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Publish-subscribe middleware is a popular technology for facilitating device-to-device communication in large-scale distributed Internet of Things (IoT) scenarios. However, the stringent quality of service (QoS) requirements imposed by many applications cannot be met by cloud-based solutions alone. Edge computing is considered a key enabler for such applications. Client mobility and dynamic resource availability are prominent challenges in edge computing architectures. In this paper, we present EMMA, an edge-enabled publish-subscribe middleware that addresses these challenges. EMMA continuously monitors network QoS and orchestrates a network of MQTT protocol brokers. It transparently migrates MQTT clients to brokers in close proximity to optimize QoS. Experiments in a real-world testbed show that EMMA can significantly reduce end-to-end latencies that incur from network link usage, even in the …