Preserving Referential Constraints in XML Document Association Relationship Update

Transcription

1 Preserving Referentil Constrints in XML Document Assocition Reltionshi Udte Eric Prdede 1, J. Wenny Rhyu 1, Dvid Tnir 2 1 Dertment of Comuter Science nd Comuter Engineering, L Trobe University, Bundoor VIC 3083, Austrli ekrdede, wenny}@cs.ltrobe.edu.u 2 School of Business System, Monsh University, Clyton VIC 3800, Austrli Dvid.Tnir@infotech.monsh.edu.u Abstrct. In this er we roose the usge of W3C-stndrdized query lnguge, XQuery, to ccommodte XML Udte. Our min im is to enble the udte without violting the semntic constrint of the documents. The focus of the udte is on the ssocition reltionshi. Therefore, we distinguish the reltionshi tye bsed on the number of rticints, crdinlity, nd dhesion. We lso roose the ming of the ssocition reltionshi in the concetul level to the XML Schem. We use XML Schem for structure vlidtion, even though the lgorithm cn be used by ny schem lnguges. 1 Introduction In the lst few yers the interest in storing XML Documents in the ntive XML Dtbses (NXD) hs emerged ridly. The min ide is to store the documents in their nturl tree form. However, it is well-known tht mny users still refer to use DBMS tht re bsed on estblished dt models such s Reltionl Model for their document storge. One reson is the incomleteness of NXD query lnguge. Mny rorietry XML query lnguges nd even W3C-stndrdized lnguges still hve limittions comred to the Reltionl Model SQL. One of the most imortnt limittions is the lck of the udte oertions suort [8]. Different NXD lies different strtegies for XML udtes. Very frequently fter udte oertions, the XML document contins mny dngling references, loses the key ttribute, hs unnecessry dulictions, nd mny other roblems tht indicte very low dtbse integrity. In our best knowledge, there is no XML query lnguge tht hs considered the integrity issues tht emerge from the udte oertions. We suggest this s n imortnt issue to rise nd to investigte further. This er rooses the XQuery [11] for udte oertions. It is bsed on the bsic oertions firstly mentioned in [9]. Our min contribution is tht we hve included semntic constrints in the oertions. It focuses on the udte for ssocition reltionshi, which cn be defined s reference reltionshi between one to nother

2 node/document in n XML tree. We will distinguish different ssocition tye bsed on the number of rticints nd on the crdinlity. The XQuery udte considers different trget contents whether it is key node or key reference node. By doing this, we remove the ossibilities of low integrity dt fter ech udte oertion. For XML udte we need structure vlidtion nd we roose the use of XML Schem [10]. Thus, rt of the er is llocted to develoing trnsformtion on the ssocition reltionshi into XML Schem, in wy tht suits our roosed XQuery. 2 XML Document Udte: An Overview There re severl strtegies for udting XML documents in NXDs [1][8]. At the time of writing, there re three min strtegies: (i) use rorietry udte lnguge tht will llow udting within the server [3][7], (ii) use XUdte, the stndrd roosed by XML DB inititive for udting distinct rt of document [2][5], nd (iii) use XML API fter retrieve the document out of the dtbse [4]. It is imortnt to mention tht none of these re concerned with the semntic constrint of the udted XML document. Different strtegies hve limited the dtbse interchngebility. To unite these different strtegies, [9] hs tried to roose the udte rocesses for XML Documents into n XML lnguge. These rocesses re embedded into XQuery nd thus, cn be used for ny NXD tht hs suorted this lnguge. In [9] the udte is embedded in the XQuery FLOWR exression. Note the difference between the originl XQuery (Fig.1) nd its extension [ (Fig. 2). The UPDATE cluse secifies the node trgets tht will be udted. Inside the UPDATE cluse, we determine the oertion tyes. FOR $binding1 IN th exression LET $binding := th exression WHERE redicte, ORDER BY redicte, results Fig. 1. XQuery FLOWR Exressions FOR...LET...WHERE... UPDATE $trget DELETE $child INSERT content [BEFORE AFTER $child] REPLACE $child WITH $content,subo}* } Fig. 2. XQuery UPDATE Extensions Nonetheless, even with this roosl there is bsic question to nswer. We do not know how the udte oertions cn ffect the semntic correctness of the udted XML Documents. Secificlly for the ssocition reltionshi, this roosl does not mintin the referentil constrint between the ssocited nodes/documents. This fct hs motivted us to tke the toic ste further.

