[rabbitmq-discuss] Correct way of determining which node is master?

Simon MacMullen simon at rabbitmq.com
Wed Feb 22 09:58:27 GMT 2012

On 21/02/12 19:04, Matt Pietrek wrote:
> In a clustered scenario (3 brokers) with mirrored queues, what is the
> correct way to determine which node is the "master" via inspection?

There isn't one. As in, there isn't really an overall master node.

> On approach would be to look at the list of queues and see which node is
> hosting the queue (as opposed to slave_nodes). However, I'm unsure if
> there's some scenario where queue 'A' is hosted by one broker, while
> queue 'B' is hosted by another.

Absolutely. The original master node *for a given queue* is the one that 
the client that first declared the queue was connected to. But then they 
can fail over to other nodes pretty arbitrarily.

> Another approach is to go off of whichever node has the statistics
> database, but I'm unsure if there's a 1:1 correlation there.

Again, this is the first node with mgmt to come up, or some other node 
at random in the case of failover.

> Background info, in case it matters:
> We're using keepalived to maintain a virtual IP that all clients connect
> to. We'd obviously like keepalived to put the VIP on whatever node is
> the master rather than on a slave node. Our understanding is that if the
> master has messages that aren't in the slave node, a client that
> connects via the VIP to the slave node and tries to read messages won't
> see them as they're only on the master.

No, that's not correct. Messages are *always* read from the master node 
(for a given queue).

Cheers, Simon

Simon MacMullen
RabbitMQ, VMware

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