[wplug] OT - request for comment on an idea

[Bill Moran]

Christopher DeMarco <cmd at alephant.net> wrote:

> > On the secondary server I want to have a copy of our
> > website/db/email systems.  The services will not be running by
> > default but will have up-to-date data.  The reason for this is, I
> 
> How will you synchronize the content?  Typically the content synch is
> more of a challenge than the high availability.  Please explicate your
> plans; I (as well as other list members, I suspect) have decent
> experience in this realm.

This can be the hardest part.  Web content can often be quite simple, since
it usually dosn't change very often.  A simple rsync script run every 15
minutes might be enough to keep the content synced without any major
network overhead (rsync uses very little bandwidth if nothing's changed)

email can be very difficult.  It changes so often that it's not easy to
keep organized.  Any time you switch between the two machines, there's
the chance that messages will get stuck on one or the other.

If you're using pop3, you can just do an rsync like you did with the web,
but (depending on your mail load) it could be considerably more bandwidth.
If you're using IMAP (like we are) then it's probably not practical to do
rsync.

As I said, the biggest problem is that when the DNS switches over, any
mails received, but not downloaded will be stuck on the other machine, and
when it switches back, your rsync script will obliterate any emails stuck
on the backup machine.  I'm not saying it can't be done, I'm just saying
that it will require some careful planning to avoid problems.

-- 
Bill Moran
Potential Technologies
http://www.potentialtech.com