Siobhán North

Issues in implementing a model checker for z

In this paper we discuss some issues in implementing a model checker for the Z specification language. In particular, the language design of Z and its semantics, raises some challenges for efficient model checking, and we discuss some of these issues here. Our approach to model checking Z specifications involves implementing a translation from Z into the SAL input language, upon which the SAL toolset can be applied. In this paper we discuss issues in the implementation of this translation algorithm and illustrate them by looking at how the mathematical toolkit is encoded in SAL and the resultant efficiency of the model checking tools.

Z2SAL - Building a Model Checker for Z

In this paper we discuss our progress towards building a model-checker for Z. The approach we take in our Z2SAL project involves implementing a translation from Z into the SAL input language, upon which the SAL toolset can be applied. The toolset includes a number of model-checkers together with a simulator. In this paper we discuss our progress towards implementing as complete as a translation as possible, the limitations we have reached and the optimizations we have made. We illustrate with a small example.

Z2SAL: a translation-based model checker for Z

Despite being widely known and accepted in industry, the Z formal specification language has not so far been well supported by automated verification tools, mostly because of the challenges in handling the abstraction of the language. In this paper we discuss a novel approach to building a model-checker for Z, which involves implementing a translation from Z into SAL, the input language for the Symbolic Analysis Laboratory, a toolset which includes a number of model-checkers and a simulator. The Z2SAL translation deals with a number of important issues, including: mapping unbounded, abstract specifications into bounded, finite models amenable to a BDD-based symbolic checker; converting a non-constructive and piecemeal style of functional specification into a deterministic, automaton-based style of specification; and supporting the rich set-based vocabulary of the Z mathematical toolkit. This paper discusses progress made towards implementing as complete and faithful a translation as possible, while highlighting certain assumptions, respecting certain limitations and making use of available optimisations. The translation is illustrated throughout with examples; and a complete working example is presented, together with performance data.

COPAS—A conversational Pascal system

Terminal systems usually permit a user to interact with a running program, if only by using the terminal for input and output. It is possible to extend the interaction to include the compilation phase. Program text is translated as it is supplied and edits are checked immediately they are made. A system displaying these characteristics can be called conversational. COPAS is such a system.

A performance evaluation of a new bitmap-based XML processing approach over RDBMS

This paper presents a comprehensive performance analysis of PACD; a novel bitmap-based XML processing approach introduced earlier to resolve several performance issues identified in existing XML database technology. The study evaluated three performance aspects of XML database techniques including query processing, XML updates and scalability. Each of these aspects has been tested using various measures and compared with some representative alternative approaches. Despite its narrow domain for the order-access queries and its high cost in terms of the number IO-read operations, PACD almost performed well in terms of query processing, resource consumption during XML updates and has shown acceptable scalability over a variety of XML database categories.

Advances in Databases

This paper surveys recent trends in network-hosted end-user services, with an emphasis on the increasing complexity of those services due to the convergence of the conventional telephony, wireless, and data networks. In order to take full advantage of these services, private and corporate users will need personalized ways of accessing them. Support for this personalization will involve advances in both data and policy management.

A Prime Number Approach to Matching an XML Twig Pattern including Parent-Child Edges.

Twig pattern matching is a core operation in XML query processing because it is how all the occurrences of a twig pattern in an XML document are found. In the past decade, many algorithms have been proposed to perform twig pattern matching. They rely on labelling schemes to determine relationships between elements corresponding to query nodes in constant time. In this paper, a new algorithm TwigStackPrime is proposed, which is an improvement to TwigStack (Bruno et al., 2002). To reduce the memory consumption and computation overhead of twig pattern matching algorithms when Parent-Child (P-C) edges are involved, TwigStackPrime efficiently filters out a tremendous number of irrelevant elements by introducing a new labelling scheme, called Child Prime Label (CPL). Extensive performance studies on various real-world and artificial datasets were conducted to demonstrate the significant improvement of CPL over the previous indexing and querying techniques. The experimental results show that the new technique has a superior performance to the previous approaches.

TwigStackPrime: A Novel Twig Join Algorithm Based on Prime Numbers

The growing number of XML documents leads to the need for appropriate XML querying algorithms which are able to utilize the specific characteristics of XML documents. A labelling scheme is fundamental to processing XML queries efficiently. They are used to determine structural relationships between elements corresponding to query nodes in twig pattern queries (TPQs). This article presents a design and implementation of a new indexing technique which exploits the property of prime numbers to identify Parent-Child (P-C) relationships in TPQs during query evaluation. The Child Prime Label (CPL, for short) approach can be efficiently incorporated within the existing labelling schemes. Here, we propose a novel twig matching algorithm based on the well known TwigStack algorithm [3], which applies the CPL approach and focuses on reducing the overhead of storing useless elements and performing unnecessary join operations. Our performance evaluation demonstrates that the new algorithm significantly outperforms the previous approaches.

XML Labels Compression using Prefix-encodings

XML is the de-facto standard for data representation and communication over the web, and so there is a lot of interest in querying XML data and most approaches require the data to be labelled to indicate structural relationships between elements. This is simple when the data does not change but complex when it does. In the day-to-day management of XML databases over the web, it is usual that more information is inserted over time than deleted. Frequent insertions can lead to large labels which have a detrimental impact on query performance and can cause overflow problems. Many researchers have shown that prefix encoding usually gives the highest compression ratio in comparison to other encoding schemes. Nonetheless, none of the existing prefix encoding methods has been applied to XML labels. This research investigates compressing XML labels via different prefix-encoding methods in order to reduce the occurrence of any overflow problems and improve query performance. The paper also pre sents a comparison between the performances of several prefix-encodings in terms of encoding/decoding time and compressed code size.

Experiences using Z2SAL

The Z notation is a language that can be used for writing formal specifications of a system since it is based on mathematical notation and logic. However, there is less tool support for this language that one might wish for. In this paper, Z2SAL, a translator for Z which translates the Z notation into a SAL input language, is explored. The generated SAL file can be used further by an existing model checker, specifically ones provided in the SAL tool suite. This paper describes experiences during conducting several experiments on the Z2SAL translator.