A Hierarchical Architecture for Prioritized MQTT

Modern automotive systems are evolving towards software-defined vehicles, enabling seamless updates and replacements of software components throughout the vehicle lifecycle. These advancements rely on connected services, making middleware solutions for Service-Oriented Architectures (SoAs) increasingly relevant. Among the various middleware options, MQTT has emerged as a lightweight publish/subscribe protocol. However, MQTT lacks support for prioritizing diverse kinds of communication flows. To address this limitation, a recent work in the literature proposed PrioMQTT, an extended version of MQTT that incorporates message prioritization and adopts the UDP transport protocol to enhance predictability and reduce transmission delays. Despite these enhancements, the centralized broker architecture in MQTT and PrioMQTT can lead to scalability issues under high network loads. This work discusses the challenges and design choices behind a hierarchical version of PrioMQTT, named H-PrioMQTT, that aims to reduce the communication delays in automotive networks handling a large number of flows. Preliminary results obtained through simulations in a realistic in-vehicle scenario show a reduction of the maximum end-to-end delays compared to the plain PrioMQTT.