Re: [ADMIN] How to measure replication lag in logical replication setup.

From: pgsql-admin-owner@postgresql.org [mailto:pgsql-admin-owner@postgresql.org] On Behalf Of rammohan ganapavarapu
Sent: Tuesday, November 07, 2017 9:52 PM
To: pgsql-admin@postgresql.org
Subject: [ADMIN] How to measure replication lag in logical replication setup.

 

HI,

 

How to measure replication lag time in case of logical replication?

 

this query is returning empty

 

SELECT now() - pg_last_xact_replay_timestamp() AS time_lag;

 time_lag

----------

 

(1 row)

 

this is my slot details

 

SELECT * FROM pg_replication_slots;

 slot_name | plugin | slot_type | datoid | database | active | xmin | catalog_xmin | restart_lsn

-----------+--------+-----------+--------+----------+--------+------+--------------+-------------

 slot_1    |        | physical  |        |          | t      | 1983 |              | 0/50805C8

 

 

In my case slot_type is physical,  does it look ok or for logical replication the slot_type should be logical?

 

Thanks,

Ram

 

I compare confirmed_flush_lsn in PG_REPLICATION_SLOTS on the Publisher with pg_current_wal_lsn():

 

SELECT confirmed_flush_lsn, pg_current_wal_lsn(), (pg_current_wal_lsn() - confirmed_flush_lsn) AS lsn_distance         

       FROM pg_catalog.pg_replication_slots

       WHERE slot_name = 'subscriber_1';

 

slot_type for logical replication is logical.

Slot was created “automatically”, when I created “subscriber_1” subscription.

 

Regards,

Igor Neyman