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# Introduction

Our work is motivated by a couple of observations about the evolving nature of distributed applications and their implementation environments. First, the structure of distributed software is undergoing some changes. We are seeing the growth of new types federated, loosely coupled distributed applications that have several common requirements and characteristics including: (1)Event-driven Architecture: A single distributed application may be composed of separate components that all work together in a coordinated fashion. Such applications are event-oriented in nature in that we can think of changes in the overall state of the global application as being triggered by discrete changes in the state of event processing at each of the components. (2)Heterogeneity: The components must run on a variety of different platforms with varying inherent capabilities and environments. In addition, some of components themselves are reused pieces of software implemented in a variety of languages and environments. Despite all of this heterogeneity, it is desirable to be able to design and develop distributed applications in a common portable framework. (3)Mobility: The ability to dynamically move code to and among these platforms during application execution greatly enhances the ability to deploy and reconfigure complex systems. (4)Reliability: These extension and reconfiguration capabilities can be used to construct reliable systems that distribute system state in ways that enable graceful failure recovery and adaptation. (5) Security: In such highly dynamic systems, the ability to secure and control resources at several levels (global application, single node, individual process) is necessary to insure the correct behavior of applications and the viability of systems that support them.

Our second observation is that while many systems share the characteristics mentioned above, the variety of component types and platforms that must be accommodated preclude a language specific or application specific solutions. Instead we suggest that a Middleware approach based upon Tcl scripting technology provides the most flexibility in the design of such composite applications. Tcl is great "component glue". Its simplified structure and extensibility are strengths when assembling applications from disparate, heterogeneous software components 1.

This paper is about AGNI - a multi-threaded Tcl 8.1 based Middleware for scripting reconfigurable event-oriented distributed systems. AGNI builds upon the proven scripting power of Tcl by adding extensions for an abstraction we call Mobile Streams. Mobile Streams (MStreams) are a generalization of simple mobile code technologies (e.g. Java Applets) that provide code distribution and communication between clients and servers. MStreams extend today's simple notions of code mobility by incorporating state mobility, decentralized peer-to-peer communications and the ability to extend and reconfigure distributed application during execution while preserving behavioral guarantees. At a higher level, MStreams allow the separation of the logical structure of a distributed application from the physical placement of components. Our model of code mobility allows the mapping of logical application structure to physical resources (e.g. machines and processes) to occur dynamically at run time and change during the course of the active life-time of the global application.

The rest of this paper is organized as follows: Section 2 presents the MStreams programming model and system architecture and presents an introductory example. Section 3 gives a brief overview of AGNI, our prototype implementation of MStreams Middleware. Section 4 presents a simulation environment for designing applications using our system. Section 5 presents some more comprehensive applications that we have built on our system. In Section 6 we compare and contrast our work with those of others. In Section 7 we conclude and present our future plans for this project.

Next: Mobile Streams Up: AGNI: A Multi-threaded Middleware Previous: Abstract