we do no deail he peer and onology managers. More deails may be found in [26]. 3.1 Documen and Meadaa Represenaion The erm file refers o a physical represenaion sored in a peer; documen refers o he represenaion of an objec in he real world. In oher words, one documen can be sored as many files, eiher because i is replicaed or versioned. A file has a regisering and a modificaion ime. The modificaion ime is considered o define he file order over ime. Local (fileid) and global idenifiers (GFID) are used o idenify a file in a peer and in a specific locaion in he nework, respecively [26]. Documens also have idenifiers (docid) and hey are used o idenify versions and replicas of he same objec in he real world. To manage idenifiers and oher relevan informaion, he approach relies on he exensive use of meadaa. Meadaa are represened as XML files and classified in wo levels, as shown in Figure 1. In his paper, only super peer meadaa are considered. Super peer meadaa basically specify he available versions and replicas in a specific super peer (superpeerid), and he corresponding imesamps for each elemen (imesar: TS, imeend: TE) ha is found in cerain file (fileid) in a peer (peerid), as shown in Lising Curriculum Onology Meadaa Curriculum Files Adminisraive Super Peer Meadaa SpPeer 1 SpPeer 2 Peer Peer 2 Peer 3 Peer 4 Curriculum and Research Projecs Files Onology Reposiory Research Projecs Onology Meadaa Research Research Projecs Projecs Files Files <Meadaa superpeerid="sp1"> <documen docid= D1 fileid="f7" HDoc= YES > <version versionid="1" peerid="p1" regiseringtime="10/10/2005" modificaiontime="08/08/2004" duplicae="no" hashresul="d49622ddab e fd52b5"> <elemen name="auhor" TS="08/08/2004" TE="10/15/2004"/> <elemen name="address" TS="08/08/2004" TE="10/15/2004"/></version> <version versionid="2" peerid="p2" regiseringtime="11/20/2005" modificaiontime="10/16/2004" duplicae="yes" hashresul="7c00bb062edc60fa548729a3d55c04fd"> <locaionduplicae>peer 3</locaionDuplicae> </version> </Meadaa> Lising 1. Super Peer Meadaa

3 Each elemen has wo imesamps inferred from he modificaion ime of he file in which he elemen is conained. Super peer meadaa informaion is updaed whenever a new file is regisered ino a peer and is exensively used during querying process. 4. Replica and Version Manager This module is responsible for deecing replicas and versions and represening he hisory in a new srucure, called H-Doc file. 4.1 Deecion Mechanism To solve he deecion problem, a firs approach is o look for replicas and versions in he local peer. If hey are no found, he deecion is execued in he nex peer of he super peer nework. The deecion mechanism is execued whenever a file is regisered or updaed in a peer. When a file is removed, only he meadaa need o be updaed. A peer modificaion checking service is responsible for periodically waching he peer and noifying is super peer whenever a change is deeced. The replica deecion mechanism aims o verify if a file is a copy of any oher file sored in any peer belonging o he same super peer nework. In our work, a duplicae (or replica) is defined as an idenical copy of a XML file. The replica deecion is done by comparing he file hash resul wih all he hash resuls already sored in is super peer meadaa. Two files f1 and f2 are replicas if: HashFuncion(f1)=HashFuncion(f2) The version deecion mechanism aims o verify if a modified file is a version of any oher file sored in any peer belonging o he same super peer nework. Since his work assumes he linear versioning approach, his aciviy will compare he candidae file only wih he las file versions available in he super peer nework. There are wo ypes of evoluion ha are considered: Conen: <x>a S, 7</x> <x>b S, 8</x> Srucure and conen: <x>a S, 7</x> <y>b S</y> <z>8</z> In his proposal, version deecion is based on file similariy. The general idea is ha wo files wih high similariy are considered wo versions of he same documen; wo differen documens, oherwise. Le s firs consider he conen evoluion ype Conen Evoluion Suppose wo files, f1 and f2, shown in Lising 2. <hiringd>10/10/03</hiringd> <job>engineer</job> <salary>3700</salary> <address>7 S</address> <hiringd>10/10/03</hiringd> <job>manager</job> <salary>4900</salary> <address>7 S</address> Lising 2. XML Files In order o evaluae he similariy beween hese files, some feaures are observed: Diff resuls: he roo elemen in boh files has six child elemens. Using a diff algorihm, he differences beween he files are deeced. As Lising 3 shows, he conen of he elemens salary and job do no mach in he second file. In oher words, 67% of he original elemens kep unchanged in he second file. The assumpion here is he following: he bigger percenage of mached elemens, he bigger chance he files are versions of he same documen. <dela> <Deleed updae="yes" pos="0:0:3:0">3700</deleed> <Deleed updae="yes" pos="0:0:2:0">engineer</deleed> <Insered updae="yes" pos="0:0:2:0">manager</insered> <Insered