Towards a High Integrity XML Link Update in Object-Relational Database

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1 Towards a High Integrity XML Link Update in Object-Relational Database Eric Pardede, J. Wenny Rahayu, and David Taniar Department of Computer Science and Computer Engineering, La Trobe University, undoora VIC 08, ustralia {E.Pardede, W.Rahayu}@latrobe.edu.au School of usiness Systems, Monash University, Clayton VIC 800 David.Taniar@infotech.monash.edu.au bstract. With the increasing usage of XML database, XML update has become an important issue in the database community. How updates affect the XML documents need to be investigated further. In this paper we propose a methodology to maintain the integrity of updated XML documents by maintaining the consistency of XML Link. XLink and its subsequent XPointer are WC standards and used to provide referential purpose among XML documents or nodes. Since XML Link is embedded as an attribute in an XML instance, our proposal can be used for schema-less documents and for instance-based references. Our proposal is targeted for Object-Relational Storage, one of the most widely used repositories for XML document. While the XML documents are stored as a CLO XML Type, our update methodologies are implemented as a set of functions that perform checking mechanism before updates. Introduction Due to the dynamic nature of the web application, we have witnessed a growing number of XML documents that require regular update. XML update methodology has been discussed in a few works [, ]. Moreover, the researches on constraints preservation during update operations are even fewer. [9] discussed the issues on capturing semantic constraints during XML update. However, it is applicable to schema-based XML documents. Unfortunately, very frequently we have to store schema-less XML document in our database repository. For this case, the update methods in [9] face a big problem due to the non-existence of schema. Furthermore, for some cases even to the XML with schema- the constraints are not schema-based but more to be instance-based. In this paper we aim to propose a methodology to update XML document without schema bound. The methodology will preserve the referential integrity constraint that exists through some XML linking technologies: XML Linking Language (XLink) [5] and XML Pointer Language (XPointer) [6]. M. Gavrilova et al. (Eds.): ICCS 006, LNCS 980, pp , 006. Springer-Verlag erlin Heidelberg 006

2 Towards a High Integrity XML Link Update in Object-Relational Database 47 In this paper we maintain the links if the XML documents stored in XML Enabled- Database that is based on Object-Relational (OR) Repository. In this database family, the XML document is stored as a Character Large Object (CLO) XML Type. When we store an XML document in our database, we create a record in a link table. This table will take roles as a lookup repository. Every time we update an XML document, the checking mechanism in forms of PL/SQL will be triggered and access the link table. It will determine whether the update can violate the referential integrity of the database. ased on this decision, the update will be performed or cancelled. Figure illustrates the proposed work. tti CLO Type XML Documents Store XML in XML Table Store Link in Link Table xlink: #... (S D5) xlink: #... (S D6) Source Destination S S D5 D6 Checking Mechanism Update Query PL/SQL XML Data Update Methodology Fig.. Proposed Work In section we provide basic information regarding the XML linking technologies. In this section we also classify the linking based on the sources and destinations of references. In section we briefly introduce the usage of OR Database for storing schema-less XML Document. Section 4 proposes the methodologies for the update operations insertion, deletion and replacement. In section 5 we provide the implementation of our proposed methods and brief analysis. Finally our work will be concluded in section 6. XML Linking Technologies Linking in XML allows users to create a complexly structured network of distributed resources [7]. WC has been working on two important linking standards namely XLink and XPointer. While the former has been released as a recommendation [5], the latter is still a candidate for recommendation [6]. XLink is used to describe complex associations between resources identified using Unified Resource Identifiers (URI). Sometimes XLink has added XPointer component for more detailed reference. XPointer, which is built on the XML Path Language (XPath) [4], is used as the basis for the fragment identifier for any URI reference that locates a resource. The fragment may be a single XML element or a

