XWEET: XML DBMS for Web Environment

Transcription

1 XWEET: XML DBMS for Web Environment JaeMok Jeong, Sangwon Park, Tae-Sun Chung, Hyoung-Joo Kim Seoul National University Seoul, Korea Abstract XML is a standard representation format for data operated in web environment. We can integrate the heterogeneous data from several web sources into XML data. Many researchers have been working on XML processing. As a conglomerate of those works, we proposed and implemented the XWEET system. In this system, we store, extract and query XML data. XWEET has several interesting modules: XDM(XWEET data model) for representing XML data, PDM(persistent data manager), wrapper, XSI(XWEET semantic integrator), and XQP(XWEET query processor). Also, it supports an environment for generating web applications; WPG(a html/xml result generation module) and WebTP(a web-based workflow system). 1 Introduction The emerging of web changes the representation and storing method of documents. There are lots of limits to express the contents and to search the information from the existing documents. But the web overcomes all barriers, which users get the information beyond the location and time. We can see the web as a database to get the information by queries[12]. WebSQL[11], WebOQL[1] are the examples to see the web as a database. HTML is a standard language based on tag to express the documents on the web. But it is difficult to use HTML as a method to transfer the information because the purpose of HTML is for browsing documents. If many applications were designed and implemented on the web, it would be increased the importance of exchange the information between each servers. But because HTML is not adequate to exchange the information, XML is emerged to cover that needs. There are semistructured database called Lore[10] which uses OEM data model which is first proposed by TSIMMIS[5]. It has been extended to support XML data[6]. We propose XWEET 1 to manage the XML which is used as a method of information exchange in this paper. XML data can be transfered from outside the system, or can be internal data, or can be made from other data sources. XWEET is a system which evaluate XML queries from lots of XML data types on the web. There are PDM, wrapper, mediator, XQP(XWEET Query Processor), and WPG in XWEET system. It provides solution of storing the XML, translating any documents to XML, transaction system of web applications. In the next section we briefly describe the architecture and data model. 2 XWEET Data Model XDM(XWEET Data Model) is a light weight DOM[17] interface which is internal data model of XWEET. Data sources can access the data which is any format or location because of same interface. XWEET uses XML as a standard interface between each modules. But this XML data must be parsed to be used as internal objects. It could make each module be larger and more complex. To overcome this problem, small interface for XML is proposed in XWEET. Data transfer between modules which are wrapper or mediator can be implemented easily because of simple interface. XML documents can be represented as a graph like tree. XDM provides the tree traverse interfaces which are traversing parent node to child node and one s sibling node. Applications supporting XDM can be easily adapted to applications which use DOM interface because it is subset of DOM. 3 The architecture of XWEET system In this section we will explain the detail structure of XWEET system. Recently many researchers have tried to develop the system which integrates and stores the heterogeneous data. Moreover modern applications require integrated access to various information sources from tradi- 1 XWEET is pronounced as [swi:t].