updae="yes" pos="0:0:3:0">4900</insered> </dela> Lising 3. Diff resul 2 for files f1 and f2 Mached and unmached elemens: We consider he erm mached o refer o an elemen ha has he same conen in boh files (for example, name); unmached, oherwise (for example, salary). Le s ake a look a he unmached elemens salary and job. Using a (combinaion of) sring similariy funcion(s), we calculae a value ha demonsraes how similar he unmached elemens are. The more similar he respecive unmached elemens, he bigger chance he files are versions of he same documen. Elemen change relevance: Anoher imporan issue is he relevance of individual changes. Some domain conceps can change more frequenly han ohers. Le s suppose ha we have an address elemen. Two differen addresses can easily refer o he same person; however, wo differen birhdaes sugges ha we are analyzing wo differen objecs in he real world. In oher words, he change relevance is differenly weighed for differen conceps. We assume differen weighs, such as high (1), medium (0.5) and low (0). The average of weighed relevances is used o calculae file similariy. The smaller change relevance hey presen, he bigger chance he files are versions of he same documen. Based on he previous discussions, he similariy funcion simc beween wo files f1 and f2 is defined as: simc(f1,f2) = (w 1*F 1 + w 2*F 2 + w 3*F w n*f n) Where w n is a facor ha weighs he imporance of a specific feaure F n. A facor may be posiive or negaive (if i influences he similariy growh or reducion, respecively). Considering w x, w x+1, w y as posiive facors and w z, w z+1, w q as negaive facors, we assume ha w x + w x w y = 1 and 0<= w z + w z w q <= 1. In our approach, hree feaures are considered o produce he following conen evoluion similariy funcion: simc(f1,f2) = w 1*P + w 2*S + w 3*R Where: P is he percenage of mached elemens, S is he mean similariy of he unmached elemens and R is he average of domain relevances of he unmached elemens (defined by he sysem adminisraor). P and S facors (w 1 and w 2, respecively) are posiive values (he greaer hese values, he more similar he files) and R facor (w 3) is a negaive value (he smaller his value, he less relevance he change and he more similar he files). The facors (w 1, w 2,..., w n) mus be defined based on he imporance of he hree feaures in 2 We are currenly using XyDiff implemenaion [14], bu he archiecure allows changing o oher diff algorihms.

4 specific applicaions/domains and recall/precision measures [30]. The inervals of he defined variables are defined as: {P P [0,1]}, {S S [0,1]}, {R R [0,1]. Analyzing he minimum e maximum values of P, S and R, and he sum resricions for posiive and negaive facors, we conclude ha he similariy funcion produces a value simc ha ranges from -1 o 1, i.e. {simc simc [-1, 1]}. To calculae P, we use a funcion calcp ha reurns he percenage of mached elemens based on he diff resul. S is calculaed by using a (combinaion of) sring similariy funcion(s) (SrSim()) and i is defined as he average of unmached elemens (ue) similariy values. The funcion is defined in more deails as follows: simc(f1,f2) = w 1*calcP(diff(f1,f2)) + w 2* SrSim(ue1 x,ue2 x) - w 3* R(ue x) x=1 x=1 As depiced in Figure 2(a), he similariy funcion values are no uniformly disribued. To uniformly disribue he values, we sor and map he m similariy funcion resuls ino n classes. The mapping, represened in a ransformaion able, caegorizes m/n members in each class. Since we have 100 differen similariy values, his ransformaion generaes 0.01*m members in each class. Fig. 2. Similariy Funcion Values (a) and (b) Figure 2(b) shows he disribuion of he mapped uniform ransformaion. We generaed values according o he original similariy funcion, using 0.5, 0.5 and -0.5 as he weigh values, and grouped hem ino 100 classes. These classes were mapped o values [0,1], in order o uniformly disribue he funcion values. To ensure ha he mapping is correc, we generaed more values and mapped hem o his able. Afer producing he similariy values, a hreshold is used o deec versions based on hem. The hreshold generaion is an ongoing work and i is no deailed in his paper. Furher sudy is sill needed o assess which hreshold is beer respec o precision and recall Srucure and Conen Evoluion Suppose wo files, f3 and f4, shown in Lising 4. <hiringd>10/10/03</hiringd> <job>engineer</job> <salary>3700</salary> Lising 4.XML Files In order o evaluae he similariy beween hese files, he discussions abou diff resuls and elemen change relevance <salary>4500</salary> <address>7 S</address> in he las sub-secion are sill valid. Anoher feaure is also observed: Added and removed elemens: using a diff algorihm, he differences beween he files are deeced. Analyzing he files and he diff resuls, we can see ha he f4 has added one elemen (address) and has removed wo elemens (job and hiringd). Le s refer added o he elemens in he firs siuaion and deleed o he elemens in he