That seems like it would work, but what happens if there is a lot of data
such as MySQL databases that is tough to combine, if two servers have
different updates after a reconnect?

----- Original Message -----
From: "Jan Harkes" <jaharkes_at_cs.cmu.edu>
To: <codalist_at_TELEMANN.coda.cs.cmu.edu>
Sent: Tuesday, May 09, 2000 3:35 PM
Subject: Re: High Availability


> On Tue, May 09, 2000 at 06:46:44PM +0200, Nicolas Huillard wrote:
> > I'm finishing reading Dr Le Blanc's presentation of Coda
> > (http://www.mcc.ac.uk/~zlsiial/coda/text47.html) and also wonders
> > if... (quote :) "Finally, I wonder whether Coda's success as
> > high-availability software is impaired by the dependence of a cell on
> > a single System Control Machine." I am really concerned about this
> > issue, because my goal is to build High Availability...
> > Does anyone have an idea ?
>
> The coda clients and servers do not rely on the existence of an SCM. The
> only reason that machine is special is because of the way we propagate
> updates to the user and volume databases. They are read/write on the
> SCM, and the other servers check for updated files on the SCM once every
> 30 seconds.
>
> If you lose the SCM, all servers can still rely on their local copy. But
> in order to add a volume or user, you need to restore the old SCM. Or
> designate one of the other servers (all of them should have all of the
> required data) as the replacement SCM. To do this is is necessary to
> update the /vice/db/scm file on all servers, and restart the
> updateclnt/updatesrv processes. Not as good as a quorum based replicated
> database server, but a lot simpler for the common case.
>
> Jan
>