3 3 Assocition Reltionshis in XML Document Assocition reltionshi is reference reltionshi between one object with nother object in system. For n XML document, the object is node or document. Semnticlly, this reltionshi tye cn be distinguished by some constrints such s number of rticint tye, crdinlity, nd dhesion. These constrints cn be esily identified in XML Dt Model such s in Semntic Network Digrm [2]. We will show running exmle describing different ssocition reltionshi constrints (see Fig. 3). FcultyID FACULTY FcultyNme [1..N] SubjCode SubjNme SubjDesc SUBJECT TechBy (reference Stff) s[0..n] wek dhesion SubjPrereq (reference Subject) StffNme s[0..n] wek dhesion StffAddress [1..N] STAFF StffOffice (reference Office) s[0..1] wek dhesion s[0..n] wek dhesion [1..N] OfficeNumber StffReserch (reference Reserch) OFFICE OfficeLoc StffID [1..N] OfficePhone ReserchCode RESEARCH ReserchNme [1..N] ReserhMember (reference Stff) ReserchDesc Fig. 3. Assocition Tyes in XML Document Tree The number of rticints cn be distinguished into unry nd binry. In the former, n object refers to nother object of the sme tye. For exmle, there is one unry reltionshi, where node SubjPrereq in Subject document refers to nother subject. In the binry reltionshi, the rticints come from different tyes. In our exmle bove, there re: Subject:Stff, Stff:Office, nd Stff:Reserch. Assocition crdinlity identifies the number of objects of the sme tye to which single object cn relte. For exmle, stff cn tech no subject t ll to mny subjects [0..N]. In the digrm the defult crdinlity is [1..1]. Finlly, ssocition dhesion identifies whether the ssocited objects must or must not coexist nd dhere to ech other. In the digrm the defult dhesion is strong dhesion. For exmle, Stff hs wek dhesion ssocition to Subject, which mens tht the former does not require the ltter to exist.

4 4 Ming Assocition Reltionshi to XML Schem XQuery udte will require schem to vlidte the document structure. We select XML Schem becuse s well s its recision in defining the dt tye, XML Schem hs n effective bility to define the semntic constrints of document. It includes the crdinlity, dhesion, nd referentil integrity constrint deicted in Fig. 3. First, the crdinlity constrint is very strightforwrd nd well-known. We cn determine the minoccurs nd mxoccurs fter n element declrtion. It is generl rctice tht these constrints re not used for ttribute. A rticulr ttribute will er, t most, once in n object/instnce. Second, the dhesion constrint cn esily be indicted by the use constrint. A strong dhesion hs use required nd wek dhesion hs use otionl. However, in rctice the use constrint cn only be ttched in ttribute. We will find roblem if, for exmle, the dhesion constrint is lied to n element. Fortuntely, we cn utilize the crdinlity constrint to define the dhesion constrint s well. Crdinlity [0..1] or [0..N] indictes wek dhesion, while [1..1] or [1..N] indictes strong dhesion. For consensus we rovide tble combining the two constrints tht cn be very useful for the next section. Tble 1. Adhesion nd Crdinlity Constrints in XML Schem Adhesion Crdinlity Wht to Use Strong 1:1 Attribute (use = required ) 1:N Element (minoccurs= 1 mxoccurs= unbounded ) N:N Element (minoccurs= <2.. > mxoccurs= unbounded ) Wek 0:1 Attribute (use = otionl ) 0:N Element (minoccurs= 0 mxoccurs= unbounded ) Thirdly, for the referentil integrity constrint, XML Schem rovides two otions ID/IDREF nd key/keyref. We hve chosen to use the ltter for severl resons: (i) key hs no restriction on the lexicl sce [10], (ii) key cnnot be dulicted nd thus fcilitte the unique semntic of key node, nd (iii) key/keyref cn be defined in secific element nd, thereby, heling the imlementer to mintin the reference. For our design, we roose to crete secific element under the root element clled GrouKey to store ll keys nd keyrefs. It is required to trck the th of the ssocited elements once n udte is erformed. We cnnot just declre the key/keyref inside the element where they re ered. We will exerience roblems in trcking n ssocited element since keyref cn only trce the key declred in the sme element or in its direct redecessor. For exmle, if we declre the keyref TechBy inside subject element nd key StffID inside stff element, the keyref cnnot trce the key nd thus, if we udte stff, there is no checking done to ensure the subject refers to the rticulr stff. In unry reltionshi, the key nd keyref ttribute/element is locted under one element. In binry reltionshi we hve to determine the loction of the keyref t-

