Parallel databases?

Does anyone have any suggestions for a way to keep 2 databases in sync?
Ideally updates need to be made to both... this can't be too uncommon a
requirement..... any kind of HA would need it....

I use a PHP or Perl codebase to access databases (mysql or postgres). I always
use a wrapper class to control the database connection because it makes the
code a lot more simple -- additionally I could easily insert logic in the
class to open a second connection to another database and duplicate all
inserts, updates and deletes. Alternatively the info could be queued in a log
file for delayed usage. However this is still far from a true HA setup. For
that either you need a lot of intelligent code or Oracle Parallel Server.

Andy

On Fri, 14 Apr 2000, A James Lewis wrote:

Does anyone have any suggestions for a way to keep 2 databases in sync?

Ideally updates need to be made to both... this can't be too uncommon a
requirement..... any kind of HA would need it....

I believe the drbd system can do this across a LAN. I haven't looked in
detail at it. You should be able to find info on the linux-ha.org site. I
know it mirrors blocks across multiple servers but I have no idea how well
it handles locking etc.

--
|Colin Smith: Colin.Smith@yelm.freeserve.co.uk | Windows 2000 |
|Configuration management library for Unix/Linux | AKA |
| http://www.yelm.freeserve.co.uk/libcfg/ | The W2K Bug |

On Sat, 15 Apr 2000, Colin Smith wrote:

On Fri, 14 Apr 2000, A James Lewis wrote:

Does anyone have any suggestions for a way to keep 2 databases in sync?

Ideally updates need to be made to both... this can't be too uncommon a
requirement..... any kind of HA would need it....

I've had a closer look at drbd. The home site is:
http://www.complang.tuwien.ac.at/reisner/drbd/

It will replicate the blocks on a device out to backup servers. If the
primary system fails the backup can take over. It doesn't do any
distributed locking so it's strictly a failover service at the moment.

It also looks like a real performance killer which is pretty much what
you'd expect with each block being sent over the LAN as well as to disk.
Definitely a case for a very high bandwidth low latency network (SCI, SP
switch) 100Mbit/1Gbit dedicated might be acceptable though.

--
|Colin Smith: Colin.Smith@yelm.freeserve.co.uk | Windows 2000 |
|Linux: Delivers on the promises Microsoft make. | AKA |
| http://www.linux.org/ | The W2K Bug |

James Lewis wrote:

Does anyone have any suggestions for a way to keep 2 databases in sync?

Ideally updates need to be made to both... this can't be too uncommon a
requirement..... any kind of HA would need it....

No. The way HA works is that the system is made in such a way that you
can't lose data, or that you don't lose CPU cycles. HA does not make any
assumptions on the kind of applications that are running on the system.

If you want to experiment with HA, start with building a mirroring disk on
your Linux system to get the feel of it.

Then try to asses what you really want : low down-time or 7x24 operation.
This is what determines your HA system.

If you don't want to lose CPY cycles, then you have to build a cluster
with e.g. two CPU's. These should share their mass storage. This mass
storage should be organised as RAID. With hot-plug capabilities, it is
possible to keep the system running either if a CPU goes down or if a
drive fails.

The worst thing that you can do is to base the implementation of your
application upon the fact that the system should have high-availability
requirements. That is not a database issue, but an operating system issue.

Jurgen Defurne
defurnj@glo.be