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RE: Multi-Master, referrals, and chaining

I'd probably set it up a little differently.  Something like this (very

2 of your 6 servers as suppliers/masters, accepting writes.  If you are
using a load balancer, set it up so that you have a vip that always goes
to one of these servers (the "primary" supplier), and directs traffic to
the other ("secondary" supplier) only when the primary in unavailable.
This gives you redundancy for high availability, while minimizing risk
of replication conflicts.

4 of your 6 servers as consumers.  If you are using a load balancer,
create a vip for your "read-only" ldap cluster, such that traffic is
balanced across these servers.  Referrals on these should go to the
supplier vip (not directly to either "real" supplier).  If your client
apps chase referrals, you are fine.  If not, you may need to set up
chaining to pass writes to the supplier vip.

To scale read access, add more servers to the consumer cluster (scale
horizontally).  To scale your suppliers, get bigger boxes (Scale
vertically - distributing writes across more supplier servers doesn't
help with scaling, since replication kicks in and they all see all the
write load anyway, but details of that have been discussed ad nauseum

If you are using SSL/TLS, there are some complications with naming and
making certs match up.  
If you don't use load balancers, you'll have to do some config/dns/etc
tricks, maybe some manual or scripted failover, etc.

If you are wed to your current setup - you'll probably need to do
something like:
- Make only one server the supplier
- Make the rest consumers
- Monitor the supplier, and write a script that makes all the
appropriate config changes to let you quickly promote a consumer to
supplier and redirect the remaining consumers, but this can get tricky,
I'm thinking.


> -----Original Message-----
> From: 
> openldap-software-bounces+jeff_clowser=fanniemae.com@OpenLDAP.
> org 
> [mailto:openldap-software-bounces+jeff_clowser=fanniemae.com@O
penLDAP.org] On Behalf Of Ryan Steele
> Sent: Wednesday, March 24, 2010 12:38 PM
> To: openldap-software@openldap.org
> Subject: Re: Multi-Master, referrals, and chaining
> I actually realized that my logic was slightly faulty - since 
> each of the six masters is capable of performing the write
> operation, the referral will never be chased.  Given that, is 
> it impossible to have an N-Way MultiMaster configuration
> in which writes are possible on any node, but the writes at 
> any given time are performed on only one of them to ensure
> consistency?  That is, short of doing what I'm doing now 
> (sending ALL traffic to only one master), which doesn't scale
> very well.
> Ryan Steele wrote:
> > Hey folks,
> > 
> > As it stands, I've got an environment with six slapd 
> servers (2.4.18) in an N-Way Multi-Master configuration.  At any
> > given moment, only one of these six nodes is receiving 
> client requests (reads, writes, etc.).  We use Pacemaker to
> > provide some basic high-availability services, such that if 
> the current 'master' (a term I'll use in this thread to
> > refer to the one node receiving all reads/writes from 
> clients) croaks, a shared IP/hostname resource is migrated to the
> > next available server.
> > 
> > I would like to modify this setup such that ALL of these 
> six nodes can receive client requests, but in the interest of
> > preserving data consistency, I'd like to refer writes to 
> just one of them (that one being the current 'master'), such
> > that the rest receive the update(s) via syncrepl.  From my 
> own research and reading, it seems as though I'll need a
> > combination of referrals (specifically, the 'updateref' 
> directive) and chaining, with the latter being introduced so
> > that the job of chasing the referrals is handled 
> server-side instead of being offloaded to the clients.  I have two
> > questions about this setup, though:
> > 
> > 1. In order to prevent an endless loop of referrals, I'm 
> assuming that the terminal node (the 'master') should NOT have
> > an updateref option that points to the 'master' shared 
> IP/hostname (i.e., itself), like the rest of the nodes do.  Is
> > this correct, or will slapd realize that the referral 
> target is itself, and answer authoritatively?
> > 
> > 2. If the assumption in #1 is correct, what is the most 
> advisable way to handle referrals in the situation where the
> > identity of the terminal node is subject to change?  My 
> thought was to trigger an ldapmodify operation which effectively
> > removes the olcUpdateRef option during the failover, but 
> I'm open to better ideas if they exist.
> > 
> > Thanks!
> > 
> > -Ryan
> >