3 48 E. Pardede, J.W. Rahayu, and D. Taniar collection of elements. The only limitation is that the resources must be an XML Document. Examples of the links are shown as follows. <rticle xmlns:xlink= title= Linking Language in XML Document > <firstchapter> <para> (! - --) our previous work on <article xlink:href= rticle.xml#xpointer(title( Referential Mechanism in XML Data Model > Referential Mechanism in XML Data Model </article>. Our main interest </para> <para> </para>..</firstchapter></rticle> We can associate a link in XML with directed labeled graph. The resources are the vertices and the link itself is the edge of the graph. ased on the vertices we can classify the links into three types (see Fig.): () a link between two resources in the same collection, () a link between two resources in the same database, but from different collection, and () a link between two resources from different database. COLLECTION ROOT- DTSE COLLECTION ROOT- EXTERNL SOURCES URIs URL Collection Elementi Elementi+m Elementj Elementj+n Element Fig.. XML Link Classification based on the Vertices a. Single forward link b. Multi-level forward link c. Multiple forward links d. Single reverse link C C e. Multi-level reverse link f. Multiple reverse links g. Cyclic link C C Fig.. Internal Link Classification based on Path

4 Towards a High Integrity XML Link Update in Object-Relational Database 49 mong different links, it is hard to maintain the persistent reference using link type. There are current content management systems that contain modules for checking external links regularly. However, it usually draws a high overhead cost and manual intervention and thus, is not desirable [7]. Now we have decided to cover only internal links, we are aware that we need further classification. This is necessary before we can propose the methodology to checking the validity of the update operations. There are various paths that an XML Link can take to connect XML nodes/documents. We show the paths in this section using the graphs in figure. The node depicts the document and the line depicts the link between the documents where the vertex points to destination document. Having classified the links, we can continue by proposing the update methodology in ORD as the storage. However, we will first briefly discuss the use of ORD for schema-less XML storage. Storing Schema-Less XML in Object-Relational Database ORD is increasingly popular as XML storage. It is associated with its ability to capture the OO modeling semantic and the maturity of relational implementation. With many new data structures introduced in the current SQL [, 7], the modeling power of ORD has increased. In particular, complex data structures such as row type, user-defined type and collection types can be utilized in XML storage. There are few works that have discussed the usage of ORD for XML storage. They map different schema languages such as DTD and XML Schema into the OR Schema either by flattening the trees [, 6, 8] or by maintaining the tree structure of the document [0]. These works are applicable for data-centric XML Document. In other words, we need schema along with the XML documents. In practice, sometimes we have to store a non-schema based XML documents. For this purpose, some ORD products have extended their products into XML-Enabled Database. This database allows users to store their XML documents even though they do not have a defined schema. The XML documents are usually stored as a Large Object (LO). D is one of the major relational products, developed by IM. It supports the XML by using two extended features: XML Extender and Text Extender [4]. SQL Server 000 supports the XML in three ways: FOR XML statement, XPath queries based on annotated schema and OpenXML functions []. Oracle is a major OR product that has tried to accommodate the XML data in their last three versions. t the time of writing, the latest version of Oracle 0g has supported XML Type. It is a predefined object type that can store an XML document []. The XML Type can be implemented as a CLO or as an object-relational storage []. CLO is mainly used for a document-centric XML. It can be useful if there is no schema available for the document. It supports text indexes for part document search. OR storage is used for data centric document, where some update management is required. For this storage, there are two options that can be followed. We can store the XML as a table or as a column of a table. In this paper we will use the CLO XML Type in Oracle 0g for implementation of our update methodology.

