How to check if a PostgreSQL streaming replicate server is up-to-date with the primary

On the primary, run

SELECT pg_current_wal_lsn(); -- On the primary

In my case, the result was

 pg_current_wal_lsn
--------------------
 0/E015420

On the standby, run

SELECT pg_last_wal_replay_lsn();

In my case, the result was

pg_last_wal_replay_lsn
------------------------
 0/E015420

This means that

All WAL records have been received

All WAL recores have been replayed (not just flushed) on the standby

There is no lag between primary and standby

How to check if your PostgreSQL server is part of a replication setup

If the server is down, check the entries in postgresql.auto.conf.

The following postgres.auto.conf is from a slave server:

synchronous_commit = 'local'
wal_level = 'replica'
hot_standby = on
hot_standby_feedback = 'on'
primary_slot_name = 'stby1'
max_connections = 636
primary_conninfo      = 'host=pgserver01.oric.no port=5432 user=replicate password=mysecretpassword'
promote_trigger_file  = '/tmp/MasterNow'

The master server may have a very simple postgres.auto.conf file:

# Do not edit this file manually!
# It will be overwritten by the ALTER SYSTEM command.
synchronous_commit = 'local'
wal_level = 'replica'

On both servers, if the parameter $PGDATA is set in your environment, use this statement to grep for the parameters:

grep -E '^\s*(wal_level|hot_standby|hot_standby_feedback|primary_slot_name|primary_conninfo|promote_trigger_file|max_connections|synchronous_commit)\s*=' "$PGDATA/postgresql.auto.conf"

If the server is up, use queries:

1. On the primary server, check that data is shipped over to the replica server:

[postgres@pgserver01.oric.no /pgdata/data]$ echo 'select usename,state,sync_state,sync_priority,client_hostname from pg_stat_replication;' | psql
  usename  |   state   | sync_state | sync_priority |        client_hostname
-----------+-----------+------------+---------------+--------------------------------
 replicate | streaming | async      |             0 | pgserver02.oric.no
(1 row)

2. The following query should return f (false) on primary server, t (true) on the replica server:

[postgres@pgserver01 /pgdata/data/log]$ echo "select pg_is_in_recovery();" | psql
 pg_is_in_recovery
-------------------
 f
(1 row)

[postgres@pgserver02 /pgdata/data/log]$  echo "select pg_is_in_recovery();" | psql
 pg_is_in_recovery
-------------------
 t
(1 row)

How to set up replication between two postgreSQL servers

Compared to Oracle, setup of a postgreSQL physical standby database is quite simple. With two identically provisioned posgreSQL 13 servers created, the below steps must be performed.

Master: test01-sandbox-pgsql.mydomain.com
Slave: test02-sandbox-pgsql.mydomain.com

1. Configure the master database cluster As the postgres software owner, execute the following:

echo "CREATE ROLE replicate WITH REPLICATION LOGIN password 'mypassword';" | psql
echo "grant app_user to replicate;" | psql
echo "alter system set wal_level = replica;" | psql
echo "select pg_reload_conf();" | psql
echo "alter system set synchronous_commit = local;" | psql
echo "alter system set track_commit_timestamp = on;" |  psql

2. Restart the postgres server

systemctl restart postgresql-13.service

3. Configure the slave database cluster

As the root user, stop the postgres server

systemctl stop postgresql-13.service

4. Remove any files created in the deployment of the slave. They will be replaced anyway

rm -rf /data/*

5. take a base backup of the master database cluster

pg_basebackup -h test01-sandbox-pgsql.mydomain.com -D /data/ -P -U replicate --wal-method=stream

6. Set some parameters

echo "hot_standby = on" >> /pgconfig/postgresql.auto.conf 

echo "
primary_conninfo      = 'host=masterhost.mydomain.com port=5432 user=replicate password=mypassword'
promote_trigger_file  = '/tmp/MasterNow'
" >  /pgconfig/postgresql.conf  

7. Create a file necessary for the master/slave role determination

touch /data/standby.signal

8. Start the postgres server

systemctl start postgresql-13.service

9. Verification checks

On the master, to check that everything works fine, use the dictionary view pg_stat_replication, which, according to the documentation "contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers."

 echo 'select usename,state,sync_state,sync_priority,client_hostname from pg_stat_replication;' | psql
  usename  |   state   | sync_state | sync_priority |          client_hostname
-----------+-----------+------------+---------------+-----------------------------------
 replicate | streaming | async      |             0 | test02-sandbox-pgsql.mydomain.com
(1 row)

The check the slave status, use the dictionary view pg_stat_wal_receiver. According to the documentation the view "contain only one row, showing statistics about the WAL receiver from that receiver's connected server":

psql
psql (13.6)
Type "help" for help.

postgres=# \x
Expanded display is on.
postgres=# select status,sender_host,conninfo from pg_stat_wal_receiver;
-[ RECORD 1 ]--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
status      | streaming
sender_host | test01-sandbox-pgsql.mydomain.com
conninfo    | user=replicate password=******** channel_binding=prefer dbname=replication host=test01-sandbox-pgsql.mydomain.com port=5432 fallback_application_name=walreceiver sslmode=prefer sslcompression=0 ssl_min_protocol_version=TLSv1.2 gssencmode=prefer krbsrvname=postgres target_session_attrs=any

Check that recovery is in progress:

 echo 'SELECT pg_is_in_recovery();' | psql
 pg_is_in_recovery
-------------------
 t
(1 row)

The equivalent query on the master node would state "f" (false). Read more about

pg_stat_replication

pg_stat_wal_receiver

Note that some important changes to replication were introduced in PostgreSQL version 12

Oracle vs PostgreSQL replication terminology

Functionality	Oracle	PostgreSQL
A standby server that can accept connections and serves read-only queries	Active Data Guard	Hot Standby
A standby server that cannot be connected to until it is promoted to a master server	Physical Standby Server	Warm Standby
A data-modifying transaction is not considered committed until all servers have committed the transaction.	Maximum Availability	Synchronous
Allow some delay between the time of a commit and its propagation to the other servers. Some transactions might be lost in the switch to a backup server load balanced servers might return slightly stale results.	Maximum Performance	Asynchronous
Do not allow any transactions to be unprotected at any time	Maximum Protection

PostgreSQL 11 dokumentation here

Oracle 12cR2 documentation here

Oracle streams being terminated with Oracle 18c

As noted in the "Desupported features" section of the Oracle 18c documentation, Oracle Streams will be terminated in with the release of Oracle 18c.

For replication, Oracle Corporation encourage their customers to use GoldenGate.