Wednesday, November 15, 2023

[389-users] Re: Documentation as to how replication works

I'm not sure about doc, but the basic idea iirc is that a vector clock[1] (called replica update vector) is constructed from the sequence numbers from each node. Therefore it isn't necessary to keep track of a list of CSNs, only compare them to determine if another node is caught up with, or behind the state for the sending node. Using this scheme, each node connects to each other and by asking the other node for its current ruv can determine which if any of the changes it has need to be propagated to the peer. These are sent as (almost) regular LDAP operations: add, modify, delete. The consumer server then decides how to process each operation such that consistency is preserved (all nodes converge to the same state). e.g. it might skip an update because the current state for the entry is ahead of the update. It's what nowadays would be called a CDRT scheme, but that term didn't exist when the DS was devloped.

On Wed, Nov 15, 2023, at 9:59 AM, William Faulk wrote:
I am running a RedHat IdM environment and am having regular problems with missed replications. I want to understand how it's supposed to work better so that I can make reasonable hypotheses to test, but I cannot seem to find any in-depth documentation for it. Every time I think I start to piece together an understanding, experimentation makes it fall apart. Can someone either point me to some documentation or help me understand how it works?

In particular, IdM implements multimaster replication, and I'm initially trying to understand how changes are replicated in that environment. What I think I understand is that changes beget CSNs, which are comprised of a timestamp and a replica ID, and some sort of comparison is made between the most recent CSNs in order to determine what changes need to be sent to the remote side. Does each replica keep a list of CSNs that have been sent to each other replica? Just the replicas that it peers with? Can I see this data? (I thought it might be in the nsds5replicationagreement entries, but the nsds50ruv values there don't seem to change.) But it feels like it doesn't keep that data, because then what would be the point of comparing the CSN values be? Anyway, these are the types of questions I'm looking to understand. Can anyone help, please?

William Faulk
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