PostgreSQL Replication Setup / configuration on Linux

what is the postgresql Streaming Replication (SR) ?
Streaming Replication (SR) provides the capability to continuously ship and apply the WAL XLOG records to some number of standby servers in order to keep them current. This feature was added to PostgreSQL 9.0. 

Advantages of postgresql Streaming Replication (SR):
1.Switch over/Fail over after the primary fails.
2.Data loss so less and data integrity.
3.No need more down time after primary fail.
4.synchronous mode Zero data loss.
5. Load balancing using load balancer ex.pgpool-II,pgbouncer.

6.Auto failover using repmgr.

Now We can go postgresql streaming replication setup using postgresql-10.3 version master IP: 192.168.2.2  slave IP: 192.168.2.3

NetWork Configuration :

On Master:

[postgres@master ~]$ cat /etc/redhat-release 
Red Hat Enterprise Linux Server release 7.3 Beta (Maipo)
--Netcard Entry
cd /etc/sysconfig/network-scripts/
vi ifcfg-ens33

BOOTPROTO=static
IPADDR=192.168.2.2
NETMASK=255.255.255.0
DEVICE="ens33"
ONBOOT=yes

vi /etc/sysconfig/network

NETWORKING=yes
HOSTNAME=master
GATEWAY=192.168.2.1

## Configure DNS Server
# vi /etc/resolv.conf

nameserver 8.8.8.8     # Replace with your nameserver ip
nameserver 192.168.2.1  # Replace with your nameserver ip

--Host Entry
vi /etc/hosts
192.168.2.2 master 

--Restart the network services
#systemctl restart network
        (OR)
#service network restart

FOR SLAVE SERVER:

--Netcard Entry
cd /etc/sysconfig/network-scripts/
vi ifcfg-ens33

BOOTPROTO=static
IPADDR=192.168.2.3
NETMASK=255.255.255.0
DEVICE="ens33"
ONBOOT=yes

vi /etc/sysconfig/network

NETWORKING=yes
HOSTNAME=slave
GATEWAY=192.168.2.1

## Configure DNS Server
# vi /etc/resolv.conf

nameserver 8.8.8.8     # Replace with your nameserver ip
nameserver 192.168.2.1  # Replace with your nameserver ip

--Host Entry
vi /etc/hosts
192.168.2.3 slave 

--Restart the network services
#systemctl restart network
        (Or)
#service network restart

On Master Server 192.168.2.2:-

ping 192.168.2.2
ping 192.168.2.3

## Some time ping will not work for security firewall at this time you have to use Telnet :
telnet 192.168.2.2 5432
telnet 192.168.2.3 5432

On Slave Server 192.168.2.3:-

ping 192.168.2.2
ping 192.168.2.3

## Some time ping will not work for security firewall at this time you have to use Telnet :
telnet 192.168.2.2 5432
telnet 192.168.2.3 5432

Note : 5432 is database port for EDB 5444 you can give what you given at the time of postgres installation, if not reaching the destination host issue with Firewall you have add firewall rule else if you are using csf firewall  open the Database port  of 5444 or 5432 on CSF

ON SLAVE:
stop the slave server

/usr/pgsql-10/bin/pg_ctl -D /var/lib/pgsql/10/data/ stop

--move the data directory to backup location:
mv /var/lib/pgsql/10/data/* /home/postgres/backup/

ON MASTER: postgresql.conf:

listen_addresses = 'localhost,192.168.2.2'  
wal_level = replica                     # minimal, replica, or logical
archive_mode = on 
archive_command =  'rsync -av %p /home/postgres/archive/%f && rsync -av %p postgres@192.168.2.3:/home/postgres/archive/%f'
max_wal_senders = 2 
wal_keep_segments = 10

on pg_hba.conf:

host    replication     postgres        192.168.2.3/24          md5

Restart the master server:

/usr/pgsql-10/bin/pg_ctl -D /var/lib/pgsql/10/data/ restart

taking consistancy backup:

$ psql -c "select pg_start_backup('initial_backup');"
$ rsync -cva --inplace /var/lib/pgsql/10/data/* postgres@192.168.2.3:/var/lib/pgsql/10/data/
$ psql -c "select pg_stop_backup();"

ON SLAVE: postgresql.conf:

listen_addresses = 'localhost,192.168.2.3'  
wal_level = replica                     # minimal, replica, or logical
archive_mode = on 
archive_command = '/bin/cp -av %p /home/postgres/archive/%f'
max_wal_senders = 2 
wal_keep_segments = 10
hot_standby = on

on pg_hba.conf:

host    replication     postgres        192.168.2.2/24          md5

On recovery.conf

cat /var/lib/pgsql/10/data/recovery.conf
standby_mode = 'on'      # to enable the standby (read-only) mode.
primary_conninfo = 'host=192.168.2.2 port=5432 user=postgres password=postgres'
                         # to specify a connection info to the master node.
trigger_file = '/tmp/pg_failover_trigger'
                         # to specify a trigger file to recognize a fail over.
restore_command = 'cp /home/postgres/archive/%f "%p"'
archive_cleanup_command = '/usr/pgsql-10/bin/pg_archivecleanup /home/postgres/archive/ %r'

