Summarization of XML Documents

Transcription

1 Summarization of XML Documents Hesham Elzentani, Prof. dr Mladen Veinović Abstract EXtensible Markup Language (XML) has become a standard of data exchange and representation in many applications. An XML document is usually too complex and large to understand and use. A summarized XML document of the original document is useful in such cases. This paper introduces semantic XML document summaries, which present the important information available in an XML document, three standards are given to evaluate the final summarized XML document: document size, information content, and information importance and approaches for the summarization of XML documents. well-formed XML document showed in Fig. 1 [3]. Index Terms XML Documents, Summarization. I. INTRODUCTION TO XML DOCUMENTS The extensible Markup Language (XML) was developed by an XML Working Group (originally known as the SGML Editorial Review Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996 [1]. XML is an open standard providing the means to share data and information between computers and computer programs as unambiguously as possible. Once transmitted, it is up to the receiving computer program to interpret the data for some useful purpose thus turning the data into information. Sometimes the data will be rendered as HTML. Other times it might be used to update and/or query a database and also it might be used in E-business and E-payment transactions. Originally intended as a means for Web publishing, the advantages of XML have proven useful for things never intended to be rendered as Web pages [2]. XML is a text file format for representing tree structures in a standard form. The whole structure of an XML document, if we abstract over less important details, is a tree of variable entity, in which nodes (also called elements) are labeled, leaves of the tree are text nodes, and the ordering between children of a node is significant. XML can be seen as a concrete syntax for describing such tree structures using mark-up texts. Fig. 1 shows an example of an XML document [3]. The XML specification does not fix a priori the set of allowed labels in an XML document nor it defines any semantics for labels. Only well-formed conditions are defined in particular to ensure proper nesting of elements, which allows to consider XML documents as trees. For instance, Fig. 2 gives a more visual tree representation of the previous Hesham Elzentani is a PHD student with Faculty of Informatics and Computing, University of Singidunum, 32 Danijelova, Voždovac, Beograd, Serbia ( hesham342002@gmail.com). Prof. dr Mladen Veinović is with Faculty of Informatics and Computing, University of Singidunum, 32 Danijelova, Voždovac, Beograd, Serbia ( mveinovic@singidunum.ac.rs). Fig. 1. Example of XML document. Fig. 2. Sample Tree of a Well-Formed Document. A. Elements and Attributes. Three important terms for describing basic XML syntax are elements, attributes, and documents. All three of these terms are special and important; they encompass a large portion of the conceptual playing field for XML [4]. Markup is often divided into two separate vocabulary words when we are talking about XML: elements and attributes. An element is a start tag <... > and an end tag < /... > including the stuff inside of it and an attribute is a simple name-value pair where the value is in single or double quotes. An attribute cannot stand by itself and must be inside the start tag of a given element. Two short examples followed [4]: <Food> Ice Cream </Food> <Food Flavour="Chocolate">Ice Cream </Food> The first line in the previous example is an element called

2 Food that has Ice Cream as its element content. The second line in the previous example is the same element with a name-value pair added to it (this is an attribute). The name is Flavour and the value is Chocolate. Elements may also be empty, having no element content. This is shown in the example below [4]: <HealthyFood></HealthyFood> <HealthyFood/> The first line in the previous example is an element called <HealthyFood> that has nothing inside of it. The second line is shorthand for the same empty element [4]. 5. The number of child elements. 6. Whether an element is empty or can include text. 7. Data types for elements and attributes. 8. Default and fixed values for elements and attributes. In general, XML Schema Structures specifies the XML Schema definition language and XML Schema Data types specifies extensible data types for XML. Fig. 3 shows an example of a simple XML schema document to describe an address. B. Well-formed XML documents must be well-formed by defining simple syntax rules for the legal positioning of elements and attributes. The data represented in XML documents are wellformed if the following points are achieved [4]: 1. There is exactly one root element. 