[Date Prev][Date Next] [Chronological] [Thread] [Top]

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.

I should note that if you go very far down this path, you are going to wind up re-implementing X.500 DSP and DISP:
X.500 servers have explicit knowledge references to their superior, subordinate, and peer servers
cooperating servers maintain a single connection to each other, all chaining activity is mutiplexed over this connection using DSP
when any server connection dies, its connected neighbors notice immediately

All of those features are necessary in order for what you propose to work, and they're already part of X.500.

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