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Re: Feature request: multimaster

Wouldn't conflict resolution be a must for this? Think of a provider that is not reachable by the rest of the DSAs, not because it is down, but due to a segmented network. You will end up with true multimaster where changes are going to be taken on both of them which may result in conflicts.

Cheers, Morteza

Pierangelo Masarati wrote:
We are in the process of investigating the feasibility of adding the above capability to slapd.

First of all a fundamental clarification: multimaster here means that there is a pool of DSAs that are in sync (e.g. by syncrepl). Each time, only one of them acts as the provider, but they all know of each other. As soon as the provider is not reachable any more, the consumers elect another provider among the remaining DSAs in the pool, much like replication for Berkely DB does, selecting the one with the latest contextCSN or, in case, resolving the conflict somehow (e.g. a ballot with random sleep). Appropriate measures are required to welcome the original provider back in the pool: it should become a consumer and sync with the new provider, but conflicts might occur if it was modified after losing providership.

This essentially needs the configuration of the replication (i.e. syncprov overlay and mostly the syncrepl directive, updatedn and updateref) to be modifiable run-time, via some mech to be defined.For the purpose, unless back-config is available any soon, I'd like to investigate the possibility to temporarily delegate this to back-monitor, e.g. by exposing the syncrepl and updateref directives and allowing them to be modified via protocol. Despite promotions/demotions should be performed internally, manual intervention via protocol should still be possible.

The rest of the multimaster functionality, that is the capability to accept writes anywhere in the pool, should be delegated to the chain overlay, in order to guarantee the consistency of the operation in a tansparent manner, at the cost of a delay between the successful return of the update and the actual appearance of the changes on the database.


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