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From Anindya Haldar <>
Subject Questions on HA cluster and split brain
Date Thu, 14 Jun 2018 00:52:17 GMT
I have some questions related to the HA cluster, failover and split-brain cases.

Suppose I have set up a 3 node cluster with:

A = master
B = slave 1
C = slave 2

Also suppose they are all part of same group, and are set up to offer replication based HA.

Scenario 1

B starts up and finds A
B becomes the designated backup for A
C starts up, and tries to find a live server in this group
C figures that A already has a designated backup, which is B
C keeps waiting until the network topology is changed

Q1: At this point, will the transaction logs replicate from A to C?

Now let’s say

Node A (the current master) fails
B becomes the new master

Q2: At this point will C become to new new back up for B, assuming A remains in failed state?

Q3: If the answer to Q2 is yes, B will start replicating its journals to C; is that correct?

Scenario 2 (split brain detection case)

B detects a transient network failure with A
B wants to figure out if it needs to take over and be the new master
B starts a quorum voting process

The manual says this in the ‘High Availability and Failover’ section: 

"Specifically, the backup will become active when it loses connection to its live server.
This can be problematic because this can also happen because of a temporary network problem.
In order to address this issue, the backup will try to determine whether it still can connect
to the other servers in the cluster. If it can connect to more than half the servers, it will
become active, if more than half the servers also disappeared with the live, the backup will
wait and try reconnecting with the live. This avoids a split brain situation."

Q4: At this point, which nodes are expected to participate in quorum voting? All of A, B and
C? Or A and C only (B excludes itself from the set)? When it says "half the servers”, I
read it in a way that B includes itself in the quorum voting. Is that the case?

Whereas in the ‘Avoiding Network Isolation’ section, the manual says this:

“Quorum voting is used by both the live and the backup to decide what to do if a replication
connection is disconnected. Basically the server will request each live server in the cluster
to vote as to whether it thinks the server it is replicating to or from is still alive. This
being the case the minimum number of live/backup pairs needed is 3."

Q5: This implies only the live servers participate in quorum voting. Is that correct?

Q6: If the answer to Q5 is yes, then how does the split brain detection (as described in the
quoted text right before Q4) work?

Q7: The text implies that in order to avoid split brain, a cluster needs at least 3 live/backup
PAIRS. To me that implies at least 6 broker instances are needed in such a cluster; but that
is kind of hard to believe, and I feel (I may be wrong) it actually means 3 broker instances,
assuming scenarios 1 and 2 as described earlier are valid ones. Can you please clarify?

Would appreciate if someone can offer clarity on these questions.

Anindya Haldar
Oracle Marketing Cloud

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