5 tribute/element. The dhesion constrint is used for this urose. Locte the keyref ttribute/element under the ssocited element tht hs strong dhesion towrds the other. In the cse where both ssocited elements re in strong dhesion, the keyref ttribute/element will be dded in both elements. For exmle (see Fig.3), in the reltionshi between stff nd reserch, we dd reference ReserchMember inside reserch element tht refers to StffID, nd reference StffReserch inside reserch element tht refers to ReserchCode. 5 Proosed XQuery Udte Now we hve shown how to m the concetul model into logicl XML Schem, we cn begin to roose the XQuery for Udte rocesses. The udte cn be differentited into three min rts: deletion, insertion, nd relcement. 5.1 XQuery for Delete Oertion Deletion is the simlest udte oertion, esecilly if we wnt to delete n ttribute/element tht contins simle dt. In this cse we only require the binding to the rent nd the child. We do not need to erform redicte checking on instnces. However, if the ttribute nd element lso hve roles s key/keyref the checking on instnces is required to mintin the referentil integrity constrint. For our roosed XQuery, we differentite the oertion for key nd keyref since we ut focus on the ssocition reltionshi. For deletion however, there is no constrint checking required for keyref node since it is treted like simle dt content. The following XQuery shows functions used for redictes in key deletion. Function checkkeyrefnode checks whether the trget key node hs keyref node referred to it. This function highlights one reson why we select key/keyref insted of ID/IDREF. Now the imlementer cn check only the th where the keyref is defined. Since we hve groued ll key nd keyref declrtions in the GrouKey element, we only need to check the GrouKey element. Under condition (see the function), the checkkeyrefnode function roceeds to function checkkeyrefinstnce. This function checks tht there is no instnce tht hs keyref ttribute/element referring to the trget key content. This is the reson we lso need to ss the content of the trget key node. If the function returns flse, we cnnot erform the deletion. FUNCTION checkkeyrefnode($gkbindingpth, $keynme, $keycontent) BOOLEAN LET $gkref := $gkbindingpth/keyref/[@refer] IF $gkref = $keynme THEN getkeyrefinstnce($gkbindingpth, $keynme, $keycontent) ELSE TRUE}

6 FUNCTION checkkeyrefinstnce($gkbindingpth, $keynme, $keycontent) BOOLEAN FOR $keyrefbinding IN LET $keyrefpth:=$keyrefbinding/selector/xth "$keycontent"] IF exists ($keyrefinstnce) THEN FALSE ELSE TRUE} Exmle: FOR $g IN document("fculty.xml")/fculty/groukey $ IN document("fculty.xml")/fculty/subject(@subjcode = "ADB41") $c IN $/SubjCode LET $ccontent := ADB21 WHERE checkkeyref($g, "SubjCode", $ccontent) DELETE $ (:delete the key nd the siblings:) }}} The exmle following the functions shows the key deletion Subjcode in document Fculty.xml of instnce with SubjCode equls to ADB41. We know tht there is node SubjPrereq tht refers to SubjCode. Thus, if there is ny instnce where SubjPrereq vlue refers to SubjCode ADB41, the deletion should be restricted. Note tht if the checkkeyref function returns TRUE, we will delete whole elements nd not only the key SubjCode. This is becuse we do not hve trigger-bsed delete, which will delete the whole element if the key is deleted. 5.2 XQuery for Insert Oertion Unlike deletion, insertion requires the constructor of new ttribute or new element. Beside the XQuery for key insertion, we hve to roose insertion for keyref s well, to mke sure tht the new keyref ctully refers to n existent key. Function checkkeydulicte returns TRUE if the trget is not dulicting n existing instnce. The exmle following the function shows the checking if we wnt to insert SubjCode with content ADB41. FUNCTION checkkeydulicte($bindingpth) BOOLEAN IF EXISTS($bindingPth) THEN FALSE ELSE TRUE} Exmle: FOR $g IN document("fculty.xml")/fculty(@fcid = "FSTE") $ IN $g/subject(@subjcode="adb41") LET $c:= SubjCode $ccontent:= ADB41 WHERE checkkeydulicte($) INSERT new_tt($c, $ccontent)

