RFE RATLC00738634 -- syncreplica -export analysis in syncreplica

Currently, one may give an explicit list of target replicas one wishes to synchronize: the replica-selector parameter of the syncreplica man page.

In my customization of the distribution script, I will get the list of siblings in one vob, and pass it (formatted) to the command. This way, I create one single packet for multiple destinations.

This is often (as soon as there are a few replicas of the vob, but especially when the routes to the various hosts share a common part) an improvement over the behaviour of the default sync_export_list, however, passing explicitly all the siblings is:

1. an extra step in the script
2. suboptimal in that it will force sending a packet also to replicas which would not need it (one size fits all).

The obvious scenario is the following:

• there is at the present time only one active replica in the set (e.g. for timezone reasons)
• our (import/export, not only forwarding) hub vob server thus received changes only from there, and would reexport those changes to all the other siblings
• all siblings will have their epochs updated, except the one originator, for which the packet is useless and will be skipped after reception (will thus have been shipped there for nothing).

Such an analysis is not trivial to do on the 'hub' vob server, and should in any case be made for every packet, in the script.

-> The RFE: do this analysis in syncreplica, if invoked with a vob:xxx selection. Make then one common packet, skipping the siblings which do not need it!

Note: more sophisticated behaviour yet could be thought of, trading off the size and the number of the packets (common packets if small differences in the oplogs for different destinations, distinct packets if large ones...). I do not request this, but don't exclude it either.