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Re: Is this a sensible architecture?
On 14/03/12 09:58 -0400, Aaron Richton wrote:
On Wed, 14 Mar 2012, Brett Watson wrote:
I'm currently planning a shift in our use of LDAP to incorporate
mirror mode masters for the sake of high availability. The plan is
to hide a mirror mode master pair behind a virtual IP using "sorry
server" fail-over, such that the primary mirror server takes 100%
of the load when it's up and responsive, with fall-back to the
secondary mirror server when the primary is down. In this way, the
virtual IP presents a "virtual master" to the outside world, and
the plan is for *all* outside LDAP interaction with the masters to
happen over this single virtual IP.
There will be other "slave" servers which replicate the master (to
distribute read-only load), and the plan is for them to syncrepl
from the master virtual IP. I gather that a possible alternative to
this arrangement is to have the "slave" servers act as syncrepl
consumers to *both* masters simultaneously, via their real IP
addresses. If this is indeed a valid configuration, does it convey
any advantages? The single "virtual master" approach seems
architecturally simpler, but is it considered robust by those in
the know? Thanks in advance.
Well, people test this
Test what? Replicating from the VIP to the slaves or replicating from
both masters to the slaves?
quite regularly (via OpenLDAP's included test suite) and it seems to
work without issue; there are also live sites using mirrormode and
(anecdotally) I don't believe they have an excessive defect rate
versus other installations.
As with most the-other-node-must-be-dead scenarios, the ugly edge
cases rest more on the traffic direction methods than the actual
application. You'd probably be well served by taking a careful stock
of your "virtual IP" methods -- split brain, false positives, what
have you -- and OpenLDAP will likely serve you well.
Nick Urbanik http://nicku.org 808-71011 firstname.lastname@example.org
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I disclaim, therefore I am.