Mirror Mode, MMR and replicas

[Quanah Gibson-Mount <quanah@zimbra.com>]

A number of our clients have requested "fail-over"/redundancy capabilities 
for the LDAP master, and as I'm currently working on moving our product to 
use OpenLDAP 2.4, this becomes a distinct possibility.  However, I have 
some questions about the viability/reliability/effectiveness of using 
multiple masters combined with replicas.  I don't see these answered in the 
Admin Guide.

I'll start with replication under MMR.

As I understand it, the replicas can only point at a single master.  So, if 
I have a 2 master MMR setup, I assume I would want to point half my 
replicas at master A and the other half at master B for their updates. 
This leads to a problem in my mind, in that if master A goes down, then 
half of my replica pool is now going to remain completely out of sync with 
the remaining master until master A is recovered.  Throwing a Load balancer 
in front of the two masters, and pointing the replicas at that instead, is 
not a viable option because the two masters may be getting updates in a 
different sequence, so if a replica disconnects from the LB and then 
reconnects, the updates it could get fed from whatever master the LB is 
pointing at could lead to inconsistencies.  Neither of these seem like a 
good option.  I don't see a good solution here to resolve this issue, 
either, unless the replica could somehow know which master it had been 
talking to, and drop into refresh mode if it found itself talking to a new 
master?  I'm also not clear on what happens if your replicas are 
delta-syncrepl based, rather than normal syncrepl, in the LB setup.

For Mirror Mode, I would assume you could point the replicas at the LB 
fronting the two masters, since only one master is ever receiving changes. 
I also assume delta-syncrepl would be a completely valid option for 
replication to the replicas, again because only one master is getting the 
updates, so all updates would be logged in the same sequence on both 
servers.  However, I don't know if this is correct or not, or if there are 
limitations here I haven't considered.  When I was first pondering this on 
the #openldap-devel channel in IRC, Matt Backes made a comment about 
delta-syncrepl not working with Mirror Mode.


So, basically, I'm at a loss if my understanding things is correct, on how 
I provide a consistent replicated environment for my customers, while also 
providing master/master failover.

Thanks,
Quanah





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Quanah Gibson-Mount
Principal Software Engineer
Zimbra, Inc
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