Author: wplaat

How to failover from Master to Replication node and visa versa

Pi6 192.168.2.106 (MASTER)

Pi7 192.168.2.107 (REPLICATION)

Make Replication node master

# Goto pi7

su postgres

/usr/lib/postgresql/13/bin/pg_ctl -D /data02/data promote
Restore old master

# Goto to pi6

sudo cp /data02/data/*.conf ../.

/usr/lib/postgresql/13/bin/pg_ctl -D /data02/data stop

rm -rf /data02/data/*

/usr/lib/postgresql/13/bin/pg_basebackup --pgdata=/data02/data --format=p --write-recovery-conf --checkpoint=fast --label=mffb --progress --host=pi7 --port=5432 --username=plaatsoft

rm /data02/data/standby.signal

/usr/lib/postgresql/13/bin/pg_ctl -D /data02/data start

psql -c "select * from pg_create_physical_replication_slot('standby1_slot');"
# Goto to pi7

sudo cp /data02/data/*.conf ../.

/usr/lib/postgresql/13/bin/pg_ctl -D /data02/data stop

rm -rf /data02/data/*

/usr/lib/postgresql/13/bin/pg_basebackup --pgdata=/data02/data --format=p --write-recovery-conf --checkpoint=fast --label=mffb --progress --host=pi6 --port=5432 --username=plaatsoft

/usr/lib/postgresql/13/bin/pg_ctl -D /data02/data start
Check replication
CHECK MASTER NODE

postgres=# \x

postgres=# SELECT * FROM pg_stat_replication;
postgres=# select pg_is_in_recovery();

pg_is_in_recovery

-------------------

f

(1 row)
CHECK REPLICATION NODE

postgres=# \x

postgres=# SELECT * FROM pg_stat_wal_receiver;
postgres=# select pg_is_in_recovery();

pg_is_in_recovery

-------------------

t

(1 row)
	

Execute on each Postgresql node the following actions

# Create check script
sudo vi /usr/lib/postgresql/13/bin/pgsqlchk

#!/bin/bash
# This script checks if a postgres server is healthy running on localhost. It will return:
# "HTTP/1.x 200 OK\r" (if postgres is running smoothly)
# - OR -
# "HTTP/1.x 500 Internal Server Error\r" (else)
# The purpose of this script is make haproxy capable of monitoring postgres properly
# It is recommended that a low-privileged postgres user is created to be used by this script.
# For eg. create user healthchkusr login password 'hc321';

PGBIN=/usr/pgsql-10/bin
PGSQL_HOST="localhost"
PGSQL_PORT="5432"
PGSQL_DATABASE="postgres"
PGSQL_USERNAME="postgres"
export PGPASSWORD="passwd"
TMP_FILE="/tmp/pgsqlchk.out"
ERR_FILE="/tmp/pgsqlchk.err"

# We perform a simple query that should return a few results

VALUE=`/usr/lib/postgresql/13/bin/psql -t -h localhost -U postgres -p 5432 -c "select pg_is_in_recovery()" 2> /dev/null`
# Check the output. If it is not empty then everything is fine and we return something. Else, we just do not return anything.

if [ $VALUE == "t" ]
then
/bin/echo -e "HTTP/1.1 206 OK\r\n"
/bin/echo -e "Content-Type: Content-Type: text/plain\r\n"
/bin/echo -e "\r\n"
/bin/echo "Standby"
/bin/echo -e "\r\n"
elif [ $VALUE == "f" ]
then
/bin/echo -e "HTTP/1.1 200 OK\r\n"
/bin/echo -e "Content-Type: Content-Type: text/plain\r\n"
/bin/echo -e "\r\n"
/bin/echo "Primary"
/bin/echo -e "\r\n"
else
/bin/echo -e "HTTP/1.1 503 Service Unavailable\r\n"
/bin/echo -e "Content-Type: Content-Type: text/plain\r\n"
/bin/echo -e "\r\n"
/bin/echo "DB Down"
/bin/echo -e "\r\n"
fi

# Make script executable
sudo chmod a+x /usr/lib/postgresql/13/bin/pgsqlchk

# install xinetd and telnet
sudo apt-get install xinetd telnet -y

# Create xinitd file
sudo vi /etc/xinetd.d/pgsqlchk

service pgsqlchk
{
flags = REUSE
socket_type = stream
port = 23267
wait = no
user = nobody
server = /usr/lib/postgresql/13/bin/pgsqlchk
log_on_failure += USERID
disable = no
only_from = 0.0.0.0/0
per_source = UNLIMITED
}

sudo bash -c 'echo "pgsqlchk 23267/tcp # pgsqlchk" >> /etc/services'
sudo systemctl restart xinetd

# Check if service is working
telnet 127.0.0.1 23267

Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
HTTP/1.1 206 OK
Content-Type: Content-Type: text/plain
Standby
Connection closed by foreign host.

Execute on HaProxy following actions

# Install HaProxy
sudo apt-get install haproxy -y 

sudo vi /etc/hosts
Add pi6 192.168.2.106
Add pi7 192.168.2.107

sudo vi /etc/haproxy/haproxy.cfg  
  
global  
    maxconn 100  
   
defaults  
    log global  
    mode tcp  
    retries 2  
    timeout client 30m  
    timeout connect 4s  
    timeout server 30m  
    timeout check 5s  
   
listen stats  
    mode http  
    bind *:10000  
    stats enable   
    stats refresh 10s
    stats uri /
   
listen Postgres
    bind *:5432 
    option httpchk  
    http-check expect status 200  
    default-server inter 3s fall 3 rise 2 on-marked-down shutdown-sessions  
    server pi6 pi6:5432 maxconn 100 check port 23267  
    server pi7 pi7:5432 maxconn 100 check port 23267  
   
systemctl restart haproxy  

Check HaProxy

# Install postgres client
sudo apt-get install postgresql-client

# Start psql client
psql -h localhost -U postgres  
  
postgres=# select pg_is_in_recovery();  
pg_is_in_recovery  
-------------------  
f  
(1 row)

Check HaProxy portal http://192.168.2.107:5000