2 Web Browser INTERNET External DataSource and others XWEET Web Service Manager and others Templates WPGs(WebTP) Mediator XDM Parser XML Wrapper XWS Application Modules Onthology Manager XQP XSI DATA Source Figure 1. The architecture of XWEET XWEET System Mediator XDM PDM Persistent Store tional DBMS to semistructured web repositories. XWEET system is the middle-ware system which integrates and processes the heterogeneous data in the form of XML. Figure 1 shows the overall architecture of XWEET. Wrapper extracts the important information from the external data source(for examples, HTML file in the web, e- mail, news, XML files). Data source is the lowest module of XWEET system. It works as the input source which converts several XML document and equivalent DOM structure into XDM(XWEET Data Model). XDM is explained in section 2 in detail. Data source is consist of the wrapper which extracts the information from external data sources, PDM which is the XML repository of XWEET, and XML documents, etc. PDM(Persistent Data Manager) stores XML data into RDBMS and retrieves them from RDBMS. PDM provides the interface to handle XWEET meta data. It provides the index for the rapid retrieval also. Using RDBMS as the storage engine, we can make use of the stability of RDBMS technology. XSI(XWEET Semantic Integrator) unifies the different data encoding. Because the information obtained by wrapper can be inconsistent, XSI converts the data format into a common one. Thanks to XSI, XQP can see the uniform view of XML documents. Mediator cooperates with XSI and converts the different syntax and semantic into the common format. XQP(XWEET Query Processor) processes the declarative query on the XML data from data source. Ontology Manager is in charge of the integration and reorganization on schemas and terms used in the specific application domain. Application Modules is invoked by a user request using. It processes the application specific works. WPG(Web Page Generator) generates web page, WPG can be implemented using CGI and Servlet. Templates is a part of WPG and help the easy creation of web pages. XWEET Web Service Manager manages the data and control flow among WPGs. Document(id, name, url) Node(id, name, type, docid, parid, val, attr, groupping) Attribute(id, name, elmid, val, type) 3.1 PDM Figure 2. Schema of PDM PDM[13] is a local storage system to store the XML data used by local system, which is implemented by Java/JDBC. There are two methods to store XML data to database. First method is preserving the semantics of elements when we store each elements into database. Second method is preserving the document structure when we store objects into class or table like element, document, attribute[4]. The advantage of the first method is that users can query directly by native query languages and the possibility of object clustering can be increased. But it is difficult to gather the structure information of a document without DTD. XML without DTD could be a semistructured data. If the structure of elements are changed, there is no other way to store objects except overflow node. If the XML DTD is changed to another form when we using a wrapper to store and retrieve XML data, wrapper has to be re-designed to make new schema and re-compiled to support new schema. Therefore it is hard to generate a wrapper automatically. The flaw of second method is missing the semantic information of each elements. A XQL query is translated into lots of SQL queries. Each SQL query fetches an object using OID. It diminishes join query because path expressions of XQL which means lots of join predicate are translated to non-join SQL queries. So PDM translates each XQL into simple object fetch queries like select * from Node where id = OID. SQL statements executed in PDM are precompiled so the cost of parsing query is very cheap. The id is indexed, so the performance is not quite different from object oriented database which use logical OID because the logical OID is managed by B-Tree index. 3.2 XWS The data which is not stored in PDM is processed with wrapper. Wrapper converts the lower level object(html, , etc) into XDM. Figure 3 shows the example of XML-QL. This example extracts title and price tag from bib.xml file in host. Because there is the various sources(e.g. from PDM or web page, etc) in XWEET system, we extend in clauses of XML-QL to point out the origin of sources.

3 where <book> <title> $T </title> <price> $p </price> </book> in " construct <result><booktitle>$t</booktitle> <bookprice>$p</bookprice> </result> Figure 3. Example of XML-QL Mapping Module Element Mapping Extraction Module TreeBuilder HTMLTools Node Retrieval Module Acquisition Cache Web Data Source(URL) Script File repository Figure 4. XWS architecture XWS[7](XWEET Web-wrapper System) converts the HTML file read from Web into XML file using script configuration language. The script file describes the process how to convert HTML into XML, Each script file has the same naming in the data source configuration. XWS is the library and program implemented using Perl[18], which has an good advantages of string processing. Figure 4 shows the architecture of XWS. XWS is composed of three layers: retrieval, extraction, and mapping module. Wrapper designer describes the meaningful region of HTML pages using declarative script file. The extracted data is converted into XML file using DTD generation language in Mapping module. Using the high-level operator designed by object oriented methodology, XWS extracts the significant data based on script file. If operator provided in XWS is too abstract to control the subtle part of given HTML, built-in string function or external module of Perl can be used in script file. Because the script file used in XWS is by itself perl script, it can be executed without any compilation process. In addition, if XWEET system doesn t have the necessary module for script file, it automatically download modules from the well-known repository. So the configuration file, which translate the specific HTML pages to XML, can be shared among several XWEET systems. 3.3 Mediator and XSI It is hard to determine whether two objects from the different information sources are the same. We assume that the user of XWEET system know the global schema(dtd) in advance. Even to describe the same data, a lots of representation on the data exists. For example, to describe a person s name, one can use name terminology and others use first name and last name. Above the data sources in Figure 1 lie Mediator and XSI. Mediator refines the input sources. If necessary, mediator asks XSI(XWEET Semantic Integrator) about global schema. Mediator performs conversion of DTD, elimination of duplicated element, etc. In XWEET, we have implemented relatively simple conversion routine. We think further research is needed. 3.4 XQP XQP is the module to process the declarative query on several XML data passed by data sources. XML data can be mapped semi-structured data model based on graph[14, 15]. Assuming that O is a set of infinite object identify, C is a set of infinite constant set, and O and C has no intersection. data graph DB can be defined with DB = (V E) in the semi-structured data model. V O is the set of node, and E V C V is set of directed edge. The element of XML data can be mapped into node V, the tags mapped into edge E. Query is based on the regular path expression on data path graph. Regular path expression is defined as follows. Definition 1 Regular path expression is defined to R = jaj j(r 1 :R 2 )j(r 1 jr 2 )jr. R, R 1, R 2 represents regular path expression, a 2 C does the specific constants, and does empty string. Thus, processing query can be mapped into the work to find the node of graph which satisfy regular path expression on data graph DB. The objective of XQP design is to process the query efficiently. DTD(document type definition) in XML helps to process the query efficiently. Query in Figure 5 will find and generate element, if the name element of person.xml file in PDM is the same as author element in XML file created DBLP-title wrapper. DTD on XML data provides the following information; Sub-element is optional in Person element. User name can be expressed either <name> or <first name> and <last name>. This hints helps the query processor to traverse only the node with name element, and not to traverse the node without sub-element. That is, XQP provides NodeInfo and MergeNodeInfo[2]. Using NodeInfo, query processor can determine whether it is necessary to traverse