7 }} For keyref insertion, we roose three dditionl functions. The first is for the referentil integrity constrint nd the following two re for the crdinlity constrint. The crdinlity is required since keyref cn ctully refer to more thn one key instnce. Function checkkeyinstnce sses the keyref nme nd content, nd then checks the key being referred. Function getmxoccurs nd checkmxoccurs re used to clculte the mxoccurs constrint of keyref node. If the rticulr instnce hs the mximum number of keyref, we will disble the insertion of nother keyref content. FUNCTION checkkeyinstnce($gkbindingpth, $keyrefnme, $keyrefcontent) BOOLEAN FOR $keyrefbinding IN = $keyrefnme], LET $keypth:=$gkeyrefbinding/selector/xth, "$keyrefcontent") IF EXISTS($KeyInstnce) THEN TRUE} FUNCTION getmxoccurs($xsnme, $rentnme, $childnme) INTEGER FOR $Def IN $cdef IN LET $cmxoccurs:=$cdef/mxoccurs, $cmxoccurs} FUNCTION checkmxoccurs($bindingpth, $cmxoccurs) BOOLEAN LET $instnceoccurs:=count($bindingpth) IF $cmxoccurs >= $instnceoccurs + 1 THEN FALSE ELSE TRUE} 5.3 XQuery for Relce Oertion Since relcement cn be seen s combintion of deletion nd insertion, we cn reuse the functions we hve lredy described in the lst two sub-sections. In fct, we do not need the functions for crdinlity constrints during keyref relcement s well. For relcement of key, we hve to check whether the new key content does not dulicte ny existing instnce. For relcement of keyref, we just need to check whether the new keyref content refers to vlid instnce. No crdinlity checking is required since to ccommodte new keyref, we hve deleted nother keyref. XQuery below shows the exmle of relcement for keyref TechBy element. FOR $g IN document("fculty.xml")/fculty/groukey $ IN document("fculty.xml")/fculty/subject(@subjcode = "ADB41") $c IN $/TechBy LET $cnme := TechBy $ccontent := WR01 WHERE checkkeyinstnce($g, $cnme, $ccontent)

8 REPLACE $c WITH <TechBy>WR01</TechBy> }} 6 Conclusion In this er, we roose extending XQuery to reserve semntic constrints during n XML udte. The udte oertions re divided into deletion, insertion, nd relcement. The focus in the er is on the ssocition reltionshi tye, thus ech oertion considers the key nd key reference trget node. By doing this, we cn void dtbse nomlies tht might occur using conventionl XQuery. Since Udte requires document structure vlidtion, we lso roose the trnsformtion of the reltionshi into XML Schem. The constrints ctured re crdinlity, dhesion, nd referentil integrity constrint. With this extension, XML query lnguges (in the form of XQuery) re becoming more owerful. Concurrently, it cn increse the otentil of using tree-form XML reository such s Ntive XML Dtbse. References 1. Bourett, R.: XML nd Dtbses. htt:// (2003) 2. Feng, L., Chng, E., Dillon, T.S.: A Semntic Network-Bsed Design Methodology for XML Documents. ACM Trns. Informtion System, Vol. 20, No. 4. (2002) Iedo.: Iedo XML Dtbse, htt:// (2004) 4. Jgdish, H. V., Al-Khlif, S., Chmn, A., Lkhsmnn, L. V. S., Niermn, A., Prizos, S., Ptel, J. M., Srivstv, D., Wiwttn, N., Wu, Y., Yu, C.: TIMBER: A ntive XML dtbse. VLDB Journl, Vol. 11, No. 4. (2002) Meier, W.M.: exist Ntive XML Dtbse. In Chuduri, A.B., Rwis, A., Zicri, R. (eds): XML Dt Mngement: Ntive XML nd XML-Enbled Dtbse System. Addison Wesley (2003) Robie, J.: XQuery: A Guided Tour. In Kttz, H. (ed.): XQuery from the Exerts. Addison Wesley (2004) SODA Technology.: SODA. htt:// (2004) 8. Stken, K.: Introduction to Ntive XML Dtbses. htt:// (2001) 9. Ttrinov, I., Ives, Z.G., Hlevy, A. Y., Weld, D. S.: Udting XML. ACM SIGMOD (2001) Vlist, E. V-D.: XML Schem, O Reilly, Sebstool (2002) 11. W3C: XQuery 1.0: An XML Query Lnguge. htt:// (2001) 12. XML DB: XUdte XML Udte Lnguge, htt:// (2004)