5 50 E. Pardede, J.W. Rahayu, and D. Taniar 4 Proposed Methodology In this section we propose the methodology for different updates (insertion, deletion and replacement). The methodology preserves the referential integrity constraints in the updated documents. The constraints are specified in the linking language attributes of the document. The link information is stored in a separate table to the actual XML document. The main purpose for this table is not only to store the links but also as a look up reference before an update operation. y doing so, we can prevent broken links in our XML documents. The link table only stores the internal link. These links are link type and type as it is mentioned in section. To maintain the external link, we will not have control on the content and therefore some mechanisms that monitor the changes in the content might be required. The link table also contains a column store the source of the link. This information is necessary during the update process. Source Destination T Tm T Tn T CLO Type T CLO Type T T Source Destination T Tj T5 Tk Fig. 4. XML Document and Link Storages efore we describe the methodology, we show the framework of how these methods will work. In figure 4 we have two tables, each has a CLO attribute that stores the XML. In addition to the CLO attribute, they also have an identifier attribute that can be generated every time they are inserted in the database. These identifiers are necessary for defining the source and the destination of the link attribute. In addition to the storage tables, we need additional tables that store the link information from the source to the destination. For a binary relation, we will need two link tables with different direction of source and destination (see Fig. 4). 4. Insertion Methodology Every time we insert a new XML document as a CLO in the table, we need to check whether it contains an internal link attribute. If there is an internal link, our proposed method checks the link source and destination.

6 Towards a High Integrity XML Link Update in Object-Relational Database 5 In algorithm we check whether the actual destination already exists in our database. If the link attribute contains a valid link, we need to register it in the link table. If the link attribute is invalid, we need to follow one of these options: (i) for restrict strategy, the insertion has to be cancelled, (ii) for nullify strategy, remove the link attribute from source document, (iii) for cascade strategy, insert the new document that will become a destination and register the link in the link table. Note that this algorithm is applicable not only for single forward link, but also the other two forward links. lgorithm. Insertion Update for Forward Link Level 5 C Level 6 D b..b..bi c..c..c j Level d..d..d k XML TLE Note: Element 4 b, b, bi ttributes 4 LINK TLE Operation In every level Operation (Read the element) Operation (Read the attribute) FOR LL link attribute Operation (Check XML Table) IF destination exist THEN Operation 4 (Register the link) ELSE CSE Restrict : Stop insertion CSE Nullify : Insert NULL in Link table CSE Cascade : IF there is NO cyclic link THEN repeat alg.8. for new source insert the new document, 4 ELSE insert the new document [] 4, insert reverse link Operation 5 (Move to next element) Operation 6 (Move to the next level) Unlike forward link, there is no specific checking mechanism is required for reserve links. For example in figure d, the insertion of without a will not raise any referential integrity problem. On the other hand, the insertion of with already existed is an unlikely case since during the insertion of we have already follow algorithm.

7 5 E. Pardede, J.W. Rahayu, and D. Taniar Finally, we can see a cyclic link (see figure g) as two single links. However, if we use algorithm, we end up with a recursive loop without a stop condition. Therefore, we have different condition [] as a sub-procedure. Note that we will have a two link tables with reverse values. However, it is very unlikely for us to implement cascade strategy for this one since during insertion of one document we might not have the information for the other document, which is being linked. 4. Deletion Methodology The main concern for deletion is to avoid deleting a target that is referred by other document(s). In our methodology, we perform the checking to the link table before any deletion. For reverse link we use algorithm. We need to check whether there is any internal source actually refers to the target node and perform maintenance operation according to the strategy: (i) for restrict strategy, cancel the deletion, (ii) for nullify strategy, remove the link attribute from the referring document(s), (iii) for cascade strategy, delete all referring document(s) and delete all the links to this target in all link tables. lgorithm is applicable not only for single reverse link, but also for the other two forward links. For a cyclic link, we need to add an addition checking function []. We do not show the algorithm for deletion of forward link due to the page limitation. Note if there is a case where a document has a forward link to a document and reverse link from another document, we need to perform two different checking mechanisms. lgorithm. Deletion Update for Reverse Link Level 5 C Level 6 D b..b..b i c..c..c j Level d..d..d k LINK TLE Note: Element 4 4 XML TLE b, b, b i Operation ttribute and Leaf Node

