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Mirror Mode, MMR and replicas

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.



Quanah Gibson-Mount
Principal Software Engineer
Zimbra, Inc
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