Re: How to test replication without doing a failover

[Rui DeSousa <rui@xxxxxxxxxxxxx>]

On Nov 10, 2022, at 9:59 AM, Edwin UY <edwin.uy@xxxxxxxxx> wrote:

Is it possible to force/initiate a log transaction and see if that gets replicated or not? I mean for example on Oracle we can do a switch logfile and check if that log gets shipped across and applied on the standby, is there something similar in PostgreSQL?

Yes, if you are using log shipping instead of replication you can use:  select pg_switch_wal();

However, replication doesn’t wait for a WAL file switch to replicate data; thus you can monitor it on the upstream server (master or cascading replication server) using the pg_stat_replication and pg_replication_slots views.  

I create and use the following views to monitor replication; viewing the lag by data size instead of lsn values.

create or replace view dba.replication_status

as

select pg_stat_replication.client_addr

  , pg_stat_replication.application_name

  , pg_stat_replication.sync_priority

  , pg_stat_replication.sync_state

  , pg_stat_replication.state

  , case pg_is_in_recovery()

      when true then pg_size_pretty(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), sent_lsn))

      else pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), sent_lsn))

    end as sent_lag

  , case pg_is_in_recovery()

      when true then pg_size_pretty(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), flush_lsn))

      else pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), flush_lsn)) 

    end as flush_lag

  , case pg_is_in_recovery()

      when true then pg_size_pretty(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), replay_lsn)) 

      else pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn))

    end as replay_lag

from pg_stat_replication

;

create or replace view dba.replication_slot_status

as

select slot_name

  , slot_type

  , temporary

  , active

  , active_pid

  , xmin

  , catalog_xmin

  , restart_lsn

  , case pg_is_in_recovery()

      when true then pg_size_pretty(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), restart_lsn))

      else pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn))

    end as restart_lag

  , confirmed_flush_lsn as flush_lsn

  , case pg_is_in_recovery()

      when true then pg_size_pretty(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), confirmed_flush_lsn))

      else pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), confirmed_flush_lsn))

    end as flush_lag

from pg_replication_slots

;