2. Every start tag has a matching end tag. 3. No tag overlaps another tag. 4. All elements and attributes must obey the naming constraints. C. Validity A valid XML document is a well-formed document that also conforms to the stricter rules specified in the Document Type Definition (DTD) or in the schema (XSD). The DTD describes a document s structure, specifies which element types are allowed, and defines the properties for each element. A well-formed document is not checked against an external DTD [5]. Schemas, like DTDs, describe the structure of the document. But schemas are more powerful than DTDs, because they can also describe the structure of other information, such as databases. They also provide additional information about inheritance and data types in the XML document [5]. D. Schema Languages. Schemas describe structural constraints for XML documents. There are many formalisms (called schema languages) for specifying schemas (or types ). For instance: DTD, which is part of the XML specification, XML Schema (W3C), and RELAX NG (OASIS/ISO) are actively used by various applications. Each schema language has different constraint mechanisms and different expressivenesses [3]. The purpose of an XML Schema is to define the legal building blocks of an XML document, just like a DTD. XML Schema has better support for applications, document structure, attributes, and data-typing. An XML Schema must define [9]: 1. Elements that can appear in a document. 2. Attributes that can appear in a document. 3. Which elements are child elements. 4. The order of child elements. Fig. 3. XML schema document to describe an address. E. XML Technologies. W3C produces and manages the XML specifications and technologies. The followings are some of them [7]: 1. XML Namespaces enable the XML document to contain elements and attributes without any naming collisions occurring. 2. XPointer is a system for addressing components of XML-based Internet media. 3. XSLT is a declarative, XML-based document transformation language. 4. XInclude defines the ability for XML documents to include all or part of an external file. 5. XML Signature defines the syntax and processing rules of creating digital signatures on XML content. 6. XML Encryption defines the syntax and processing rules of encrypting XML content. 7. XPath lets it possible to reach to individual parts of XML document. XPath expressions can refer to all or part of the text, data, and values in XML documents. 8. XQuery is a query and functional programming language that is designed to query collections of XML data. In fact, XQuery uses XPath as its sublanguage.

3 F. Processing XML Documents An XML document may be stored as a plain file in a file system of any operating system as well as in a database management system (DBMS). There are three ways to process an XML document: 1. SAX (Simple API for XML), SAX is an event based sequential access parser API for XML documents, in which the processing is made on a tag-at-a-time basis without the need to load the entire document into memory and provides a mechanism for reading data from an XML document [3]. 2. DOM (Document Object Model), is an API designed for structured documents in general, which the document is first entirely loaded into memory and then processed, DOM operates on the document as a whole [4]. 3. A transformation language such as XSLT, which is a language for transforming XML documents into other XML documents and describes rules for transforming a source tree into a result tree [6]. SAX or DOM have advantages and disadvantages. SAX normally requires less memory space than DOM to process a document. However, SAX processing is limited to only oneway direction. In SAX, when an element/attribute is processed, there is no way to return to it. In contrast, DOM can navigate throughout the document, in both forward (rootto-leave) direction and reverse (leave-to-root) direction. DOM will require, however, a memory space proportional to the document size which may not be suitable in many practical situations. G. XML Data Management. Both shredded and native database processing of XML documents also have advantages and disadvantages. For shredding processing, a relational database engine is normally used. In this case, a document is mapped to (a set of) tables and columns, thus breaking its native structure. For instance, a row may contain a document node and each column can store information regarding the document node (e.g., element/attribute name and/or value). Using a relational engine, one can benefit from proven features of the relational database management systems such as transaction management and query processing and reuse them. An additional software layer should be provided to enable document mapping and unmapping. This layer should provoke a non-negligible burden because, as the XML document is shredded to enable its use in a relational storage, it must be reconstructed as a result of a query. Nevertheless, a shredded document is processed as relational data, not taking into account the specific needs and idiosyncrasies of a native XML data management. Instead the processing unit being a document, it is a table. A document query in a shredded scenario is made with SQL language or SQL/XML, an extension of SQL enabling specific document operations and (a limited form of) XPath/XQuery expressions. Pure XML data management systems (XDBMS), in turn, store an XML document, keeping its entire tree structure. Normally, B-trees are used as supporting structure to hold the document order. An XDBMS uses XPath and/or XQuery for querying stored documents. Query results are also XML documents which are sent back to the user with no need for remapping [8]. II. XML DOCUMENT SUMMARIZATION extensible Markup Language (XML) has become one of the de facto standards of data representation over World Wide Web, Web services based on XML technologies also have been emerging for exchanging structured information among organizations and in mostly applications and elsewhere. More and more data are stored in XML format [10][11]. To understand these XML documents with complex structure and abundant data, a human being must spend much time to read such documents. In some cases, it is impracticable, even not impossible, for a human