Restart the slave server:

/usr/pgsql-10/bin/pg_ctl -D /var/lib/pgsql/10/data/ restart

if you get any error error like arch missing wal log …etc while restarting slave server again take the incremental physical backup

$ psql -c "select pg_start_backup('initial_backup');"
$ rsync -cva --inplace --exclude=pg_hba.conf --exclude=postgresql.conf --exclude=recovery.conf /var/lib/pgsql/10/data/* postgres@192.168.2.3:/var/lib/pgsql/10/data/
$ psql -c "select pg_stop_backup();"

I got Following error while Restarting the slave Server:

[postgres@slave root]$ /usr/pgsql-10/bin/pg_ctl -D /var/lib/pgsql/10/data/ restart
could not change directory to "/root": Permission denied
pg_ctl: PID file "/var/lib/pgsql/10/data/postmaster.pid" does not exist
Is server running?
starting server anyway
waiting for server to start....2018-05-26 20:36:34.834 IST [45453] LOG:  listening on IPv6 address "::1", port 5432
2018-05-26 20:36:34.834 IST [45453] LOG:  listening on IPv4 address "127.0.0.1", port 5432
2018-05-26 20:36:34.836 IST [45453] LOG:  listening on IPv4 address "192.168.2.3", port 5432
2018-05-26 20:36:34.840 IST [45453] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
2018-05-26 20:36:34.849 IST [45453] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
.2018-05-26 20:36:35.685 IST [45453] LOG:  redirecting log output to logging collector process
2018-05-26 20:36:35.685 IST [45453] HINT:  Future log output will appear in directory "log".

ON LOG FILE:

tail -f postgresql-2018-05-26_203635.log
2018-05-26 20:36:35.957 IST [45461] FATAL:  could not connect to the primary server: FATAL:  password authentication failed for user "postgres"
cp: cannot stat ‘/home/postgres/archive/000000010000000000000006’: No such file or directory
2018-05-26 20:36:40.964 IST [45465] FATAL:  could not connect to the primary server: FATAL:  password authentication failed for user "postgres"
cp: cannot stat ‘/home/postgres/archive/000000010000000000000006’: No such file or directory
2018-05-26 20:36:45.963 IST [45467] FATAL:  could not connect to the primary server: FATAL:  password authentication failed for user "postgres"
cp: cannot stat ‘/home/postgres/archive/000000010000000000000006’: No such file or directory
2018-05-26 20:36:50.977 IST [45471] FATAL:  could not connect to the primary server: FATAL:  password authentication failed for user "postgres"
2018-05-26 20:36:55.459 IST [45453] LOG:  received fast shutdown request
2018-05-26 20:36:55.465 IST [45457] LOG:  shutting down
2018-05-26 20:36:55.472 IST [45453] LOG:  database system is shut down

on master:
–change the postgres password because as default postgres is trust user you given md5 authentication on master so you have to give right postgres password on master then give that password in recovery.conf  file.

alter user postgres with password 'postgres';

Then Restart the slave server:

 /usr/pgsql-10/bin/pg_ctl -D /var/lib/pgsql/10/data/ restart

MONITORING STREAMING REPLICATION:

At MASTER:

1.create some dummy table check the slave server whther is replicated or not.

postgres=# create table t(id int);
CREATE TABLE

 postgres=# select pg_is_in_recovery();
 pg_is_in_recovery 
-------------------
 f
(1 row)
Note : check master server whether is in recovery mode or not, master should not be in recovery mode , slave only should be in recovery mode.

2.Using pg_stat_replication views
postgres=# select client_addr,client_hostname,client_port,state,sent_lsn,write_lsn,flush_lsn,replay_lsn,write_lag,replay_lag,flush_lag,sync_state from pg_stat_replication;
 client_addr | client_hostname | client_port |   state   | sent_lsn  | write_lsn | flush_lsn | replay_lsn | write_lag | replay_lag | flush_lag | sync_state
-------------+-----------------+-------------+-----------+-----------+-----------+-----------+------------+-----------+------------+-----------+------------
 192.168.2.3 |                 |       60000 | streaming | 0/8017A28 | 0/8017A28 | 0/8017A28 | 0/8017A28  |           |            |           | async
(1 row)

3.using linux command check the wal sender process whether started or not:

