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Re: Feature request: multimaster

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.

Since this is something we'll probably need as well, by all means go ahead with it. The back-config work has been delayed, and I don't see it being ready very soon. Hopefully with syncrepl settling down I'll have time to resume focus on back-config.

I note that one should not expect a completely general solution here, because there are many common replication environments that preclude the connectivity required to make such a scheme work. I.e., a common case we see is with replicas deployed on the opposite side of a firewall from the master, and access rules set up such that connections can only be established in one direction between the master and the slaves. In these scenarios, chaining and floating master configurations would not be allowed by the firewall policy.

Also, the requirement for all slaves to know of each other makes configuration a bit more tedious. One of the claimed features for syncrepl was to avoid the requirement that the provider be configured in advance with knowledge of each consumer. In a floating master setup, it's most logical to store/manage all of the consumer knowledge on the provider and replicate it to the consumers (this is a desired feature of back-config). Needless to say, this is a very different use case from the existing syncrepl code, and will require some new config directives to support it. When working with X.500 knowledge references we found it was easiest to have all the knowledge references configured identically; each server would automatically recognize (and ignore) the reference pointing to itself. This way all of the reference information can be replicated indiscriminately and identically to all slaves, and each DSA does the right thing with it. A separate masterID is used to denote which slot in the list of servers is the current master, and gets replicated along with the other info.

As for using chaining to accept writes anywhere in the pool - I think this is the best approach.

 -- Howard Chu
 Chief Architect, Symas Corp.       Director, Highland Sun
 http://www.symas.com               http://highlandsun.com/hyc
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