Re: Running Postgres Daemons with same data files

Just to add a tidbit on top of this, there are commercial RDBMS's that
do, or at least say they do, clustered database servers.  (cough cough
oracle, cough cough ingres)

The products are expensives as all get out, a bastard to set up, and
require truely kickass SAN storage and clustered hardware to be
meaningful.  Not, in general, something to be set up on a lark.

You can get an awful lot of what you seem to want with replication.
Have you considered doing so ?  For instance, I expect there are
replication products for postgres that would allow a disconnected client
to push transactions up to a master server, then resyncronise itself
with the master.

In terms of HA, in addition to replication solutions, you can also do a
fair to middling job of failover by having a primary and hot backup
database server on a heartbeat/STONITH setup, using a shared, redundant
SAN as a data store.  The hot backup would mount the SAN drives on
primary failure, takeover the IP, and start the postmaster.

Of course, just the SAN setup for such a beast is going to run to a
minimum of 5 figures, and any transaction that might be in progres will
be borked. Oh, and this complicates the mobile operations requirement
quite a bit too. :-(

If you feel really really really masocistic, uh, I mean ambitious, you
could start looking at what it would take to port and run a RDBMS server
in a truely clustered, high availablibility environment.  It will not be
simple.  You have issues of distributed cache coherency, distributed
locking, distributed filesystems/datastory, load balancing on a
by-transaction basis (wouldn't want to send the second part of the same
transaction to a different server ....), recovery from server node
failure, adding server nodes to a running instance, and lots of other
things I haven't thought of.

This is complicated by the fact that there really isn't any mature linux
cluster solution that gives decent support for everything you've asked
for yet.  The closest project I'm aware of is SSIC-Linux.  Look it up on
sourceforge if you're curious.

Good luck,
-- Pat

p.s.  Yes, I was brought up and spoiled on VMS clusters.  Still nothing
beats good 'ol RDB for a clustered database solution.

Bhartendu Maheshwari wrote:
> Dear Hal, Frank, Oli and all,
>
> I understand what you all trying to say, I know this is not good way of
> designing, but we are planning for using the database for the keeping
> mobile transactions and at the same time we need to provided the HA
> solutions. The one solution i derive from the discussion that uses one
> server and multiple clients but the issue in this if the system in which
> database server was running get down the its all the way no use of HA
> and load balancing, since without data the other one can't do anything.
>
> What I have in mind is the following implementation:-
>
> Step 1 :- Mount the data files from NAS server.
> Step 2 :- start the postgres with the mounted data.
> Step 3 :- Lock the data files by one server.
> Step 4 :- do the database operation.
> Step 5 :- Commit in the database files.
> Step 6 :- Unlock the database files
> Step 7 :- Now the other one can do the same way.
>
> Or if anybody have other solution for this, please suggest. How can I
> commit the data into database files and flushing cache with the latest
> data files contents? Is there any command to refresh both.
>
> Thank you Hal for
> *************************************************************************
> If you really, really do need an HA solution, then I'd hunt around for
> someone to add to your team who has extensive experience in this kind of
> thing, since it's all too easy otherwise to unwittingly leave in lots of
> single points of failure.
> *************************************************************************
>
> If you really have someone to help me in this regards, please I need his
> help in this issue and want to derive a common techniques so that
> everyone can use this.
>
> regards
> bhartendu
>