Re: GSoC 2018 - Mailing list pgsql-hackers

Hello,

On Fri, Dec 15, 2017, at 14:30, Stephen Frost wrote:
> Aleksander,
> 
> * Aleksander Alekseev (a.alekseev@postgrespro.ru) wrote:
> 
> > Regarding the difficulty of the project - in fact it's not that
> > difficult. Particularly this project can rely on external tools, e.g.
> > use Consul for service discovery and leader election based on
> > leader-lease approach (implementation [1]). Having this the only thing
> > is missing is automatic replica promoting and (optionally)
> > re-configuring of HAProxy / pgbouncer / whatever. Yes, and lots of
> > Jepsen-like test of course. I believe it's not such a complicated
> > project.

Does it make sense to address the  limitations of the logical
replication first, i.e. inability to replicate DDL, sequences and so on?

> 
> What you're talking about is rebuilding Patroni, but adding more into it
> than even Patroni tries to do, building it on Logical Replication
> instead of physical replication, and calling it simple and something
> that could get into core PostgreSQL over a 12 week GSoC project.  I've
> certainly got doubts about that, even if we decide that it'd be an
> external-to-PG project (like Patroni).
> 
> What might be interesting is seeing if Logical Replication could be
> added to Patroni as an option and then building on that..  Having
> someone involved in the Patroni project would be the right way to go
> about proposing that though to see what they think of it.  That would
> also be much more sensible as a GSoC project, since it'd be an addition
> to an existing project and not more-or-less starting a whole new
> project.

Right now logical replication and  physical replication-based HA tools
don't work together nicely, since logical replication position is not
propagated to the promoted replica (I think Craig Ringer has been
tackling this issue for a few releases already, the latest set of
patches I could find is https://commitfest.postgresql.org/15/788/).
Perhaps there is opportunity for a GSoC student to help fixing it. Until
then  we cannot use logical replication for HA, and even doing something
simpler like automating creation of logical replicas in Patroni makes
little sense, as they are doomed to be reinitialized on every failover.

-- 
Sincerely,
Alex