Best approach to prevent split brain

Hi All,

we started using keepalived and so far it looks very promising. We would
like to use it for VIP failover/HA but we are missing one important point
which is how one can avoid split brain ...

We already posted the question on Github and pqarmitage answered it with a
note that follow up should be posted onto this mailing list.

For initial question and response please see
https://github.com/acassen/keepalived/issues/408

To follow up pqarmitage answer .. we were told to not use separate
communication link for VRRP but to use the same interface where VIP is
running - this really makes sense and thank you pqarmitage for that advice.
But if I understand it correctly then even with that configuration there
could be a state that both application instances are running in MASTER mode
and therefore some clients can connect to one instance and some other
clients to the second instance and because our application has a state
which has to be replicated we always require that there is only one and
exactly one MASTER in a pair and then 0 or one HOT STANDBY instance.

To give you a better picture of our use case here is the full description:

- We have two separate boxes on which we are running our application. This
application is listening for incoming connections and doing some job. The
incoming connections are TCP based and the application has a state. To
support HA of our application we implemented a state replication between
two instances of our application. The instance can run in two modes:

A) Master - the incoming TCP-based connections are established and it is
processing all client requests and doing some job. Every state change is
replicated to second (Hot Standby) instance.

B) Hot standby - application is running but the TCP-based connections are
not established. Hot standby instance is only listening for replicated
state and updated from master instances. Once the master application stops
working hot standby application should switch to MASTER mode, start
listening for incoming TCP-based connections and start processing the
requests.

Can you provide us with more information how exactly we should configure
keepalived to best meet our requirements and more importantly to prevent
any false state which could cause some unexpected behavior of our platform?

Thank you very much for any additional advice.

Kind regards,
Petr

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