Acute: high-level programming language design for distributed computation

Existing languages provide good support for typeful programming of standalone programs. In a distributed system, however, there may be interaction between multiple instances of many distinct programs, sharing some (but not necessarily all) of their module structure, and with some instances rebuilt with new versions of certain modules as time goes on. In this paper we discuss programming language support for such systems, focussing on their typing and naming issues. We describe an experimental language, Acute, which extends an ML core to support distributed development, deployment, and execution, allowing type-safe interaction between separately-built programs. The main features are: (1) type-safe marshalling of arbitrary values; (2) type names that are generated (freshly and by hashing) to ensure that type equality tests suffice to protect the invariants of abstract types, across the entire distributed system; (3) expression-level names generated to ensure that name equality tests suffice for type-safety of associated values, e.g. values carried on named channels; (4) controlled dynamic rebinding of marshalled values to local resources; and (5) thunkification of threads and mutexes to support computation mobility. These features are a large part of what is needed for typeful distributed programming. They are a relatively lightweight extension of ML, should be efficiently implementable, and are expressive enough to enable a wide variety of distributed infrastructure layers to be written as simple library code above the byte-string network and persistent store APIs. This disentangles the language runtime from communication intricacies. This paper highlights the main design choices in Acute. It is supported by a full language definition (of typing, compilation, and operational semantics), by a prototype implementation, and by example distribution libraries

KLAIM: A Kernel Language for Agents Interaction and Mobility

We investigate the issue of designing a kernel programming language for mobile computing and describe KLAIM, a language that supports a programming paradigm where processes, like data, can be moved from one computing environment to another. The language consists of a core Linda with multiple tuple spaces and of a set of operators for building processes. KLAIM naturally supports programming with explicit localities. Localities are first-class data (they can be manipulated like any other data), but the language provides coordination mechanisms to control the interaction protocols among located processes. The formal operational semantics is useful for discussing the design of the language and provides guidelines for implementations. KLAIM is equipped with a type system that statically checks access rights violations of mobile agents. Types are used to describe the intentions (read, write, execute, etc.) of processes in relation to the various localities. The type system is used to determine the operations that processes want to perform at each locality, and to check whether they comply with the declared intentions and whether they have the necessary rights to perform the intended operations at the specific localities. Via a series of examples, we show that many mobile code programming paradigms can be naturally implemented in our kernel language. We also present a prototype implementaton of KLAIM in Java

Process Creation and Full Sequential Composition in a Name-Passing Calculus

This paper presents the underlying theory for a process calculus featuring process creation and sequential composition, instead of the more usual parallel composition and action prefixing, in a setting where mobility is achieved by communicating channel names. We discuss the questions of scope and name binding, raised by the interaction of mobility and sequential composition. Substitution of names is integrated as a syntactic operator in the calculus. We present an axiomatic theory for the calculus and show its soundness and completeness w.r.t. bisimulation equivalence

Modelling and simulation framework for reactive transport of organic contaminants in bed-sediments using a pure java object - oriented paradigm

Numerical modelling and simulation of organic contaminant reactive transport in the environment is being increasingly relied upon for a wide range of tasks associated with risk-based decision-making, such as prediction of contaminant profiles, optimisation of remediation methods, and monitoring of changes resulting from an implemented remediation scheme. The lack of integration of multiple mechanistic models to a single modelling framework, however, has prevented the field of reactive transport modelling in bed-sediments from developing a cohesive understanding of contaminant fate and behaviour in the aquatic sediment environment. This paper will investigate the problems involved in the model integration process, discuss modelling and software development approaches, and present preliminary results from use of CORETRANS, a predictive modelling framework that simulates 1-dimensional organic contaminant reaction and transport in bed-sediments

or, being first-class is not enough

A monad is presented which is suitable for writing concurrent programs in a purely functional programming language. In contrast to, for instance, the IO monad [Launchbury, Peyton Jones 94], the primitives added to the functional language are not represented as built-in functions operating on the monad, but rather by Perry-style constructors [Perry 90] of a distinguished algebraic data type. Therefore, monadic expressions representing concurrent computations are not only first-class objects of the language; in addition, they may even be decomposed. A number of examples show that decomposability of concurrent code is crucial for the purely functional construction of more powerful concurrency abstractions like rendezvous, remote procedure call, and critical regions from the primitives. The paper argues that this technique helps to remedy a recurrent dilemma in the design of concurrent programming languages, namely, how to keep the language small, coherent, and rigorously defined, yet to provide the programmer with all the communication constructs required. It is suggested that functional languages are not only capable of describing concurrent programs, but that in terms

Mobile Computation with Functions

The practice of computing has reached a stage where computers are seen as parts of a global computing platform. The possibility of exploiting resources on a global scale has given rise to a new paradigm -- the mobile computation paradigm -- for computation in large-scale distributed networks. Languages which enable the mobility of code over the network are becoming widely used for building distributed applications. This thesis explores distributed computation with languages which adopt functions as the main programming abstraction and support code mobility through the mobility of functions between remote sites. It aims to highlight the benefits of using languages of this family in dealing with the challenges of mobile computation. The possibility of exploiting existing static analysis techniques suggests that having functions at the core of a mobile code language is a particularly apt choice. A range of problems which have impact on the safety, security and performance of systems are discussed here. It is shown that types extended with effects and other annotations can capture a significant amount of information about the dynamic behaviour of mobile functions and offer solutions to the problems under investigation. The thesis presents a survey of the languages Concurrent ML, Facile and PLAN which remain loyal to the principles of the functional language ML and hence inherit its strengths in the context of concurrent and distributed computation. The languages which are defined in the subsequent chapters have their roots in these languages. Two chapters focus on using types to statically predict whether functions are used locally or may become mobile at runtime. Types are exploited for distributed calltracking to estimate which functions are invoked at which sites in the system. Compilers for mobile code languages would benefit from such estimates in dealing with the heterogeneity of the network nodes, in providing static profiling tools and in estimating the resource-consumption of programs. Two chapters are devoted to the use of types in controlling the flow of values in a system where users have different trust levels. The confinement of values within a specified mobility region is the subject of one of these. The other focuses on systems where values are classified with respect to their confidentiality level. The sources of undesirable flows of information are identified and a solution based on noninterference is proposed

Open issues in semantic query optimization in relational DBMS

After two decades of research into Semantic Query Optimization (SQO) there is clear agreement as to the efficacy of SQO. However, although there are some experimental implementations there are still no commercial implementations. We first present a thorough analysis of research into SQO. We identify three problems which inhibit the effective use of SQO in Relational Database Management Systems(RDBMS). We then propose solutions to these problems and describe first steps towards the implementation of an effective semantic query optimizer for relational databases