Configuring more than one hot standby server

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Configuring more than one hot standby server

Santhosh Kumar-2
Hi Community,
 I read about "hot stand by" server and as far as the document goes, it does not explicitly explain about, whether is it possible to configure more than on database as a Hot stand by server. If we can configure more than one hot stand by server, which database will take the role of master, in case the original master instance crashes? Does a leader election happen?

Regards,
KRS
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Re: Configuring more than one hot standby server

Kyotaro Horiguchi-4
At Tue, 19 May 2020 22:39:20 +0530, Santhosh Kumar <[hidden email]> wrote in
> Hi Community,
>  I read about "hot stand by" server and as far as the document goes, it
> does not explicitly explain about, whether is it possible to configure more
> than on database as a Hot stand by server. If we can configure more than
> one hot stand by server, which database will take the role of master, in
> case the original master instance crashes? Does a leader election happen?

You can configure two or more standbys for one primary and cofigure
some of or oall of them as synchronous standbys.  Howerver, PostgreSQL
doesn't choose the next-primary automatically.

A replication cluster involving three or more servers can be
configured in a couple of ways.  You may configure them in a cascade
replication chain, or in fanned-out configuration, or mixture of the
two.

If you made a cascaded replication set, you don't have a choice other
than the first standby in the chain.

   before: [primary] - [standby 1] - [standby 2] ...
   after : [old standby 1] - [standby 2] ...

If you made a fanned-out replication set. You can configure some of or
all of them to be synchronous standbys, which is guaranteed to have
the same data for all-commited transactions on the primary.  If you
configured priority-mode synchronous standbys, you can choose the next
primary among the first n active standbys.  If you didn't configured
synchronous standbys, you would need to check up the all standbys and
choose the most-"advanced" one. That is, the next primary is the
standby having the largest receive-LSN.  You need to reconstruct the
replication set anyways.

   before: [primary] -+- [standby 1] LSN = x - 8
                      +- [standby 2] LSN = x - 2  <= the next primary
                      +- [standby 3] LSN = x - 6

   after : [old standby 2] -+- [standby 1]
                            +- [standby 3]

That is the same for the case of quorum-mode synchronous standbys
setting.


regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center