[MonetDB-users] Dual instances of MonetDB for 1 dbfarm - is it possible?

Stefan Manegold Stefan.Manegold at cwi.nl
Wed Sep 3 10:31:06 CEST 2008

On Wed, Sep 03, 2008 at 09:38:46AM +0200, Fabian Groffen wrote:
> Hi Matthew,
> On 02-09-2008 21:56:40 -0400, McKennirey.Matthew wrote:
> > While we look forward to the availability of a new real-time
> > replication strategy for MonetDB, we were wondering if it would be
> > plausible to configure two instances of MonetDB, on different machines,
> > to point to the same dbfarm.
> I assume you mean not only using the same dbfarm, but also using the
> same databases.  MonetDB locks the database it is using, so unless NFS
> locking or something is malfunctioning you should see this doesn't work.

At different points in time (i.e., not concurrently) two different instances
of MonetDB can (technically) very well share the same dbfarm --- provided
the two instances of MonetDB are binary compatible. In fact, multiple
instances of MonetDB can even concurrently share the same dbfarm, provided
they all use a different database (dbname).
MonetDB locks the database such that only a single instance can use a
particular database at a time.

> > Only one instance would be used at a time; one instance would be the
> > primary instance and the second instance would only be used if the
> > first failed to respond (at which time we would stop sending requests
> > to the primary instance and raise an alert for the system
> > adminsitrator)
> Sounds like a "failover".
> > We can provide for the replication of the database data at the file
> > system layer (ZFS) but are still susceptible to a failure of MonetDB or
> > the machine it is running on.
> The danger of the ZFS solution here is that you get a copy, that doesn't
> include locks.  We once had some thoughts of supporting read-only
> databases (think of a LiveCD), but for your use that sounds not quite
> like what you need either.
> I'm wondering why you actually want to "failover" from another machine.
> Does it mean that merovingian isn't able to cover up mserver crashes?
> Does mserver take the entire machine down?  Or are there other
> conditions (network?) that lead to this multi machine failover strategy?

I do share Fabians concerns.


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| Dr. Stefan Manegold | mailto:Stefan.Manegold at cwi.nl |
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