Thread: pg_receivelwal vs synchronous

Hi,

PostgreSQL 11.4

Given,

postgresql.conf:
----------------
wal_level = replica
synchronous_commit = remote_apply
synchronous_standby_names = '*'


CREATE ROLE repluser WITH LOGIN REPLICATION PASSWORD 'mypwd';
SELECT pg_create_physical_replication_slot('replica1');


Execute

pg_receivewal -D /tmp/wal/ -S replica1 --synchronous -p 5432 -h 
localhost -U repluser -W --dbname="application_name=replica1"

gives

LOG:  standby "replica1" is now a synchronous standby with priority 1

Then

psql -c 'CREATE DATABASE test' postgres

which hangs (committed locally).

postgres=# SELECT * FROM pg_replication_slots ;
  slot_name | plugin | slot_type | datoid | database | temporary | 
active | active_pid | xmin | catalog_xmin | restart_lsn | 
confirmed_flush_lsn

-----------+--------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------
  replica1  |        | physical  |        |          | f         | t 
  |      12502 |      |              | 0/1655508   |
(1 row)

postgres=# SELECT * FROM pg_stat_replication;
   pid  | usesysid | usename  | application_name | client_addr | 
client_hostname | client_port |         backend_start         | 
backend_xmin |   state   | sen
t_lsn  | write_lsn | flush_lsn | replay_lsn |    write_lag    | 
flush_lag    |   replay_lag    | sync_priority | sync_state

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

-------+-----------+-----------+------------+-----------------+-----------------+-----------------+---------------+------------
  12502 |    16384 | repluser | replica1         | ::1         | 
         |       45816 | 2019-06-26 11:00:45.098276-04 |              | 
streaming | 0/1
655508 | 0/1655508 | 0/1655508 |            | 00:00:00.000267 | 
00:00:00.000267 | 00:02:14.938836 |             1 | sync
(1 row)


So, replica1 is active and in sync mode, but replay_lsn is never 
updated, and replay_lag keeps increasing.


What am I missing here ?

Thanks in advance !

Best regards,
  Jesper

Hi Magnus,

On 6/27/19 8:37 AM, Magnus Hagander wrote:
>> What am I missing here ?
>>
> 
> I believe your problem is remote_apply.
> 
> pg_receivewal never *applies* any WAL, so it just updates the write and
> flush locations. Notice how the replay_lsn remains NULL.
> 
> So you need synchronous_commit to be 'on' or 'remote_write', not
> 'remote_apply'.
> 

Yeah, remote_write works :)

I'll a patch for the pg_receivewal documentation to make this more clear.

Thanks !

Best regards,
  Jesper