4 where <person> <name>$p</name> < >$e</ > </person> in "PDM:person.xml" <item> <authorlist><author>$p</author> </authorlist> in "DBLP-title:title=database" </item> construct <result> < > $E </ > </result> Figure 5. Extended XQL used in XQP sub-element or not. And MergeNodeInfo structure of each node has the label information about every sub element. In addition, XQP uses query optimization technique using materialized view. If the query submitted by user may contain the view of the previous query result, the previous query result is used for optimization. XQP provides 2 - index-join[3] technique to exploit materialized view. For an efficient processing on a materialized view, we extend 2-index[16] technique into 2 -index-join. And we implemented an algorithm to rewrite query into another query using materialized view. 3.5 WPG and templates Electronic business processing has been drastically grown since the advent of the World Wide Web. Middletier application use the web page generator to publish its result on web. CGI(Common Gateway Interface) is used in common as WPGs. CGI has an advantages over the other approaches; it can be easily implemented by any languages. Perl, C++, and C is the prevalent language these days for CGI program. The CGI program is executed at the arrival of every corresponding. Servlet, active server page, and PHP is developed to reduce the overhead of the process invocation. But because this technology is developed for the general application based on web, it didn t provide the solution about business processing and transaction processing yet. In addition, software development methodology such as object oriented paradigm and component programming hard to be applicable to CGI programming environment. And it is hard to reuse the existing program and modules[8]. To solve this problem, we have implemented WebTP, web transaction server system in XWEET[9]. To preserve the consistency of the system, WebTP makes use of web page logging, HRSN( Request Sequence Number) and cookie logging. Work concepts in WebTP make it possible to implement the complex web application in the structured way. Web application with several page can be treated in one transaction boundary. Each page function is numbered according to execution order. If user requests the wrong pages, WebTP prevents the access of the wrong page and guarantee the consistency of the system. Due to this context management, WebTP provides save point and partial rollback in web application. 4 Conclusion There has been much interest recently in integrating heterogeneous information sources. In this paper, we presents XWEET system, which is middle-ware system to manage and access the heterogeneous data from several information source in uniform way. In this system, we store, extract and query XML data. XWEET has several interesting modules: XDM(XWEET data model) for representing XML data, PDM(persistent data manager), wrapper, XSI(XWEET semantic integrator), and XQP(XWEET query processor). Also, it supports an environment for generating web applications; WPG(a html/xml result generation module) and WebTP(a web-based workflow system). For the future works, we intend to enhance index technique in XWEET system; regular expression processing in multi-path index. And we plan to provide GUI tools to manage and tune overall XWEET system. XWS system will have heuristic module to cope with changing the target HTML page. References [1] G. Arocena and A. Mendelzon. WebOQL: Restructuring documents, databases and webs. In Proc. of 14th. Intl. Conf. on Data Engineering (ICDE 98), [2] T.-S. Chung and H. Kim. Enhancing query evaluation of xml using document type definitions. Technical report, Seoul National University, Feb (in Korean). [3] T.-S. Chung and H. Kim. Enhancing query evaluation of xml with materialized views. Technical report, Seoul National University, Feb (in Korean). [4] D. Florescu and D. Kossman. A Performance Evaluation of Alternative Mapping Schemes for Storing XML Data in a Relational Database. INRIA Technical Report No.3680, May [5] H. Garcia-Molina, J. Hammer, K. Ireland, Y. Papakonstantinou, J. Ullman,, and J. Widom. Integrating and Accessing

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