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 http://www.openldap.org/lists/openldap-devel/200806/msg00041.html
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 (glue) database.
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?
-- Rein Tollevik Basefarm AS