Tagged Sets: A Secure and Transparent Coordination Medium

A simple and effective way of coordinating distributed, mobile, and parallel applications is to use a virtual shared memory (VSM), such as a Linda tuple-space. In this paper, we propose a new kind of VSM, called a tagged set. Each element in the VSM is a value with an associated tag, and values are read or removed from the VSM by matching the tag. Tagged sets exhibit three properties useful for VSMs: 1. Ease of use. A tagged value naturally corresponds to the notion that data has certain attributes, expressed by the tag, which can be used for later retrieval. 2. Flexibility. Tags are implemented as propositional logic formulae, and selection as logical implication, so the resulting system is quite powerful. Tagged sets naturally support a variety of applications, such as shared data repositories (e.g., for media or e-mail), message passing, and publish/subscribe algorithms; they are powerful enough to encode existing VSMs, such as Linda spaces. 3. Security. Our notion of tags naturally corresponds to keys, or capabilities: a user may not select data in the set unless she presents a legal key or keys. Normal tags correspond to symmetric keys, and we introduce asymmetric tags that correspond to public and private key pairs. Treating tags as keys permits users to easily specify protection criteria for data at a fine granularity. This paper motivates our approach, sketches its basic theory, and sketches some implementation issues.