MQTT-ST: a Spanning Tree Protocol for Distributed MQTT Brokers

Abstract

MQTT, one of the most popular protocols for the IoT, works according to a publish/subscribe pattern in which multiple clients connect to a single broker, generally hosted in the cloud. However, such a centralised approach does not scale well considering the massive numbers of IoT devices forecasted in the next future, thus calling for distributed solutions in which multiple brokers cooperate together. Indeed, distributed brokers can be moved from traditional cloud-based infrastructure to the edge of the network (as it is envisioned by the upcoming MEC technology of 5G cellular networks), with clear improvements in terms of latency, for example. This paper proposes MQTT-ST, a protocol able to create such a distributed architecture of brokers, organized through a spanning tree. The protocol uses in-band signalling (i.e., reuses MQTT primitives for the control messages) and allows for full message replication among brokers, as well as robustness against failures. We tested MQTT-ST in different experimental scenarios and we released it as open-source project to allow for reproducible research.

D-MQTT: design and implementation of a pub/sub broker for distributed environments

“…In our previous work [1], we proposed MQTT-ST, a protocol for interconnecting MQTT brokers in a tree topology automatically, based on a combination of the Mosquitto bridging feature and the Spanning Tree Protocol. The protocol allows to create an overlay network among the brokers and to elect a root node for traffic forwarding, taking into account inter-broker latency and broker physical resources.…”

“…Therefore, an overlay network structured as a loop-free spanning tree should be imposed over the brokers. Here we reuse a modified version of our recently proposed work MQTT-ST [1], which allows to obtain such a loop-free topology in a network of MQTT brokers. The process follows the well known Spanning Tree Protocol used for layer 2 switches, and works according to a two-steps process:…”

“…The REQUEST/RESPONSE exchange is also used to identify potential broker disconnection from the system. In particular, the REQUEST message is retransmitted with a stop-and-wait approach with timeout T o equal 3 Differently from what proposed in [1], the REQUEST messages is not transmitted as part of the MQTT application layer, since there is no MQTT connection among brokers at this stage…”

“…In our previous work [1], after the tree creation, publish messages are flooded to all brokers in the network, with the overlay network used only to avoid message loops among brokers. In this work we take a step further, and we develop a routing protocol which forwards publish messages only to brokers who are interested into such messages, i.e., who have clients subscribed to the publication topics.…”

“…• Automatic broker discovery: in contrast to other available solutions for interconnecting MQTT brokers (e.g., MQTT bridging), which require a static, a-priori knowledge of all brokers, we propose a mechanism to automatically discover brokers, speeding up the creation and management of the distributed architecture. • Optimised overlay network: we make efficient use of our previous work MQTT-ST [1] to create a spanning tree among the brokers joining the distributed architecture. The spanning tree is created automatically by the brokers, taking into account parameters such as inter-broker latency and computational/memory resources.…”