Re: [PATCH] libpq: Allow specifying multiple host names to try to connect to - Mailing list pgsql-hackers

On Wed, Jul  8, 2015 at 12:24:37PM -0400, Robbie Harwood wrote:
> > You update the documentation just for  psql but your change effects any 
> > libpq application if we go forward with this patch we should update the 
> > documentation for libpq as well.
> >
> > This approach seems to work with the url style of conninfo
> >
> > For example
> >   postgres://some-down-host.info,some-other-host.org:5435/test1
> >
> > seems to work as expected but I don't like that syntax I would rather see
> > postgres://some-down-host.info:5435/test1,postgres://some-other-host.org:5435/test1
> >
> > This would be a more invasive change but I think the syntax is more usable.
> 
> I agree with this; it seems to me that it's more powerful to be able to
> specify complete urls for when they may differ.
> 
> For the non-url case though, I don't see a clean way of doing this.  We
> could always, e.g., locally bind port specification to the closest host
> specification, but that seems nasty, and is still less powerful than
> passing urls (or we could just do the same for all parameters, but
> that's just a mess).
> 
> Might it be reasonable to only allow the multi-host syntax in the
> url-style and not otherwise?

First, I agree this is a very useful feature that we want.  Many NoSQL
databases are promoting multi-host client libraries as HA, which is kind
of humorous, and also makes sense because many NoSQL solution are
multi-host.

I can see this feature benefitting us for clients to auto-failover
without requiring a pooler or virtual IP reassignment, and also useful
for read-only connections that want to connect to a read-only slave, but
don't care which one.  The idea of randomly selecting a host from the
list might be a future feature.

I agree we should allow the specification of multiple hosts, e.g. -h
"host1,host2", but anything more complex should require the URL syntax,
and require full URLs separated by commas, not commas inside a single
URL to specify multiple host names, as shown above.  If repeating
information inside each URL is a problem, the user can still use
connections-specific options to controls things, e.g. by using -p 5433,
it is not necessary to specify the port number in the URLs:
$ psql -p 5433  postgres://localhost/test,postgres://localhost/test2

I realize this is libpq-feature-creep, but considering the complexities
of a pooler and virtual IP address reassignment, I think adding this
makes sense.  The fact that other DBs are doing it, including I think
VMWare's libpq, supports the idea of adding this simple specification.

Can someone work on a patch to implement this?

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + Everyone has their own god. +