[KLUG Members] Postgres Cluster

[Adam Tauno Williams]

> I wouldn't evan use the word cluster to describe a backup situation like
> this.  It's splitting hairs a bit.  I would only use the word cluster to
> describe a situation where each machine has live data ( such as the one
> described ) and each machine is accepting querries (unlike the one
> described).  The application being used could be responsible for only
> performing update querries against a machine which would be a master.
> Clustering with multiple masters get's really sticky because of record
> locking.  Man what a pain.  I could not evan imagine trying to lock the
> same record on 1 or many replicants for a multiple master configuration.

Yep, agree.  I've looked at MySQL and PostgreSQL's "cluster" support.
And I don't think any of then qualify as a "cluster" in the way most
people intuitively think of the term.  They either user enourmous
amounts of RAM to keep data resident or they are simply replicants.

> I use unidata and would definitely not call what I do a cluster.
> Unidata keeps a transaction log.  I periodically (every 15 minutes)
> rsync the transaction log over to a backup unidata server.  If there is
> a major catastrophe on the main server, a script can be run to play the
> transacitions into the backup from the last night.  The maximinum amount
> of transactions I can lose is 15 minutes worth.

Right.  Our Informix database keeps transactions logs and automatically
spools a log to tape when it complete.  The logs + a full backup can
restore/replay the entire database back to the point when the
transaction log was closed.

> I like talking word semantics. So I ask the question is ldap with
> replicant servers an ldap "cluster".  I'd say yes, but no-one I have
> ever heard has called ldap servers with replicants a cluster.  Why???

Because they are not a cluster, they are a master (or set of masters)
and a hive of replicants.  A cluster is a peer-to-peer construct where
nodes have functional equality or at least the potential for functional
equality.

> Does the dictionary say that a cluster must be on a data repository to
> some xxx standard??  I would guess not.  Language can be fun.
> Unfortunately, I have never attempted to cluster postgres servers, so an
> answer to your direct question is absent from my experience.
> > Hey all.. I was looking at the Postgres documentation and their definition
> > of a Database Cluster is different from what I am use to with MS SQL. The
> > definition in the manual basically states a cluster is multiple databases
> > run by a single instance of Postgres. A cluster in the terms of my thinking
> > is a setup of multiple SQL installs/machines that replicate from a Main
> > point incase one machine/install fails the next machine takes over until the
> > primary machine can be repaired and then replicate the data from the backup
> > machine before it becomes live again.. can postgres do this and if so can
> > someone point me to documentation on it. 

I think so,  take a look at "slony".
http://gborg.postgresql.org/project/slony1/projdisplay.php
http://candle.pha.pa.us/main/writings/pgsql/replication.pdf

Slony is "Slony-I is a "master to multiple slaves" replication system
with cascading and failover."

Still not what I'd call a cluster, but it gives you redundancy.  

I think if you want a true cluster your options are: Oracle or DB2