In this paper, we present Reo, which forms a paradigm for composition of software components based on the notion of mobile channels. Reo is a channel-based exogenous coordination model in which complex coordinators, called connectors, are compositionally built out of simpler ones. The simplest connectors in Reo are a set of channels with well-defined behaviour supplied by users. Reo can be used as a language for coordination of concurrent processes, or as a ‘glue language’ for compositional construction of connectors that orchestrate component instances in a component-based system. The emphasis in Reo is just on connectors and their composition, and not on the entities that connect to, communicate and cooperate through these connectors. Each connector in Reo imposes a specific coordination pattern on the entities (for example, components) that perform I/O operations through that connector, without the knowledge of those entities. Channel composition in Reo is a very powerful mechanism for construction of connectors. We demonstrate the expressive power of connector composition in Reo through a number of examples. We show that exogenous coordination patterns that can be expressed as (meta-level) regular expressions over I/O operations can be composed in Reo out of a small set of only five primitive channel types.

In this paper we introduce constraint automata and propose them as an operational model for Reo, an exogenous coordination language for compositional construction of component connectors based on a calculus of channels. By providing composition operators for constraint automata and defining notions of equivalence and refinement relations for them, this paper covers the foundations for building tools to address concerns such as the automated construction of the automaton for a given component connector, equivalence checking or containment checking of the behavior of two given connectors, and verification of coordination mechanisms.

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C e n t r u m v o o r W i s k u n d e e n I n f o r m a t i c a Software ENgineeringA coinductive calculus of component connectors

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C e n t r u m v o o r W i s k u n d e e n I n f o r m a t i c a Software ENgineeringAbstract behavior types: a foundation model for components and their composition The notion of Abstract Data Type (ADT) has served as a foundation model for structured and object oriented programming for some thirty years. The current trend in software engineering toward component based systems requires a foundation model as well. The most basic inherent property of an ADT, i.e., that it provides a set of operations, subverts some highly desirable properties in emerging formal models for components that are based on the object oriented paradigm.We introduce the notion of Abstract Behavior Type (ABT) as a higher-level alternative to ADT and propose it as a proper foundation model for both components and their composition. An ABT defines an abstract behavior as a relation among a set of timed-data-streams, without specifying any detail about the operations that may be used to implement such behavior or the data types it may manipulate for its realization. The ABT model supports a much looser coupling than is possible with the ADT's operational interface, and is inherently amenable to exogenous coordination. We propose that both of these are highly desirable, if not essential, properties for models of components and their composition.To demonstrate the utility of the ABT model, we describe Reo: an exogenous coordination language for compositional construction of component connectors based on a calculus of channels. We show the expressive power of Reo, and the applicability of ABT, through a number of examples. IntroductionAn Abstract Data Type (ADT) defines an algebra of operations with mathematically well-defined semantics, without specifying any detail about the implementation of those operations or the data structures they operate on to realize them. As such, ADT is a powerful abstraction and encapsulation mechanism that groups data together with their related operations into logically coherent and loosely-dependent entities, such as objects, yielding better structured programs. ADT has served as a foundation model for structured and object oriented programming for some thirty years.The immense success of object oriented techniques has distracted proper attention away from critical evaluation of some of its underpinning concepts from the perspective of their utility for components. We propose that the most basic inherent property of an ADT, i.e., that it provides a set of operations in its interface, subverts some highly desirable properties in emerging models for component based systems. This is already evident in the current attempts at extending the object oriented models into the realm of components (see, e.g., Sections 3 and 5).We introduce the notion of Abstract Behavior Type (ABT) as a higher-level alternative to ADT and propose it as a proper foundation model for both components and their composition. An ABT defines an abstract behavior as a relation among a set of timed-data-streams, without specifying any detail about ...

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