being to read the whole XML document when the document is very large and complex. So it is necessary to present a human being with a summarized form of the original complex and large XML document. Such a summarized XML document is also useful in other applications: querying XML documents, comparing two XML documents, displaying or storing XML documents in a mobile or embedded device which has limited CPU processing ability, display screen and storage spaces, etc. Although, such a summarized XML document is very useful, it is difficult to generate a good summarized XML document. So the challenge of summarizing XML documents is how to generate such a summarized XML document. A summarized XML document should grasp the core information of the original document so that a human being can have a basic understanding of the original document. Of course, such a summarized XML document should have less size than the original document considering the storage space and complexity. A good summarized XML document can be evaluated by the following three standards [10]: 1. Document Size: The first goal of summarizing XML document is to obtain an XML document with an acceptable size comparing with the original one according to specific applications. In general, an XML document of smaller size is more readable than a larger one for a human being. Suppose the sizes of the original and the summarized XML document are S original bytes and S summarized bytes, respectively, so the summarized ratio of a summarized XML document respect to the original one is given in (1). = (1) 2. Information Content: A perfect summarized XML document should contain the entire information content of the original one, i.e., it is equivalent to the original one in the aspect of information content. But in reality, it is impossible for a summarized XML document with less size to contain the entire information content of the original document which

4 has no redundant information. Suppose the number of element values of the original XML document without redundant information and the summarized XML document are C original and C summarized, respectively, so the information content ratio of a summarized XML document respect to the original one is given in (2). parent and/or sibling may sometimes be of importance. For example Fig. 4, in the context of movies, we cannot have a <prodyear> without also having the <title> as well. However, we can have the <title> without the <prodyear>. = (2) 3. Information Importance: As a summarized XML document cannot contain the entire information content of the original one in most cases, it is necessary and practicable to contain the most important information of the original one. Although, there are two kinds of summaries can be generated [24]: 1. A generic summary, summarizes the entire contents of the document. 2. A query-biased summary, summarizes those parts of the document which are relevant to the user s query. A. Challenges in XML Summarization. A summary is useful if, at the very least, it helps the user to decide whether a particular document is worth looking into its entirety or not. The best summary would encapsulate most or all the salient points of the document and it could in many cases serve as a replacement for the original document. However, generating the best summary involves satisfying two contradictory goals: maximum coverage and minimum space. Good balance between the size of the summary and its coverage is required (for example see Fig. 4 and Fig. 5). To achieve these goals must applying the following steps [24]: 1. Informativeness: The notion of informativeness is intuitive. For a unit of information (Elements, Text) to be informative to the user, it has to be important in the document and presented concisely to the user. This follows directly from the goal of summarization which is to present the salient points of the document in a concise manner to the user. 2. Non-redundancy: Redundancy in XML data is very explicit. From Fig. 4, the same element, say, <keyword> could occur multiple times, once for each keyword associated with a given movie. Clearly, it is not necessary to repeat all occurrences of the element in the summary, but instead, concisely represent it using a single element. 3. Coverage: Coverage is closely related to informativeness and refers to the amount of information in the summary as opposed to the data. We may choose, for example, to leave out certain elements since they are not important enough. While this may improve the readability of the summary, it simultaneously reduces the coverage. 4. Coherence: The context of a element in terms of its Fig. 4. XML Document of a movie. Fig. 5. A Concise Summary of Fig. 4. III. RELATED WORKS Text summarization uses Extraction and abstraction methods. The summarization process performs analyzing the source text, determining its salient points, and synthesizing an appropriate output. Text summarization can be indicative, informative, or critical. In single document text summarization, a ranking framework is better function than a classification framework. In general, text summarization focused on free-flowing texts in text datasets, which is not always applicable to XML summarization as the structure information and semantic information [12][13]. XML schema summarization is one related topic which

