I've been trying to figure out why syncrepl used on a backend that is
subordinate to a glue database with the syncprov overlay should save the
contextCSN in the suffix of the glue database rather than the suffix of
the backend where syncrepl is used. But all I come up with are reasons
why this should not be the case. So, unless anyone can enlighten me as
to what I'm missing, I suggest that this be changed.
The problem with the current design is that it makes it impossible to
reliably replicate more than one subordinate db from the same remote
server, as there are now race conditions where one of the subordinate
backends could save an updated contextCSN value that is picked up by the
other before it has finished its synchronization. An example of a
configuration where more than one subordinate db replicated from the
same server might be necessary is the central master described in my
previous posting in
My idea as to how this race condition could be verified was to add
enough entries to one of the backends (while the consumer was stopped)
to make it possible to restart the consumer after the first backend had
saved the updated contextCSN but before the second has finished its
synchronization. But I was able to produce it by simply add or delete
of an entry in one of the backends before starting the consumer. Far to
often was the backend without any changes able to pick up and save the
updated contextCSN from the producer before syncrepl on the second
backend fetched its initial value. I.e it started with an updated
contextCSN and didn't receive the changes that had taken place on the
producer. If syncrepl stored the values in the suffix of their own
database then they wouldn't interfere with each other like this.
There is a similar problem in syncprov, as it must use the lowest
contextCSN value (with a given sid) saved by the syncrepl backends
configured within the subtree where syncprov is used. But to do that it
also needs to distinguish the contextCSN values of each syncrepl
backend, which it can't do when they all save them in the glue suffix.
This also implies that syncprov must ignore contextCSN updates from
syncrepl until all syncrepl backends has saved a value, and that
syncprov on the provider must send newCookie sync info messages when it
updates its contextCSN value when the changed entry isn't being
replicated to a consumer. I.e as outlined in the message referred to above.
Neither of these changes should interfere with ordinary multi-master
configurations where syncrepl and syncprov are both use on the same
I'll volunteer to implement and test the necessary changes if this is
the right solution. But to know whether my analysis is correct or not I
need feedback. So, comments please?