Set Up a Highly Available PostgreSQL 12 Cluster on CentOS8

This guide will walk you through the steps to set up a highly available PostgreSQL release 12 cluster using Patroni and HAProxy on CentOS 8. These steps can also be applied if you are running an earlier or later release of CentOS, RHEL or Rocky Linux in your environment.

 

We will use following software components to set up PostgreSQL highly available, fault-tolerant cluster:

 

• Patroni is a cluster manager used to customize and automate deployment and maintenance of PostgreSQL HA (High Availability) clusters.
• PgBouncer is an open-source, lightweight, single-binary connection pooler for PostgreSQL. PgBouncer maintains a pool of connections for each unique user, database pair. It’s typically configured to hand out one of these connections to a new incoming client connection, and return it back in to the pool when the client disconnects.
• etcd is a strongly consistent, distributed key-value store that provides a reliable way to store data that needs to be accessed by a distributed system or cluster of machines. We will use etcd to store the state of the PostgreSQL cluster.
• HAProxy is a free and open source software that provides a high availability load balancer and reverse proxy for TCP and HTTP-based applications that spreads requests across multiple servers.
• Keepalived implements a set of health checkers to dynamically and adaptively maintain and manage load balanced server pools according to their health. When designing load balanced topologies, it is important to account for the availability of the load balancer itself as well as the real servers behind it.
  

 

The main purpose of this guide is to make PostgreSQL highly available, and to avoid any single point of failure.

 

Prerequisites

To follow this tutorial along, you will need a minimum of 3 (physical or virtual) machines installed with CentOS 8.

HOSTNAME	IP ADDRESS	PACKAGES TO BE INSTALLED
patroni1	192.168.10.1	PostgreSQL, PgBouncer, Patroni, etcd, haproxy, keepalived
patroni2	192.168.10.2	PostgreSQL, PgBouncer, Patroni, etcd, haproxy, keepalived
patroni3        192.168.10.3    PostgreSQL, PgBouncer, Patroni, etcd, haproxy, keepalived

Set correct timezone on each node:

 

sudo timedatectl set-timezone Asia/Karachi

Set hostname on each node like below:

sudo hostnamectl set-hostname patroni1

Edit /etc/hosts file:

 

sudo nano /etc/hosts

Add your nodes name and their respective IP Addresses in /etc/hosts file like below:

 

192.168.10.1 patroni1
192.168.10.2 patroni2
192.168.10.3 patroni3

Do not forget to replace highlighted text with yours. Save and close the editor when you are finished.

 

Make sure you repeat the same on each node before proceeding to next.

 

If you have all the prerequisites in place, you may proceed with the following steps to set up your PostgreSQL HA cluster.

 

Disable Selinux (Optional)

If you are familiar with SELINUX, you should configure it accordingly. If you don't have much expertise, it is recommended to disable selinux to avoid any difficulty during your cluster setup.

 

Log in to your each node with a non-root sudo user privileges and edit /etc/selinux/config file with any of your favorite text editor:

 

sudo nano /etc/selinux/config

Change SELINUX=enforcing to SELINUX=disabled

SELINUX=disabled

Save and close the editor when you are finished.

The /etc/selinux/config will look (after the changes) similar to like as shown in image below:

 

Make sure you repeat the same on each node before proceeding to next.

Reboot all nodes to make the selinux changes effect:

 

sudo shutdown -r now

Configure Firewalld

The ports required for operating PostgreSQL HA cluster using (patroni/pgbouncer/etcd/haproxy) are the following:

• 5432 PostgreSQL standard port.
  
• 6432 PgBouncer standard port.
  
