Re: Cascading replication: should we detect/prevent cycles?

On Tue, Dec 18, 2012 at 7:03 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> 2. should we warn the user, or refuse to start up?

One nice property of allowing cyclicity is that it's easier to
syndicate application of WAL to a series of standbys before promotion
of exactly one to act as a primary (basically, to perform catch-up).
One could imagine someone wanting a configuration that was like:

+------------>r2
| |
r1 <-----------+

This is only one step before:

r1------------>r2

or

r2------------>r1

(and, most importantly, after the cycle quiesces one can choose either one)

For my use, I'm not convinced that such checks and warnings are useful
if delivered by default, and I think outright rejection of cyclicity
is harmful.

--
fdr