Re: [MLIST] Re: [mail] Re: Big 7.4 items - Replication - Mailing list pgsql-hackers

Another concern I have with multi-master systems is what happens if the 
network splits in 2 so that 2 master systems are taking commits for 2 
separate sets of clients.  It seems to me that to re-sync the 2 databases 
upon the network healing would be a very complex task or impossible task.

On Sunday 15 December 2002 04:16 am, Al Sutton wrote:
> Many thanks for the explanation. Could you explain to me where the order or
> the writeset for the following scenario;
>
> If a tranasction takes 50ms to reach one database from another, for a
> specific data element (called X), the following timeline occurs
>
> at 0ms, T1(X) is written to system A.
> at 10ms, T2(X) is written to system B.
>
> Where T1(X) and T2(X) conflict.
>
> My concern is that if the Group Communication Daemon (gcd) is operating on
> each database,  a successful result for T1(X) will returned to the client
> talking to database A because T2(X) has not reached it, and thus no
> conflict is known about, and a sucessful result is returned to the client
> submitting T2(X) to database B because it is not aware of T1(X). This would
> mean that the two clients beleive bothe T1(X) and T2(X) completed
> succesfully, yet they can not due to the conflict.
>
> Thanks,
>
> Al.
>
> ----- Original Message -----
> From: "Darren Johnson" <darren@up.hrcoxmail.com>
> To: "Al Sutton" <al@alsutton.com>
> Cc: "Bruce Momjian" <pgman@candle.pha.pa.us>; "Jan Wieck"
> <JanWieck@Yahoo.com>; <shridhar_daithankar@persistent.co.in>;
> "PostgreSQL-development" <pgsql-hackers@postgresql.org>
> Sent: Saturday, December 14, 2002 6:48 PM
> Subject: Re: [mail] Re: [HACKERS] Big 7.4 items - Replication
>
> > >b) The Group Communication blob will consist of a number of processes
>
> which
>
> > >need to talk to all of the others to interrogate them for changes which
>
> may
>
> > >conflict with the current write that being handled and then issue the
> > >transaction response. This is basically the two phase commit solution
>
> with
>
> > >phases moved into the group communication process.
> > >
> > >I can see the possibility of using solution b and having less group
> > >communication processes than databases as attempt to simplify things,
> > > but this would mean the loss of a number of databases if the machine
> > > running
>
> the
>
> > >group communication process for the set of databases is lost.
> >
> > The group communication system doesn't just run on one system.  For
> > postgres-r using spread
> > there is actually a spread daemon that runs on each database server.  It
> > has nothing to do with
> > detecting the conflicts.  Its job is to deliver messages in a total
> > order for writesets or simple order
> > for commits, aborts, joins, etc.
> >
> > The detection of conflicts will be done at the database level, by a
> > backend processes.  The basic
> > concept is "if all databases get the writesets (changes) in the exact
> > same order, apply them in a
> > consistent order, avoid conflicts, then one copy serialization is
> > achieved.  (one copy of the database
> > replicated across all databases in the replica)
> >
> > I hope that explains the group communication system's responsibility.
> >
> > Darren
> >
> >
> >
> >
> >
> >
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