A distributed event notification service (ENS) is a middleware architecture commonly used to provide applications with scalable and robust publish/subscribe communication primitives. A distributed ENS can route events toward subscribers using multiple paths with different lengths and latencies; as a consequence, subscribers can receive events out of order. In this paper, we propose a novel solution for out-of-order notification detection on top of an existing topic-based ENS. Our solution guarantees that events published on different topics will be either delivered in the same order to all the subscribers of those topics or tagged as out-of-order. The proposed algorithm is completely distributed and is able to scale with the system size while imposing a reasonable cost in terms of notification latency. Our solution improves the current state of the art solutions by dynamically handling subscriptions/unsubscriptions and by automatically adapting with respect to topic popularity changes.
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