Re: Replication for high availability?

[Steve Schultze <sjs@gospelcom.net>]

On Thu, 9 Aug 2001, Pierangelo Masarati wrote:
> David Wright wrote:
> >
> > (This question is unrelated to my other posts.) My LDAP server went was
> > down for ~hour today, which caused chaos at my site, because it does
> > login authentication. I would like to avoid such problems in the future
> > by setting up a cheap box that replicates its database and could be used
> > in its place. The trouble is, how do I configure my client machines to
> > use the slave in the event of the master's failure? Can anyone suggest
> > some neat trick involving DNS or virtual interfaces or some such magic?
>
> If your clients use the ldap_init initialization function (most do)
> then you only have to feed them a list of hosts; the library tries
> automatically all the hosts until one succeeds.

This is not, in my experience, a very dependable way of providing
redundancy.  The problem which you will likely encounter is that much
software re-uses LDAP connections, and the host is not re-checked and
failed over between uses of the same connection.  This means that if your
master goes down, then your clients will probably have many open
connections to that server which it will not intelligently fail over to
the backup(s) you specified in the list of hosts.  Instead, they will just
keep failing.

My experience with this is solely limited to PADL's nss_ldap, but it is
certainly true that in that case it is totally unnacceptable to try to
provide redundancy simply by specifying multiple hosts.  I believe that
this sort of problem will manifest itself in many other situations as
well.  I don't know the technical specifics as well as others on this
list, so anyone else can feel free to fill in the details or correct me if
I'm wrong.

We chose the hardware (cluster) route.  We have one ip for the cluster,
and it does failover for us automatically.

 --
Steve Schultze - sjs@gospelcom.net
      "My cat's breath smells like cat food." --Ralph