Efficient Notification Ordering for Geo-Distributed Pub/Sub Systems

A distributed event notification service (ENS) is at the core of modern messaging infrastructures providing applications with scalable and robust publish/subscribe communication primitives. Such ENSs 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 ordered notifications on top of an existing distributed topic-based ENS. Our solutions guarantees that each pair of events published in the system will be notified in the same order to all their target subscribers independently from the topics they are published in. It endows a distributed timestamping mechanism based on a multistage sequencer that produces timestamps whose size is dynamically adjusted to accommodate changing subscriptions in the system. An extensive experimental evaluation based on a prototype implementation shows that the timestamping mechanism is able to scale from several points of view (i.e., number of publisher and subscribers, event rate). Furthermore, it shows how the deployment flexibility of our solution makes it perform better in terms of timestamp size and timestamp generation latency when the system load exhibits geographic topic popularity, that is, matching subscriptions and publications are geographically clustered. This makes our solution particularly well suited to be deployed in geo-distributed infrastructures.