5 summarizes XML schemas rather than XML documents. A schema summary can be of great help, providing a succinct overview of the entire schema, and making it possible to explore in depth only the relevant schema components [14]. XML structure summarization is another related topic which summarizes XML structures rather than XML documents. Clustering XML documents by structure. Modeling the XML documents as rooted ordered labeled trees, to improve the performance of the distance calculation and at the same time to maintain or even improve its quality [15]. The significant XML disadvantage is document size: tagging a set of data increases the space needed to store it, the bandwidth needed to transmit it, and the time needed to parse it, so compression techniques (Relax NG, XMill, gzip, bzip2, Huffmann, etc.) based on the document type are another related topics to reduce the document size [16][17]. Another topics focused on constructing XML summarization for XML efficient query estimation: StatiX explores schema transformation and schema validation to obtain statistics for query selectivity estimation in XML documents. StatiX uses histograms to summarize both the structure and values in an XML document [18]. TreeSketch synopses can produce fast, accurate approximate answers for XML documents. Unlike earlier techniques focusing solely on selectivity estimation, TreeSketch synopses are much more effective in capturing the complete tree structure of the underlying XML database [19]. Bloom histogram is a framework for XML path selectivity estimation in a dynamic context. Compared to other alternatives, such as Path Tree and Markov Table, it is of smaller size yet offers superior accuracy [20]. Xseed is a method to estimate cardinality of Xpath queries, it is accurate, robust, efficient, and adaptive to memory budgets. Xseed starts from a very small kernel, and then incrementally updates information of the synopsis. With such an incremental construction, a synopsis structure can be dynamically configured to accommodate different memory budgets. Cardinality estimation based on Xseed can be performed very efficiently and accurately [21]. Veronica Mayorga and Neoklis Polyzotis, proposed a method to summarize XML data streams other than XML documents. They introduced a technique for approximately answering a complex aggregate query over an XML stream using limited memory. The main novelty of the proposed technique is that it supports XML queries with any combination of the common XPath axes, namely, ancestor, descendant, parent, child, following, preceding, followingsibling, and preceding-sibling. At the heart of our method lies an efficient transform that reduces a continuous XML query to an equi-join query over relational streams [22]. A semi-automatic method to summarize XML collections where the user can specify semantically relevant features for an XML collection in a template, and define rules for summarization. The system assists the user in generating one or several such templates, selects applicable templates for a given collection, and applies them for automatic summarization [23]. Maya Ramanath and Kondreddi Sarath Kumar, focused on the generation of generic summaries of XML documents and a method of XML document summarization based on document itself alone. They proposed techniques to automatically generate concise, readable summaries of XML documents subject to a memory budget. The resulting summary is shorter but conveys all the important information from the original document also they addressed the three main issues which arise in producing such meaningful and concise summaries: which elements or text units are important and should be included in the summary, how can the selected elements and text be presented in a concise and coherent manner? and how to generate a semantic summary for different memory budgets? [24]. EXsum (Element-wise XML summarization) is framework for the summarization of XML document properties, which can capture statistical information of all important XPath axes related to (the nodes having) the same element name in a document [25]. IV. CONCLUSION The summarized XML document can help a human being to understand the original large and complex document well and present a clear and core information of the original one. In general concept, XML summarization still remain an open problem. This paper introduced a brief introduction to XML documents, XML Schema Languages, XML technologies, XML documents parsers using SAX, DOM or transformation languages (XSLT,..etc.). Also introduced XML document summarization, a good summarized XML document can be evaluated by Document Size, Information Content and Information Importance. There are two kinds of summaries which can be generated: A query-biased summary and A generic summary. To generate best summary must satisfy two contradictory goals: maximum coverage and minimum space, achieving these goals by Informativeness, Non-redundancy, Coverage and Coherence methods. Finally, paper introduced summarization's related works. REFERENCES [1] W3C Recommendation, " Extensible Markup Language (XML) 1.1(Second Edition) ", 2006, [2] Eric Lease Morgan, "Getting Started with XML: A Manual and Workshop", [3] Pierre Geneves, " Logics for XML ", [4] Blake Dournaee, " XML Security ", [5] Borland Software Corporation (JBuilder), "XML Developer s Guide", [6] W3C Recommendation, "XSL Transformations (XSLT) Version 2.0", 23 January 2007, [7] W3C Recommendation, "XML Technology", [8] Jose de Aguiar Moraes Filho, "Summarizing XML Documents: Contributions, Empirical Studies, and Challenges", [9] W3schools.com, [10] Teng Lv and Ping Yan, "A Framework of Summarizing XML Documents with Schemas", The International Arab Journal of Information Technology, Vol. 10, No. 1, January 2013.