second siuaion. These conceps are similar o he ideas presened in [29], which consider plus, minus and common elemens for measuring similariy beween a documen and a DTD. We consider he erm mached o refer o an elemen ha has he same srucure and conen in boh files (for example, name and phone); unmached, for hose elemens ha he conen has changed (for example, salary). Similar o he ideas presened for he conen evoluion, he following feaures are considered o produce he srucure evoluion similariy funcion: sime(f3,f4) = simc(f3,f4) + w 4*A + w 5*D Where: simc is he conen similariy value, A is he percenage of added elemens and D is he percenage of deleed elemens. A and D facors (w 4 and w 5, respecively) are negaive values (he smaller hese values, he more similar he files). The inervals of he defined variables are defined as: {A A [0,1], {D D [0,1]. Analyzing he minimum e maximum values of simc, A, D, and he sum resricions for posiive and negaive facors, we conclude ha he similariy funcion produces a value sime ha [-3, 2]. To calculae A, we use a funcion calca ha reurns he percenage of added elemens, based on he diff resul. To calculae D, we use a funcion calcd ha reurns he percenage of removed elemens, based on he diff resul. sime(f3,f4) =simc(f3,f4) - w 4*calcA(diff(f3,f4)) w 5*calcD(diff(f3,f4)) The similariy values are no uniformly disribued. Similarly, he process deailed in he previous secion is applied on he resuls o uniform hese values. Also, he hreshold process presened in Secion is sill valid. Whenever a new version or replica is deeced, he imesamps described in he super peer meadaa need o be updaed. Meadaa updaing is described in [26]. 4.2 A Consolidaed Hisorical Represenaion Afer deecing he versions, he sysem sores hem in a new physical file, which conains he enire hisory of a documen. The documen hisory is named consolidaed hisorical represenaion and represened in H-Doc files. H- Doc files are sored in he respecive super peer where he original versions are regisered. Timesamps are responsible for validaing daa in specific versions. H-Doc represenaions are generaed only for frequenly accessed and evolved files. The goal is o provide faser query processing for queries ha ask hisorical rerieval. The H-Doc generaion process is deailed in [27]. Lising 6 shows he H-Doc file generaed for Lising 4. Consider ha f3 and f4 have 01/01/2004 and 01/01/2005 as modificaion imes, respecively.

5 <employee TS= 01/01/2004 TE=NOW > <name TS= 01/01/2004 TE= NOW >Marcos</name> <hiringd TS=01/01/2004 TE= 12/31/2004 >10/10/03</hiringD> <job TS= 01/01/2004 TE= 12/31/2004 >engineer</job> <salary TS= 01/01/2004 TE= 12/31/2004 >3700</salary> <salary TS= 01/01/2005 TE= NOW >4500</salary> <phone TS= 01/01/2004 TE= NOW > </phone> <address TS= 01/01/2005 TE= NOW >7 S</address> Lising 5. H-Doc File In DeVX environmen, he generaion of he H-Doc file is done by XVersion ool, a currenly implemenaion work [27], based on diff algorihms and imesamps. 5. Query Processor Afer deecing replicas and versions, emporal queries may be posed on he original files locaed in he peers or on he hisorical represenaion sored in he super peers. 5.1 Querying he Original Files To evaluae which files mus be accessed o answer a query, our approach relies on meadaa described in Secion 3.1. The query submission works as follows: he user poses a query in a specific peer (named querying peer). This query belongs o a specific domain. Looking a he super peer meadaa, i is possible o see how o access he hisory or versions of an elemen or documen. Considering he super peer meadaa described in Lising 1, some emporal rerieving examples are described below: 1. Rerieve he version v i of an elemen e j for insance, ge he firs version (versionid="1", line 3) of he elemen auhor (elemen name="auhor", line 6). By searching he version number represened in meadaa, he sysem can verify ha he firs version of he queried elemen is found in peer 1 (peerid="p1", line 3) locaed a super peer 1 (superpeerid="sp1", line 1). Thus, he sysem mus access his file and reurn he resuls. 2. Rerieve he hisory of an elemen e j for insance, ge he hisory of he elemen address. To answer his query, he sysem searches he meadaa, looking for all he versions (versionid) of he elemen address (elemen name="address"). The las version of his elemen is represened by TE=now. Anoher possibiliy for his query is o check if here is a generaed H-Doc represenaion for his file (aribue HDoc="YES", line 2). In his case, he sysem can access his file in he super peer, as described in he nex secion. 