Hi,
On Wed, Dec 18, 2019 at 1:47 PM White, David via FreeIPA-users
<freeipa-users(a)lists.fedorahosted.org> wrote:
Reviewing the FreeIPA documentation for deployment recommendations, I read:
“generally, it is recommended to have at least 2-3 replicas in each datacenter”.
A couple of months ago, when we initially designed and deployed FreeIPA / IdM, we decided
to deploy 3 nodes into each of our two datacenters, for a total of 6 servers.
I have re-deployed that cluster 4-5 different times over the past few months for various
reasons as we’ve continued to test things and prepare for use in our production
environment.
In master-master database clusters (such as a MariaDB Galera cluster), you never want to
have an even number of servers, to avoid the potential for a split-brain scenario.
https://galeracluster.com/library/documentation/weighted-quorum.html
Clusters that have an even number of nodes risk split-brain conditions. If should you
lose network connectivity somewhere between the partitions in a way that causes the number
of nodes to split exactly in half, neither partition can retain quorum and both enter a
non-primary state.
Is this a scenario that FreeIPA could run into?
Not at all.
The 2-3 replica per datacenter recommendation is there to provide
redundancy as a single replica would not provide that.
A network (even) split in a FreeIPA cluster does not have the same
outcome as in a MariaDB Galera cluster.
The main impact is that LDAP entries (IPA objects) added on one side
of the now-split cluster will (obviously) not be replicated to the
other side until the network link is available again.
Badly configured clients could also take time to failover.
This is obviously a less-than-ideal situation that should not last for
too long as the replication backlog could grow to difficult to manage
levels easily.
But there will not be any split brain.
François
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