6 [11] Dilek Basci and Sanjay Misra, "Entropy as a Measure of Quality of XML Schema Document", The International Arab Journal of Information Technology, Vol. 8, No. 1, January [12] Hahn U. and Mani I., "The Challenges of Automatic Summarization", Journal of Computer, vol. 33, no. 11, pp , [13] Amini M., Tombros A., Usunier N., and Lalmas M., "Learning-Based Summarization of XML Documents", Information Retrieval, vol. 10, no.3, pp , [14] Yu C. and Jagadish H., "Schema Summarization", in Proceedings of the 32 nd International Conference on Very Large Data Bases VLDB, Korea, pp , [15] Theodore Dalamagas, Tao Cheng, Klaas-Jan Winkel, Timos Sellis, "A Methodology for Clustering XML Documents by Structure", Information Systems, vol. 31, no. 3, pp , [16] Christopher League and Kenjone Eng, "Schema-Based Compression of XML Data with Relax NG", journal of computers, vol. 2, no. 10, December [17] Sebastian Maneth, Nikolay Mihaylov, Sherif Sakr, "XML Tree Structure Compression", in Proceedings of the 3rd International Workshop on XML Data Management Tools and Techniques, Italy, pp , [18] Juliana Freire, Jayant R. Haritsa, Maya Ramanath, Prasan Roy, and Jerome Simeon, "StatiX: Making XML Count", in Proceedings of the International Conference on Management of Data, USA, pp , [19] Neoklis Polyzotis, Minos Garofalakis and Yannis Ioannidis, "Approximate XML Query Answers", in Proceedings of SIGMOD International Conference on Management of Data, France, pp , [20] Wei Wang, Haifeng Jiang, Hongjun Lu and Jeffrey Xu Yu, "Bloom Histogram: Path Selectivity Estimation for XML Data with Updates", in Proceedings of the 30 th International Conference on Very Large Data Bases VLDB, Canada, pp , [21] Ning Zhang, M. Tamer Ozsu, Ashraf Aboulnaga and Ihab F. Ilyas, "XSEED: Accurate and Fast Cardinality Estimation for XPath Queries", in Proceedings of the 22nd International Conference on ICDE, USA, pp. 61, [22] Veronica Mayorga and Neoklis Polyzotis, "Sketch-based Summarization of Ordered XML Streams", in Proceedings of IEEE 25 th International Conference on ICDE, China, pp , [23] Gudrun Fischer and Igor Jacy Lino Campista, "A Template-Based Approach to Summarize XML Collections", in Proceedings of Lernen, Wissensentdeckung and Adaptivit, Germany, pp , [24] Maya Ramanath and Kondreddi Sarath Kumar, "A Rank-Rewrite Framework for Summarizing XML Documents", in Proceedings of 2 nd International Workshop on Ranking in Databases, ICDE Workshop, Mexico, pp , [25] Jose de Aguiar and Theo Harder, "EXsum - An XML Summarization Framework", IDEAS , September 10-12, Coimbra, Portugal.