5.2 Querying he H-Doc File Consider a documen D as a n-uple D = (roo, e 1, e 2,..., e n ) and an elemen e in his documen as a 3-uple E=(TS, TE, <conen>), where TS and TE denoe he imesamps. Temporal resricions are applied based on a specific dae x or on an inerval x and y (x<y). Some emporal clauses are: 1. Selec_Before (E, x): reurns he elemens e ha are valid in H-Doc file before x (elemens whose TS < x); 2. Selec_Afer (E, x): reurns he elemens e ha are valid in H-Doc file afer x (elemens whose TE>x); 3. Selec_Beween (E, x, y): reurns he elemens e ha are valid in H-Doc file beween x and y (elemens whose TS<=y and TE>=x); 4. Selec_Now (E): reurns he elemens e ha are valid in H- Doc file in curren ime (elemens whose TE=now); The same clauses are defined for rerieving enire documens, such as Selec_Before (D, x), Selec_Afer (D, x) and ohers. Query capabiliies based on XQuery language [20] have been implemened in our ool named XVersion. This ool generaes he H-Doc documen and allows basic emporal queries over he hisorical file. More deails abou XVersion may be found in [27]. 6. Oher Applicaions This paper focuses on version and replica deecion problem in P2P sysems. Alhough his is he moivaing scenario for our sysem and experimens, we expec ha our proposal can be used in oher applicaions, such as: Web page ranking: ranking mehods usually involve he locaion and frequency of keywords in a web page. Search engines verify if he searched keywords appear close o he page op (headline or in he firs few paragraphs). Frequency is also considered by analyzing how ofen keywords appear in relaion o oher words in a web page [25]. Anoher facor ha may be considered for ranking is he incoming link degree (i.e. he number of links ha poin ou o a page p). However, new p versions may have a small incoming link degree, mainly because of he pages ha were poining o p are no aware of he new version. In such conex, version and replica deecion may be useful for ranking new versions even if hey have low incoming degrees. Plagiarism deecion: Digial files may be easily copied, eiher parially or compleely. One way o deec plagiarism is by comparing file checksums, which is simple and suffices for reliably deecing exac copies. However, deecing parial copies is more complicaed [22]. By using he mechanism proposed in his paper, similar files are idenified. The hreshold definiion mus be in accordance o such applicaion. For insance, parial copies mus be idenified wih a low hreshold, whereas complee copies mus be deeced wih a higher hreshold. Sofware clone idenificaion: replicaed code can arise during he developmen and evoluion of sofware sysems and i has a negaive impac on heir mainenance. The deecion ges difficul mainly because of small differences, such as reformaing, code and variable name changes [24]. Exisen deecion mechanisms usually rely on he use of a parser, bu his approach is dependen on he programming language synax. The classical plain-ex represenaion of code is convenien for programmers bu requires parsing o uncover he deep srucure of he program. Represening code in a srucured forma, as XML documens [28], permis easy specificaion of numerous sofware-engineering analyses by leveraging on he abundance of XML ools and echniques. In his conex, he proposed mechanism may be used for sofware clone deecion.

6 7. Concluding Remarks This paper focused on deecion and managemen of XML replicas and versions in P2P conexs. The relevance of such problem is quie eviden in many scenarios, such as plagiarism deecion, web page ranking, sofware clone idenificaion, assuring link permanence in Web documens, and enhancing search in P2P sysems. To increase efficiency and effeciveness in such sysems, his paper briefly described he proposed archiecure and funcionaliies of he DeVX environmen. We have proposed a simple srucure for represening meadaa which can be used for managing and querying he available files. A documen similariy funcion used as he basic idea in he deecion mechanism was also described. The proposal requires no inervenion by he user. The user is only requesed o updae he documen and regiser he file; he sysem deecs prior versions or duplicaes, generaes idenifiers and manages all he relaed meadaa. The curren sae of he projec is as follows. We have already implemened XVersion, a ool for represening and querying documen hisory. Basic rerieval capabiliies have been implemened, allowing simple emporal queries over he hisorical represenaion. As fuure work, we are going o incorporae he deecion mechanism in DeVX environmen. The compleion of he deecion mechanism will allow us o measure improvemens on seleced esbeds, including JXTA [31]. Resuls will be presened in he conference. References 1. Aberer, K. and Hauswirh, M.. An Overview on Peer-o-Peer Informaion Sysems. Workshop on Disribued Daa and Srucures, Paris, France, Yang, B. and Garcia-Molina, H.. Efficien Search in Peer-o-Peer Neworks. In: Proceeding of he Inl. Conf. on Disribued Compuing Sysems, Vienna, Ausria, Wesfechel, B., Munch, B. P., and Conradi, R. A Layered Archiecure for Uniform Version Managemen. IEEE Trans. Sofware Eng., 27(12): , Chien, S-Y., Tsoras, V. J., Zaniolo, C. (2001). XML Documen Versioning. SIGMOD Records, Vol. 30 Number 3, Sep. 5. 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