Re: Failover replication building a new master

On 06/05/2018 05:43 AM, Tom Loder wrote:
> I am using Postgres 10.4, and using replication, I have managed to set up four servers, with one running as a master and the other three running with streaming replication from the master.
>
> I have used the command:
> psql -c "ALTER SYSTEM SET synchronous_standby_names TO 'FIRST 1(S2,S3,S4)';" so that at least one of the three slave clusters are SYNCED to the master.
>
> If I run the following command I get the following back:
>
> SELECT pid, usename, application_name as name,state, client_addr, sent_lsn, write_lsn, flush_lsn, replay_lsn, sync_priority, sync_state FROM pg_stat_replication;
>
> pid | usename | name | state | sent_lsn | write_lsn | flush_lsn | replay_lsn | sync_priority | sync_state
> -------+---------+-------+-----------+-------------+-----------+-----------+-----------+------------+---------------+------------
> 26215 | replica | S3 | streaming | 0/6000000 | 0/6000000 | 0/6000000 | 0/6000000 | 2 | potential
> 26200 | replica | S4 | streaming | 0/6000000 | 0/6000000 | 0/6000000 | 0/6000000 | 3 | potential
> 26186 | replica | S2 | streaming | 0/6000000 | 0/6000000 | 0/6000000 | 0/6000000 | 1 | sync
>
> When I take out my master server (S1) I setup S2 as the master using a bash script and I update the recovery.conf files on the other two servers to change the replication to run from S2.

I am not sure what is going on here as I am not that familiar with
synchronous replication. I do think it would aid those that can help if
you provided the contents of :

1) The Bash script

2) The recovery.conf file.

Also are there relevant log entries for S2, S3 and S4?

> I have changed the setting for synchronouse_standby_names using:
>
> psql -c "ALTER SYSTEM SET synchronous_standby_names TO 'FIRST 1(S1,S3,S4)';" hoping that one of the remaining two or possibly the old master will SYNC to the new master (S2).
>
> However when I check on S2 I get the following output from pg_stat_replication:
>
> pid | usename | name | state | sent_lsn | write_lsn | flush_lsn | replay_lsn | sync_priority | sync_state
> ------+---------+-------+---------+-------------+-----------+-----------+-----------+------------+---------------+------------
> 6418 | replica | S4 | startup | 0/D000098 | 0/D000098 | 0/D000098 | 0/D000098 | 3 | potential
> 6417 | replica | S3 | startup | 0/D000098 | 0/D000098 | 0/D000098 | 0/D000098 | 2 | potential
>
> If I also get the original master (S1) running and set it up as a Slave this also will not "SYNC". How can I get the standby servers to SYNC to the new Master (S2) without doing a new BaseBackup from S2 to the other servers?
>
> Thanks
>
> Tom
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--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com