8 Towards a High Integrity XML Link Update in Object-Relational Database 5 lgorithm. continued In every level Operation (Read the element) Operation (Check the attribute and leaf node) FOR LL attribute and leaf node Operation (Check the Link Table) IF exist as destination THEN CSE Restrict : Stop deletion CSE Nullify : Operation 4 (remove the link in Link Table) CSE Cascade : IF there is NO cyclic link [] THEN repeat algorithm 8. with new source document delete the new source document, 4 ELSE delete the new source document 4, remove the reverse link ELSE () Operation 5 (Move to next element) Operation 6 (Move to the next level) 4. Replacement Methodology For replacement operation, we need to ensure that the old value deletion satisfies deletion checking mechanism and the new value insertion satisfies the insertion checking mechanism. For forward link, we use algorithm, which is very similar to algorithm. This algorithm checks whether the new value is a link or not. If it is a link, whether the link refers to a valid link and act according to the predefined strategy. This algorithm also applies for multiple forward link. While the algorithm for forward link replacement almost resembles the forward link insertion, the algorithm for reverse link replacement almost resembles the reverse link deletion (see algorithm ). The main concern is whether the old value being replaced is actually being referred by other document(s). We do not show the algorithm for replacement in this paper due to the page limitation. 5 Implementation of Update Methodology We have fully implemented the proposed update methodology. We use Oracle 0g as the XML-Enabled DMS. Further release of Oracle can also be used for this implementation. In this product our XML documents are stored in tables as CLO XML Type. Therefore, schema is optional. The proposed methods are implemented as PL/SQL triggers. Inside these triggers we will use some built-in functions specifically available for CLO object [] that are based on standard SQL/XML [5]. For the case study, we use XML documents that contain information of article, person and publication. efore we implement the methodologies, we perform the preparation of the storages. There are three repositories for XML Documents, each for different content. In addition we will need link tables for every set of link direction. We implement the methodology for each operation and every link type with different maintenance strategy. Not all strategies and operations are implemented. The decision of not implementing some strategies in some operations is based on the usability and practicality of the checking mechanism. For example, we do not need

9 54 E. Pardede, J.W. Rahayu, and D. Taniar reverse link insertion check since it will not affect the referential integrity problem. nother example, we do not implement cascade strategy for forward link insertion since in practice we do not always have full information for the referred document. Due to the page limitation, we do not show the actual implementation in this paper. We only show the summary. Table summarises the implementation of the checking methodologies for insertion (I), deletion (D), and replacement (R) updates. The ones with ticks are those that are likely to be found in practice and thus, are implemented. Table. Insertion Update Methodology Implementation Restrict Nullify Cascade I D R I D R I D R Single Forward Link a a a a a a r a a Multiple Forward Link a a a a a a r a a Multi-Level Forward Link a a a a a a r a a Single Reverse Link r a a r a a r a r Multiple Reverse Link r a a r a a r a r Multi-Level Reverse Link r a a r a a r a r Cyclic Link a a a a a a r a r y implementing the update methodology using XML-Enabled Database, we have utilized the technology that is considered mature. Users are also familiar to the implementation and the standard is used by most vendors. It is also backed up with the well-accepted relational model foundation. Nevertheless, it still has a shortcoming. In our implementation, the links in the tables are in document level. Thus, we do not know the element/attribute in the document that actually been referred. In addition, the links need different tables based on the source and destination. If we want to avoid this shortcoming, another option is by storing this XML type using Native XML Database. However, it is still not supported by widelyused standard [8]. 6 Conclusion We propose checking methodology to preserve the referential integrity during an update of XML. The proposal checks the references that are implemented by using XLink and its subsequent XPointer. Since these reference mechanisms do not require a schema, our methods can be used in schema-less XML documents as well as in instance-based reference in XML documents. We propose a set of checking functions that are triggered before the actual update takes place. The checking functions check the destination of the link, register or remove the link in the database, and perform other operations before deciding whether the update can be proceeded or not. Our methodologies are targeted for XML-Enabled Database as the storage. Our methodologies avoid the referential integrity violation that can be caused by updating the XML links inside an XML document. These methods have added some new functionality to the current XML data management practice and they are usable both for database and document communities.

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