VerneMQ [0] could be interesting for you, they claim that:

> VerneMQ is a high-performance, distributed MQTT broker. It scales horizontally and vertically on commodity hardware to support a high number of concurrent publishers and consumers while maintaining low latency and fault tolerance.

I'm not sure about their licensing at the moment [1], to me it's a bit confusing.

[0] https://vernemq.com/

[1] https://vernemq.com/blog/2019/11/26/vernemq-end-user-license...

Mosquitto is not, we use RabbitMQ which runs on Erlang/OTP and has mqtt plug-ins. Mosquitto is single threaded. It's fast for sure but doesn't scale out. They can interoperate though as Mosquitto has configurable bridging.

Recently wanted to also create a cluster of mqtt brokers and found this medium tutorial stringing Docker Swarm, RabbitMQ, Consul and HAProxy together very helpful[0].

I was not happy with the static ip's for haproxy and opted to register the mqtt/tls services in consul and use the consul dns resolver with hapraxy.

[0]: https://medium.com/hepsiburadatech/implementing-highly-avail...

I'm running a cluster of Aedes backed by redis.

https://github.com/moscajs/aedes

I was inspired by this post by Bolt

https://medium.com/bolt-labs/developing-software-for-bolt-sc...

I work for Cedalo AG developing Mosquitto. Cedalo will be offering a version of Mosquitto with failover support as a paid for feature soon.

There are broker implementations that support clustering. Emqx comes to mind.

There are lots of brokers and Eclipse Amlen is coming soon: https://www.eclipse.org/amlen/

(disclosure: I work on Amlen)

Have a look at Apache Artemis. Well architected multi-protocol broker with clustering.