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Subject: Re: Postgres HA
From: John Scalia <jayknowsunix@gmail.com>
In-Reply-To: <20180105220042.0978d4c1@firost>
Date: Fri, 5 Jan 2018 20:07:16 -0500
Cc: Azimuddin Mohammed <azimeiu@gmail.com>, pgsql-admin@postgresql.org,
 pgsql-general@postgresql.org
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References: 
 <CAKUuFd7x8FAg0V8+mStBeXcctpbykTz5uf0J0dtdEgfW9DL3Hg@mail.gmail.com>
 <20180105220042.0978d4c1@firost>
To: "Jehan-Guillaume (ioguix) de Rorthais" <ioguix@free.fr>
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What he said, and you also may want to look at pgpool-II. I=E2=80=99ve had f=
airly good luck with that and Tatsuo (the author) hangs out here occasionall=
y too.
=E2=80=94
Jay

Sent from my iPad

> On Jan 5, 2018, at 4:00 PM, Jehan-Guillaume (ioguix) de Rorthais <ioguix@f=
ree.fr> wrote:
>=20
> On Fri, 5 Jan 2018 13:07:10 -0600
> Azimuddin Mohammed <azimeiu@gmail.com> wrote:
>=20
>> Hello,
>> I am little confused with how HA works in postgres. Reading the article
>> which state as below "*If the primary server fails and the standby server=

>> becomes the new primary, and then the old primary restarts, you must have=
 a
>> mechanism for informing the old primary that it is no longer the primary.=

>> This is sometimes known as STONITH (Shoot The Other Node In The Head),
>> which is necessary to avoid situations where both systems think they are
>> the primary, which will lead to confusion and ultimately data loss.*
>>=20
>> *Many failover systems use just two systems, the primary and the standby,=

>> connected by some kind of heartbeat mechanism to continually verify the
>> connectivity between the two and the viability of the primary. It is also=

>> possible to use a third system (called a witness server) to prevent some
>> cases of inappropriate failover, but the additional complexity might not b=
e
>> worthwhile unless it is set up with sufficient care and rigorous testing.=
*
>> *PostgreSQL does not provide the system software required to identify a
>> failure on the primary and notify the standby database server. Many such
>> tools exist and are well integrated with the operating system facilities
>> required for successful failover, such as IP address migration."*
>>=20
>> Can someone explain how the HA failback will take place
>=20
> The failback need either to rebuild the old master as a standby (rsync,
> pg_basebackup, restore PITR, ...) or to use pg_rewind to rewind the old ma=
ster
> to a point where it can catch up with the new master.
>=20
> Some tools tries to automate failback using pg_rewind (patroni, repmgr), b=
ut I
> have no experience with them.
>=20
>> and what open source tools we can use to make sure once the primary serve=
r
>> which failed over to slave will mark itself as slave.
>=20
> There's a lot of open source tools to build some HA around PgSQL: Repmgr,
> Patroni (based on etcd or zookeeper), PAF (based on Pacemaker), etc. You w=
ill
> have to spend a lot of time to make extensive tests, understand them, pick=
 one
> and document your cluster.
>=20
> Regards,
>=20