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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