Feature request: multimaster

[Pierangelo Masarati <ando@sys-net.it>]

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.

p.



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