Cluster in MQTT for IoT and Push Notification

Question

I have started reading some details about MQTT protocol and its implementation. I came across the term 'cluster' a lot. Can anyone help me understand what does 'cluster' mean for MQTT protocol?

In this comparison of various MQTT protocol, there is a column for the term 'cluster'

fraschbi · Accepted Answer · 2017-07-19T13:27:04.443

Forwarding messages with topic bridge loops will not result in a true MQTT broker cluster, which will lead to drawbacks lined out above.
A true MQTT broker cluster is a distributed system that represents one logical MQTT broker. A cluster consists of various individual MQTT broker nodes, that are typically installed on separate physical or virtual machines and or connect over a network.
Typical advantages of MQTT broker clusters include:

Elimination of the single point of failure
Load distribution across multiple cluster nodes
The ability for clients to resume sessions on any broker cluster
Scalability
Resilience and fault tolerance - especially useful in cloud environments

I recommend this blogpost, if you're looking for a more detailed explanation.

score -1 · Answer 2 · answered Jul 18 '17 at 07:59

-1

A cluster is a collection of MQTT brokers set up to bridge all topics between each other so that a client can connect to any one of the cluster members and still publish and receive messages to all other clients no matter which cluster member they are connected to.

A few things to be aware of:

Topic bridge loops, where a message is published to one cluster member which is then forwarded to another cluster member, then another and finally back to the original. If this happens the original broker doesn't have a way to know it originally pushed this to the other cluster members so the message and end up in a loop. Shared message state databases or using a single bridging replication broker can fix this.
Persistent subscriptions/sessions, unless brokers have a pooled session cache then clients will not retain session or subscription status if they connect to a different cluster member when reconnecting.

answered Jul 18 '17 at 07:59

hardillb

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Which MQTT Broker would you use for mobile push notifications? – fosUffe Jul 18 '17 at 08:10
MQTT is the protocol – hardillb Jul 18 '17 at 08:10
Sorry I meant Broker – fosUffe Jul 18 '17 at 08:11
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A real MQTT broker cluster does not have the drawbacks outlined (topic bridge loops and persistent sessions that can't resume on any node). I believe the definition here is more about bridging and not clustering. Also a cluster is about eliminating the single point of failure in MQTT which bridging does not solve since only messages are distributed. – Dominik Obermaier Jul 19 '17 at 07:23
I disagree with this answer See my response. – fraschbi Jul 19 '17 at 12:02

Cluster in MQTT for IoT and Push Notification

2 Answers2