[postgres@mster ~]$ ps -ef|grep postgres
root      82472  82437  0 08:34 pts/1    00:00:00 su postgres
postgres  82473  82472  0 08:34 pts/1    00:00:00 bash
root      94549  94494  0 20:36 pts/3    00:00:00 su postgres
postgres  94550  94549  0 20:36 pts/3    00:00:00 bash
postgres  94582      1  0 20:36 pts/3    00:00:00 /usr/pgsql-10/bin/postgres -D /var/lib/pgsql/10/data
postgres  94584  94582  0 20:36 ?        00:00:00 postgres: logger process
postgres  94586  94582  0 20:36 ?        00:00:00 postgres: checkpointer process
postgres  94587  94582  0 20:36 ?        00:00:00 postgres: writer process
postgres  94588  94582  0 20:36 ?        00:00:00 postgres: wal writer process
postgres  94589  94582  0 20:36 ?        00:00:00 postgres: autovacuum launcher process
postgres  94591  94582  0 20:36 ?        00:00:00 postgres: stats collector process
postgres  94592  94582  0 20:36 ?        00:00:00 postgres: bgworker: logical replication launcher
postgres  94741  94582  0 20:43 ?        00:00:00 postgres: wal sender process postgres 192.168.2.3(60000) streaming 0/8017B08
postgres  95178  94550  0 21:08 pts/3    00:00:00 ps -ef
postgres  95179  94550  0 21:08 pts/3    00:00:00 grep --color=auto postgres

–you can calculate using this linux command how many wal sender is replicated to slave.

$ ps -ef|grep sender
postgres   7585   3383  0 15:59 ?        00:00:00 postgres: wal sender process postgres 192.168.2.2(42586) streaming 0/18017CD8
postgres   7598   6564  0 15:59 pts/2    00:00:00 grep --color=auto sender

AT SLAVE SERVER:
1.check the previously created table whether is replicated or not

postgres=# \dt
             List of relations
 Schema |     Name      | Type  |  Owner
--------+---------------+-------+----------
 public | qrtransaction | table | postgres
 public | t             | table | postgres
(2 rows)

yes! it is successfully replicated the table.

–check the slave mode using following function whether is recovery mode or not

postgres=# select pg_is_in_recovery();
 pg_is_in_recovery 
-------------------
 t
(1 row)

–and try to insert the valuse on slave server, slave is a read only mode it will not permitted write transaction.

postgres=# insert into t values(1);
ERROR:  cannot execute INSERT in a read-only transaction

2. using pg_stat_replication view:

postgres=# select status,receive_start_lsn,received_lsn,last_msg_send_time,latest_end_lsn,latest_end_time,conninfo from pg_stat_wal_receiver ;
  status   | receive_start_lsn | received_lsn |        last_msg_send_time        | latest_end_lsn |         latest_end_time          |                                                           conninfo
-----------+-------------------+--------------+----------------------------------+----------------+----------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 streaming | 0/6000000         | 0/8017B08    | 2018-05-26 21:26:34.577733+05:30 | 0/8017B08      | 2018-05-26 21:06:32.309548+05:30 | user=postgres password=******** dbname=replication host=192.168.2.2 port=5432 fallback_application_name=walreceiver sslmode=prefer sslcompression=1 krbsrvname=postgres target_session_attrs=any
(1 row)

3.using linux command you can monitor wether wal receiver is started or not.this linux command will be helpful for finding out the postgres data directory as well as postgres utility path means bin path

[postgres@slave data]$ ps -ef|grep postgres
root       3971   3573  0 10:27 pts/0    00:00:00 su postgres
postgres   3972   3971  0 10:27 pts/0    00:00:00 bash
root      45410  45321  0 20:36 pts/1    00:00:00 su postgres
postgres  45411  45410  0 20:36 pts/1    00:00:00 bash
postgres  45606      1  0 20:43 pts/1    00:00:00 /usr/pgsql-10/bin/postgres -D /var/lib/pgsql/10/data
postgres  45607  45606  0 20:43 ?        00:00:00 postgres: logger process
postgres  45608  45606  0 20:43 ?        00:00:00 postgres: startup process   recovering 000000010000000000000008
postgres  45610  45606  0 20:43 ?        00:00:00 postgres: checkpointer process
postgres  45611  45606  0 20:43 ?        00:00:00 postgres: writer process
postgres  45612  45606  0 20:43 ?        00:00:06 postgres: wal receiver process   streaming 0/8017B08
postgres  45613  45606  0 20:43 ?        00:00:00 postgres: stats collector process
postgres  45995  45411  0 21:16 pts/1    00:00:00 ps -ef
postgres  45996  45411  0 21:16 pts/1    00:00:00 grep --color=auto postgres

4.this command will be helpful for how many wal segment get postgres receiver 

[postgres@slave data]$ ps -ef|grep receiver;
postgres  45612  45606  0 20:43 ?        00:00:06 postgres: wal receiver process   streaming 0/8017B08
postgres  46018  45411  0 21:18 pts/1    00:00:00 grep --color=auto receiver

5.If the slave is up in hot standby mode, you can tell the time in seconds the delay of transactions applied on the slave with this query:

postgres=# select now() - pg_last_xact_replay_timestamp() AS replication_delay;
 replication_delay
-------------------
 00:18:35.207663
(1 row)

Note: above timing is noted for last 18 minutes slave not get any transaction

Know More About Replication:  https://www.tutorialdba.com/search/label/Replication

cascaded replication setup/configuration : https://www.tutorialdba.com/2018/03/how-to-configure-cascade-replication-on.html

For PostgreSQL Support :