"Net Tree Inc." <nettreeinc at gmail.com> writes:
> I just need answer that tells me how PostgreSQL and Slony would
> react if such issues happens. Answer can simply just to provide me a
> web link for me to read further. Thanks!

These aren't notably PostgreSQL or Slony-I questions; they are
questions about how you choose to manage the configuration for your
applications.

That doesn't make them dumb questions, just things for which I don't
believe we can provide any prescriptive answers.

Historically, I know that we ("Afilias folk") have faced exactly these
issues as questions of how to deploy already-designed applications
that weren't designed with these questions in mind.

It's something of a struggle: you need to design the ability to
configure these things into applications in order for it NOT to be a
fairly horrible process to do various forms of failover.

Web applications frequently are pretty "horrible" in this fashion,
particularly when they try to be "user friendly" by putting a GUI in
front of the configuration such that you pretty much have to log into
the application in order to reconfigure it.

The thing I'd counsel is to make sure that the configuration used to
control how your apps access databases is centralized in such a way
that you can change it without having to "jump through burning hoops."

Plausible options include:

 - DB connectivity might be defined in a file that can be easily
   rewritten by a script.  ("XML document containing 18 other
   things" is not an example of such a thing...)

 - App configuration never changes; it points to a proxy (e.g. -
   pgbouncer, pgpool) which redirects it to the "real" database

   You can reconfigure pgbouncer/pgpool without "burning hoops" :-)

 - Standardize on a host name and port number; app configuration
   never changes...  But this points to a CNAME so that you may
   readily repoint to an alternate server via changing the CNAME
   in your DNS configuration.

There are probably other options; these three are reasonable ones off
the top of my head.
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Christopher Browne
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