Cluster forming with Mosquitto broker

Question

I am using Mosquitto broker to implement MQTT protocol. But I am unable to find how clustering can be done in case of mosquitto brokers. Also is there any limitation on number of clients those can be served with one broker.

Answer 1

You can't do clustering with mosquitto.

Some other MQTT brokers out there which support clustering, including HiveMQ. HiveMQ has an elastic cluster ability with auto discovery and a distributed masterless architecture and works very well on cloud providers like AWS or Azure.

You can see a full list of all brokers which support clustering here.

Disclaimer: I'm a developer of HiveMQ, so this answer may be biased.

Answer 2

Original source of this answer: Horizontal scaling for brokers. I just added the plugin support.

Two functionalities of mosquitto broker combined could be used to setup a n node cluster.

1. Mosquitto bridge support.
2. Plugin support - mosquitto-auth-plug(1)

The bridge functionality is used ONLY to synchronize messages between all mosquitto brokers while the mosquitto-auth-plugin could be used to save authorization and ACL in a single database back end.

While setting up the bridge, notice the usage of in/out bridge topics to avoid forwarding loops(2). The pattern is of format

topic pattern [[[ out | in | both ] qos-level] local-prefix remote-prefix]

Quoting mosquitto.conf man page(3), For incoming topics, the bridge will prepend the pattern with the remote prefix and subscribe to the resulting topic on the remote broker. When a matching incoming message is received, the remote prefix will be removed from the topic and then the local prefix added. And vice versa for an outgoing topic

Sample mosquitto.conf for a 3 node cluster is below. To be load tested with mqtt-malaria(4) and more importantly the effect on clients connecting with clean_session flag to false, .

                                          +-------------------+
                       +------------------> BRIDGE BROKER     <------------+
                       |                  | 192.168.1.1       |            |
                       |                  +--------^----------+            |
                       |                           |                       |
                       |                           |                       |
                       |                           |                       |
  Broker A             |             Broker B      |                       |   Broker C
                       |                           |                       |
+----------------------+--+       +----------------+--------+           +--+----------------------+
| connection A            |       | connection B            |           | connection C            |
|                         |       |                         |           |                         |
| address 192.168.1.1:1883|       | address 192.168.1.1:1883|           | address 192.168.1.1:1883|
|                         |       |                         |           |                         |
| topic # out 2 "" A/     |       | topic # out 2 "" B/     |           | topic # out 2 "" C/     |
| topic # in 2 ""  B/     |       | topic # in 2 ""  A/     |           | topic # in 2 ""  A/     |
| topic # in 2 ""  C/     |       | topic # in 2 ""  C/     |           | topic # in 2 ""  B/     |
+----------------------^--+       +----------------^--------+           +--+----------------------+
                       |                           |                       ^
                       |                           |                       |
                       |                           |                       |
                       |                   +-------+---------+             |
                       +-------------------+ HA PROXY        +-------------+
                                           +-----^--^--------+
                                                 |  |
                                                 |  |
                                                 +  +