syncrepl with hardware content switch

["Brett @Google" <brett.maxfield@gmail.com>]

Hello,

I was wondering if anybody is using syncrepl in the context of a
hardware content switch or redundant environment.

That is, having a "hidden" syncrepl master, and 1-N syncrepl clients
which receive updates from the master, and those client nodes (and
only those nodes) are visible via a content switch, for the purposes
of load sharing and redundancy (more predominantly the latter).

I am considering the edge case where a connection is redirected to a
client, and :

a) client has no current data (new node introduced)
b) client decides it needs to do a full refresh - perhaps it was down
and missed a large number of updates

The problem is, that while a replica is b) significantly incomplete or
a) has no data at all, it should not be given any ldap requests by the
content switch.

A standard content switch either blindly send connections round-robin
to nodes 1-N, or if it determines that a server is "listening" (say by
sending a SYN probe) before it sends through the ldap request. Few
content switches are smart enough to examine the ldap failure code, as
most just operate on tcp streams and don't do content inspection, so
doing ldap content inspection is even less likely.

So this means that during the time a replica is initializing, and ldap
requests are going to incorrectly get "no results" where the answer
should be "not applicable" and the content switch or ldap client
should have tried again, getting another (already initialized) server.

Ideally (in a content switch environment at least), the ldap server
should not listen for requests while it is re-synchronising, but in
the case of syncrepl push replication, replication can happens over
the same port as ldap client requests.

One answer would be if syncrepl could happen over it's own port, as
then there could then be the option of not accepting (not listening?)
or refusing connections on the client port, whilst syncrepl is
(re)building on the syncrepl port.

Alternatively, there could be a "health" port, which only accepted a
connection and maybe returned "OK" if the replica was "healthy", this
port could be specified as a "probe" port on the content switch, to
determine the health of a syncrepl client.

I was just wondering how other people are dealing with this issue and
thier content switches.

Cheers
Brett