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Constraint satisfaction problems (CSPs) provide a unified framework for studying a wide variety of computational problems naturally arising in combinatorics, artificial intelligence and database theory. To any finite domain D and any constraint language Γ (a finite set of relations over D), we associate the constraint satisfaction problem CSP(Γ): an instance of CSP(Γ) consists of a list of variables x1, x2,..., x n and a list of constraints of the form "(x 7, x2,..., x5) ∈ R" for some relation R in Γ. The goal is to determine whether the variables can be assigned values in D such that all constraints are simultaneously satisfied. The computational complexity of CSP(Γ) is entirely determined by the structure of the constraint language Γ and, thus, one wishes to identify classes of Γ such that CSP(Γ) belongs to a particular complexity class. / In recent years, logical and algebraic perspectives have been particularly successful in classifying CSPs. A major weapon in the arsenal of the logical perspective is the database-theory-inspired logic programming language called Datalog. A Datalog program can be used to solve a restricted class of CSPs by either accepting or rejecting a (suitably encoded) set of input constraints. Inspired by Dalmau's work on linear Datalog and Reingold's breakthrough that undirected graph connectivity is in logarithmic space, we use a new restriction of Datalog called symmetric Datalog to identify a class of CSPs solvable in logarithmic space. We establish that expressibility in symmetric Datalog is equivalent to expressibility in a specific restriction of second order logic called Symmetric Restricted Krom Monotone SNP that has already received attention for its close relationship with logarithmic space. / We also give a combinatorial description of a large class of CSPs lying in L by showing that they are definable in symmetric Datalog. The main result of this thesis is that directed st-connectivity and a closely related CSP cannot be defined in symmetric Datalog. Because undirected st-connectivity can be defined in symmetric Datalog, this result also sheds new light on the computational differences between the undirected and directed st-connectivity problems.

A Domain-Specific Modeling Approach This study has presented a component-based domain modeling approach that provides an environment for simplifying and accelerating software development and analysis, and improves software reusability, maintainability, and productivity. With highlevel design abstraction, constraints of application domains, and the guidance of domain rules, the proposed component-based framework offers an effective solution to modeling and automating the development and deployment of software application. Meta-modeling will be used in this study to define the domain notations, rules, and constraints for component composition within a specific domain context. A domain-specific graphical design environment will also be proposed to simplify and accelerate the software development by simply dragging and dropping pre-built components with minimal programming effort. The modeling of components can be further extended with the specification of their dependability and real-time constraints. / Related work -- Component composition -- Domain-specific modeling -- Model-based component composition environment for a specific domain -- Mobile service creation framework (MCSF) -- A model-driven approach to implementing dependable component-based mobile services -- A model-driven approach to implementing component-based real-time mobile services / Related work -- Component composition -- Domain-specific modeling -- Model-based component composition environment for a specific domain -- Mobile service creation framework (MCSF) -- A model-driven approach to implementing dependable component-based mobile services -- A model-driven approach to implementing component-based real-time mobile services. / Department of Computer Science

Today's dynamic language systems have grown to include features that resemble features of operating systems. It may be possible to improve on both by unifying a language system with an operating system. Complete unification does not appear possible in the near-term, so an intermediate system is described. This intermediate system uses a common call graph to allow components in arbitrary languages to interact as easily as components in the same language. Potential benefits of such a system include significant improvements in interoperability, improved reusability and backward compatibility, simplification of debugging and some administrative tasks, and distribution over a cluster without any changes to application code.

This dissertation presents three different approaches to representing Regular Formal languages, i.e., regular expressions, finite acceptors and regular grammars. We define how each method is used to represent the language, and then the method for translating from one representation to another of the language. A toolkit is then presented which allows the user to input their definition of a language using any of the three models, and also allows the user to translate the representation of the language from one model to another. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pitermaritzburg, 2014.

Asynchronous exception handling is a useful and sometimes necessary alternative form of communication among threads. This thesis examines and classifies general concepts related to asynchrony, asynchronous propagation control, and how asynchronous exception handling affects control flow. The work covers four advanced topics affecting asynchronous exception-handling in a multi-threaded environment. The first topic is concerned with the non-determinism that asynchronous exceptions introduce into a program's control-flow because exceptions can be propagated at virtually any point during execution. The concept of asynchronous propagation control, which restricts the set of exceptions that can be propagated, is examined in depth. Combining it with a restriction of asynchrony that permits propagation of asynchronous exceptions only at certain well-defined (poll) points can re-establish sufficient determinism to verify a program's correctness, but introduces overhead, as well as a delay between the delivery of an asynchronous exception and its propagation. It also disturbs a programmer's intuition about asynchronous propagation in the program, and requires the use of programming idioms to avoid errors. The second topic demonstrates how a combined model of full and restricted asynchrony can be safely employed, and thus, allow for a more intuitive use of asynchronous propagation control, as well as potentially improve performance. The third topic focuses on the delay of propagation that is introduced when a thread is blocked, i.e., on concurrency constructs that provide mutual exclusion or synchronization. An approach is presented to transparently unblock threads so propagation of asynchronous termination and resumption exceptions can begin immediately. The approach does not require additional syntax, simplifies certain programming situations, and can improve performance. The fourth topic explores usability issues affecting the understanding of (asynchronous) exception handling as a language feature. To overcome these issues, tools and language features are presented that help in understanding exception handling code by providing additional run-time information, as well as assist in testing. For all topics, the necessary extensions to the syntax/semantics of the language are discussed; where applicable, a prototypical implementation is presented, with examples that demonstrate the benefits of the new approaches.