This paper presents SIENA, an event notification service that we have designed and implemented to exhibit both expressiveness and scalability. We describe the service's interface to applications, the algorithms used by networks of servers to select and deliver event notifications, and the strategies used Effort sponsored by the Defense Advanced Research Projects Agency, and Air Force Research Laboratory, Air Force Materiel Command,USAF, under agreement numbers F30602-94-C-0253, F3060297 -2-0021, F30602-98-2-0163, F30602-99-C-0174, F30602-00-2-0608, and N66001-00-8945; by the Air Force Office of Scientific Research, Air Force Materiel Command, USAF, under grant number F49620-98-1-0061; and by the National Science Foundation under Grant Number CCR-9701973. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Defense Advanced Research Projects Agency, Air Force Research Laboratory, or the U.S. Government

This paper factors out the common denominator underlying these variants: full decoupling of the communicating entities in time, space, and synchronization. We use these three decoupling dimensions to better identify commonalities and divergences with traditional interaction paradigms. The many variations on the theme of publish/subscribe are classified and synthesized. In particular, their respective benefits and shortcomings are discussed both in terms of interfaces and implementations

Content-based subscription systems are an emerging alternative to traditional publish-subscribe systems, because they permit more flexible subscriptions along multiple dimensions. In these systems, each subscription is a predicate which may test arbitrary attributes within an event. However, the matching problem for content-based systems — determining for each event the subset of all subscriptions whose predicates match the event — is still an open problem. We present an efficient, scalable solution to the matching problem. Our solution has an expected time complexity that is sub-linear in the number of subscriptions, and it has a space complexity that is linear. Specifically, we prove that for predicates reducible to conjunctions of elementary tests, the expected time to match a random event is no greater than O(N 1;) where N is the number of subscriptions, and is a closed-form expression that depends on the number and type of attributes (in some cases, 1=2). We present some optimizations to our algorithms that improve the search time. We also present the results of simulations that validate the theoretical bounds and that show acceptable performance levels for tens of thousands of subscriptions. 1

carzanig @ cs.colorado.edu This paper describes the design of SIENA, an Internet-scale event notification middleware service for distributed event-based applications deployed over wide-area networks. SIENA is responsible for selecting the notifications that are of in-terest to clients (as expressed in client subscriptions) and then delivering those notifications to the clients via access points. The key design challenge for SIENA is maximizing expressiveness in the selection mechanism without sacrific-ing scalability of the delivery mechanism. This paper focuses on those aspects of the design of SIENA that fundamentally impact scalability and expressiveness. In particular, we de-scribe SIENA'S data model for notifications, the covering re-lations that formally define the semantics of the data model, the distributed architectures we have studied for SIENA'S im-plementation, and the processing strategies we developed to exploit the covering relations for optimizing the routing of notifications. 1.

In this paper, we argue that there is a need for an event-based middleware to build large-scale distributed systems. Existing publish/subscribe systems still have limitations compared to invocation-based middlewares. We introduce Hermes, a novel event-based distributed middleware architecture that follows a type- and attribute-based publish/subscribe model. It centres around the notion of an event type and supports features commonly known from object-oriented languages like type hierarchies and supertype subscriptions. A scalable routing algorithm using an overlay routing network is presented that avoids global broadcasts by creating rendezvous nodes. Fault-tolerance mechanisms that can cope with different kinds' of failures in the middleware are integrated with the routing algorithm resulting in a scalable and robust system.

This paper presents an algorithm for content-based forwarding, an essential function in content-based networking. Unlike in traditional address-based unicast or multicast networks, where messages are given explicit destination addresses, the movement of messages through a content-based network is driven by predicates applied to the content of the messages. Forwarding in such a network amounts to evaluating the predicates stored in a router's forwarding table in order to decide to which neighbor routers the message should be sent. We are interested in finding a forwarding algorithm that can make this decision as quickly as possible in situations where there are large numbers of predicates and high volumes of messages. We present such an algorithm and give the results of studies evaluating its performance.

This paper proposes a routing scheme for contentbased networking. A content-based network is a communication network that features a new advanced communication model where messages are not given explicit destination addresses, and where the destinations of a message are determined by matching the content of the message against selection predicates declared by nodes. Routing in a content-based network amounts to propagating predicates and the necessary topological information in order to maintain loop-free and possibly minimal forwarding paths for messages. The routing scheme we propose uses a combination of a traditional broadcast protocol and a contentbased routing protocol. We present the combined scheme and its requirements over the broadcast protocol. We then detail the content-based routing protocol, highlighting a set of optimization heuristics. We also present the results of our evaluation, showing that this routing scheme is effective and scalable.

Publish/subscribe systems have demonstrated the ability to scale to large numbers of users and high data rates when providing content-based data dissemination services on the Internet. However, their services are limited by the data semantics and query expressiveness that they support. On the other hand, the recent work on selective dissemination of XML data has made significant progress in moving from XML filtering to the richer functionality of transformation for result customization, but in general has ignored the challenges of deploying such XML-based services on an Internet-scale. In this paper, we address these challenges in the context of incorporating the rich functionality of XML data dissemination in a highly scalable system. We present the architectural design of ONYX, a system based on an overlay network. We identify the salient technical challenges in supporting XML filtering and transformation in this environment and propose techniques for solving them.

We argue that the needs of many classes of modern applications, especially those targeted at mobile or wireless computing, demand the services of content-based publish/subscribe middleware, and that this middleware in turn demands a new kind of communication infrastructure for its proper implementation. We refer to this new communication infrastructure as content-based networking. The service model of this network must directly support the interface of an advanced content-based publish/subscribe middleware service. At the same time, the implementation must be architected as a true distributed network, providing appropriate guarantees of reliability, security, and performance. We do not propose content-based networking as a replacement for IP, nor do we advocate an implementation of a publish/subscribe middleware at the network level (i.e., within routers). Instead, we argue that contentbased networking must be designed according to established networking principles and techniques. To this end, in this paper, we formulate the foundational concepts of content-based networking, and relate them to the corresponding concepts in traditional networking. We also briefly review our experience with content-based publish/subscribe middleware and suggest some open research problems in the area of content-based networking.

Abstract. Publish-subscribe systems are evolving toward using content-based subscription rather than subject-based subscription. A key problem in implementing such systems is that a straightforward mapping from matching sets to multicast groups produces a number of groups that rapidly grows beyond practical limits. This paper proposes a set of alternative algorithms for solving this problem, by: (1) using a smaller set of overbroad multicast groups, judiciously chosen to minimize imprecision; (2) issuing multiple multicasts to appropriately chosen clusters; or (3) sending an event over multiple hops each involving a multicast to a set of neighbors. We evaluate these algorithms on a simulated wide-area network. We find that (1) a simple flooding algorithm is viable over an extensive range of conditions; and (2) under conditions of high selectivity and high regionalism of subscriptions, the other approaches mentioned above perform significantly better; however, the specific algorithm to use depends upon the economics of deployment. 1