Emmanuel Lecharny wrote:
Howard Chu wrote:
Is there a doc somewhere that describes the replication protocol?
Here you are :
http://cwiki.apache.org/confluence/display/DIRxSRVx11/Mitosis
These pages are really very rough.
Thanks. I see you've read through all the previous failures on this path...
We encountered similar issues when developing the multimaster support
in OpenLDAP. Our solution was simply to assign unique server IDs to
each master, and record the last known modification for each known
server ID. This way local modifications don't overwrite the (lack of)
state from other servers.
Another issue to consider when bringing up a new replica - you may
want to disallow all queries completely, until the replica has fully
sync'd up. If you allow searches during this initial phase, a client
is likely to get a partial set of results. In some cases, that may be
worse than getting no result...
The pb is that if you have a lot of replication, it may not be a viable
approach. Also if you want to implement the 'paging control', as it's up
to the client to request the next results, you have no good solutions...
At some point, I think we should consider that a LDAP server is not like
a RDBMS system with a full transaction system, and you must assume that
dirty read is a standard mode. That does not mean we shouldn't consider
blocking incoming requests until a sync-up is done when a server is
restarted.
In OpenLDAP I've recommended that a replica chain all incoming requests to a
provider while it's in startup mode. There can be transient inconsistencies at
later points in its runtime, and we live with those. But if you're bringing up
a brand new server, it's best to pretend it doesn't even exist until it's at
least 80-90% sync'd. Obviously you can manage this outside of LDAP (e.g., if
you have a DNS pool or hardware load-balancer directing clients to servers,
don't add the new replica to the pool until it's ready) for a new server.
This is not a simple problem...
Heh heh. So very true.
--
-- Howard Chu
Chief Architect, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
