xmile: An Incremental Code Mobility System based on XML Technologies

Logical mobility ranges from simple data mobility, where information is transferred, through code mobility that allows the migration of executable code, to mobile agents, in which code and data move together. Several application domains need a more exible approach to code mobility than the one that can be achieved with Java and with mobile agents in general. This exibility can either be required as a result of low network bandwidth, scarce resources, and slow or expensive connectivity, like in mobile computing settings, or scalability requirements like in applications on several thousand clients that have to be kept in sync and be updated with new code fragments. We show how to achieve more ne-grained mobility than in the approaches using mobile agents and Java class loading. We demonstrate that the unit of mobility can be decomposed from an agent or class level, if necessary, down to the level of individual statements. We can then support incremental insertion or substitution of, possibly small, code fragments and open new application areas for code mobility such as management of applications on mobile thin clients, for example wireless connected PDAs or mobile phones, or more in general distributed code update and management. This work builds on the formal foundation for ne-grained code mobility that was established in [3]. That paper develops a theoretical model for ne-grained mobility at the level of single statements or variables and argues that the potential of code mobility is submerged by the capability of the most commonly used language for code mobility, i.e., Java. We focus on an implementation of ne-grained mobility using standardized and widely available technology. It has been identi ed that mobile code is a design concept, independent of technology and can be embodied in various ways in di erent technologies. The eXtensible Markup Language (XML) [1] can be exploited to achieve code mobility at a very ne-grained level. XML has not been designed for code mobility, however it happens to have some interesting characteristics, mainly related to exibility, that allow its use for code migration. In particular, we will exploit the tree structure of XML documents and then use XML related technologies, such as XPath and the Document Object Model (DOM) to modify programs dynamically. The availability of this technology considerably simpli es the construction of application-speci c languages and their interpreters. XML provides a exible approach to describe data structures. We now show that XML can also be used to describe code. XML DTDs (i.e., Data Type De nition) are, in fact, very similar to attribute grammars. Each element of an XML DTD corresponds to a production of a grammar. The contents of the element de ne the right-hand side of the production.