Re: Manual failover cluster

Hi Saul,

Hope you are doing well. My apology for no response for a longer time.

Pgbackrest helps build a streaming replication. While performing role
reversal(switchover), it is mandatory to set recover_target_timeline to
latest in recovery.conf(in data directory). Steps to perform switchover is
as below.

1) Stop master database
2) Promote the slave database
3) Prepare the recovery.conf file on the stopped master node, and set
recover_target_timeline to latest in that file
4) Start the stopped master database; it will automatically come up as a
slave
5) Check status of the database.

Additionally, entries in pg_hba.conf is required.

Hope this helps.

Regards,
Ninad Shah

On Mon, 23 Aug 2021 at 23:12, Saul Perdomo <saul(dot)perdomo(at)gmail(dot)com> wrote:

> Sorry, I misspoke there - I meant to say that since one should not count
> on the standby-failover process to always run smoothly (whether it's due to
> hardware, operator, automated scripts, or software issues), DB backups
> should also be in place if at all possible.
>
> On Mon, Aug 23, 2021 at 1:37 PM Saul Perdomo <saul(dot)perdomo(at)gmail(dot)com>
> wrote:
>
>> Hi Moishe,
>>
>> Since we use pgbackrest ourselves, this is the process I followed to set
>> up something similar on PG 10:
>>
>>
>> https://pgstef.github.io/2018/11/28/combining_pgbackrest_and_streaming_replication.html
>>
>> (Not knowing much [if at all] about the reason for your requirements, I
>> would recommend looking into enabling PITR backups in addition to the hot
>> standby[s], which as you've seen are not bulletproof.)
>>
>> Anyway, just wanted to add that your experience roughly matches ours --
>> it seems that an attempt to promote the standby isn't always smooth. My
>> sample size is almost certainly smaller than yours, about half a dozen
>> attempts. In the past I've attributed this to a poor understanding on my
>> part of the behind-the-scenes of the process, and chalked it up to having
>> made a mistake or other in the manual steps (mainly in the order they are
>> executed). That being said, if you find a way to faithfully reproduce the
>> issue, I'm sure the community will want to know, there is certainly an
>> expectation that the failover is reliable from the PG software side of
>> things, as long as there are no hardware nor operator issues! Again, not
>> knowing a lot about your setup, my first instinct would be to troubleshoot
>> your automated scripts, you might find that you need to change the order
>> things are run when on server B vs server A, for example..
>>
>> On Fri, Aug 20, 2021 at 9:23 AM Hispaniola Sol <moishap(at)hotmail(dot)com>
>> wrote:
>>
>>> Team,
>>>
>>> I have a pg 10 cluster with a master and two hot-standby nodes. There is
>>> a requirement for a manual failover (nodes switching the roles) at will.
>>> This is a vanilla 3 node PG cluster that was built with WAL archiving
>>> (central location) and streaming replication to two hot standby nodes. The
>>> failover is scripted in Ansible. Ansible massages and moves around the
>>> archive/restore scripts, the conf files and the trigger and calls `
>>> pg_ctlcluster` to start/stop. This part _seems_ to be doing the job fine.
>>>
>>> The issue I am struggling with is the apparent fragility of the process
>>> - all 3 nodes will end up in a "good" state after the switch only every
>>> other time. Other times I have to rebase the hot-standby from the new
>>> master with pg_basebackup. It seems the issues are mostly with those nodes,
>>> ending up as slaves after the roles switch runs.
>>> They get errors like mismatch in timelines, recovering from the same WAL
>>> over and over again, invalid resource manager ID in primary checkpoint
>>> record, etc.
>>>
>>> In this light, I am wondering - using what's offered by PostgreSQL
>>> itself, i.e. streaming WAL replication with log shipping - can I expect to
>>> have this kind of failover 100% reliable on PG side ? Anyone is doing this
>>> reliably on PostgreSQL 10.1x ?
>>>
>>> Thanks !
>>>
>>> Moishe
>>>
>>