Re: Proposal: Implement failover on libpq connect level.

On Tue, Sep 1, 2015 at 7:50 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
wrote:

> Robert Haas wrote:
> > On Wed, Aug 19, 2015 at 9:41 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > > That sort-of ties into what seems to me the main objection to this
> > > proposal, namely that there is already a way to do this sort of thing:
> > > DNS-based load balancing. All the clients think they connect to
> > > db.mycompany.com, but which server they actually get is determined by
> > > what IP address the DNS server tells them to use.
> >
> > But that kinda sucks. I mean, suppose I have three servers, A, B, and
> > C. I point db.mycompany.com to A, which is the master; then A dies.
> > Under your proposal, whatever script I use to control failover now has
> > to change the DNS records to repoint db.mycompany.com to B, my new,
> > and newly-promoted, new master. It's quite possible that some
> > machines on the network, or some processes, will have the old IP
> > address cached, and it may be several minutes before those caches time
> > out. In the meantime, I'm down: even if I bounce the application
> > servers, they may just try to reconnect to A.
>
> The solution to this part seems to be to lower the TTL, which seems
> easy enough.
>

It seems easy enough. Until you actually try to do it, and discover
platforms and appservers that completely ignore the ttl by default (like,
uh, java, which you may have run into - at least it used to do that), or
enforces a lower minimum of longer than you want in order to decrease load,
and things like that.

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/