Mobile Computation with Functions
Kırlı, Zeliha D
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.