syncrepl multicast MMR

[Howard Chu <hyc@symas.com>]

Been thinking this would be worth trying for a while now. Set a config 
option for syncprov to send Persist messages to a multicast group 
instead of the original TCP session. All the consumers would also join 
the group and listen for updates. This would also exercise the cldap:// 
support in libldap.

Implementation details: since datagrams are unreliable, we need to 
include sequence numbers on each message, which the consumer can check 
to make sure it hasn't missed an update. Moreover, it should be able to 
send a request to the provider to resend (over the TCP session) the 
message corresponding to a given sequence number.

(Currently I envision using a small circular array in the provider to 
remember the last N messages for potential retransmit.)

Config: both consumer and provider will need to be configured with a 
particular multicast group ID. It should be possible to participate in 
more than 1 group at a time (in which case, an update must be explicitly 
sent to each active group) but in general, I expect a cluster of 
cooperating MMR servers to all use a single multicast group, and so any 
update will only need to be forwarded to the network once.

Thoughts?

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