Re: Proposal: Implement failover on libpq connect level.

On Tue, Sep 1, 2015 at 8:12 PM, Andres Freund <andres@anarazel.de> wrote:

> On 2015-09-01 14:07:19 -0400, Robert Haas wrote:
> > But I think it's quite wrong to assume that the infrastructure for
> > this is available and usable everywhere, because in my experience,
> > that's far from the case.
>
> Especially when the alternative is a rather short patch implementing an
> otherwise widely available feature.
>

But that won't actually help in the case described by Robert: if the master
server A failed, the client has no idea if B or C would become the new
master.

Unless it actually tries to connect them in turn and check for the result
of pg_is_in_recovery().  I think that brings enough complexity for keeping
this outside of libpq.  Also think about all the extra flexibility people
will likely want to have: number of retries, delay between retries, delay
backoff, etc., to the point we'll have to support some sort of client code
retry_policy_callback.

For read-only clients you might want to include a number of slave
hostnames, and let the connector choose one, but then again you can't
achieve load-balancing on the client side, you're better off using
round-robin DNS.  To add or remove a slave you only need to update DNS, and
not configuration on all the clients.

For the master failover I think a common technique is to just move the
floating IP address from the old master to the new one.  This doesn't
require touching the DNS record.

--
Alex