• 8008 patroni rest api port required by HAProxy to check the nodes status
• 2379 etcd client port required by any client including patroni to communicate with etcd
• 2380 etcd peer urls port required by the etcd members communication
• 5000 HAProxy frontend listening port, establishing connection to backend database servers via pgbouncer port 6432
• 5001 HAProxy frontend listening port, establishing connection to backend replica database servers via pgbouncer port 6432
• 7000 HAProxy stats dashboard
  

 

You can allow required ports from firewalld using the following command:

 

sudo firewall-cmd --zone=public --add-port=5432/tcp --permanent
sudo firewall-cmd --zone=public --add-port=6432/tcp --permanent
sudo firewall-cmd --zone=public --add-port=8008/tcp --permanent
sudo firewall-cmd --zone=public --add-port=2379/tcp --permanent
sudo firewall-cmd --zone=public --add-port=2380/tcp --permanent
sudo firewall-cmd --permanent --zone=public --add-service=http
sudo firewall-cmd --zone=public --add-port=5000/tcp --permanent
sudo firewall-cmd --zone=public --add-port=5001/tcp --permanent
sudo firewall-cmd --zone=public --add-port=7000/tcp --permanent
sudo firewall-cmd --add-rich-rule='rule protocol value="vrrp" accept' --permanent
sudo firewall-cmd --reload

Make sure you repeat the same on each node before proceeding to next.   

 

Install Required Repository

Type below command to install extra packages for enterprise Linux repository:

 

sudo dnf -y install epel-release

sudo dnf -y install yum-utils

Make sure you repeat the same on each node before proceeding to next.

Install PostgreSQL

For this guide, we will install PostgreSQL release 13 like below: 

 

sudo dnf -y install https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm

sudo yum-config-manager --enable pgdg12

sudo dnf -y install postgresql12-server postgresql12 postgresql12-devel

sudo ln -s /usr/pgsql-12/bin/* /usr/sbin/

Make sure you repeat the same on each node before proceeding to next.

Install etcd

Create etcd.repo file like below:

sudo nano /etc/yum.repos.d/etcd.repo

Add following:

 

[etcd]
name=PostgreSQL common RPMs for RHEL / Rocky $releasever - $basearch
baseurl=http://ftp.postgresql.org/pub/repos/yum/common/pgdg-rhel8-extras/redhat/rhel-$releasever-$basearch
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG
repo_gpgcheck = 1

Save and close the editor when you are finished.

 

Type following command to install etcd:

 

sudo dnf -y install etcd

Make sure you repeat the same on each node before proceeding to next.

 

Configure etcd

Edit /etc/etcd/etcd.conf configuration file on your first node (patroni1) in our case, to make the required changes:

sudo mv /etc/etcd/etcd.conf /etc/etcd/etcd.conf.orig

sudo nano /etc/etcd/etcd.conf

Add following configuration: 

 

ETCD_NAME=patroni1
ETCD_DATA_DIR="/var/lib/etcd/patroni1"
ETCD_LISTEN_PEER_URLS="http://192.168.10.1:2380"
ETCD_LISTEN_CLIENT_URLS="http://0.0.0.0:2379"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://192.168.10.1:2380"
ETCD_INITIAL_CLUSTER="patroni1=http://192.168.10.1:2380,patroni2=http://192.168.10.2:2380,patroni3=http://192.168.10.3:2380"
ETCD_INITIAL_CLUSTER_STATE="new"
ETCD_INITIAL_CLUSTER_TOKEN="etcd-cluster"
ETCD_ADVERTISE_CLIENT_URLS="http://0.0.0.0:2379"
ETCD_ENABLE_V2="true"

Do not forget to replace highlighted text with yours. Save and close the editor when you are finished.

 

Edit /etc/etcd/etcd.conf configuration file on your second node (patroni2) in our case, to make the required changes:

sudo mv /etc/etcd/etcd.conf /etc/etcd/etcd.conf.orig

sudo nano /etc/etcd/etcd.conf

Add following configuration:

 

ETCD_NAME=patroni2
ETCD_DATA_DIR="/var/lib/etcd/patroni2"
ETCD_LISTEN_PEER_URLS="http://192.168.10.2:2380"
ETCD_LISTEN_CLIENT_URLS="http://0.0.0.0:2379"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://192.168.10.2:2380"
ETCD_INITIAL_CLUSTER="patroni1=http://192.168.10.1:2380,patroni2=http://192.168.10.2:2380,patroni3=http://192.168.10.3:2380"
ETCD_INITIAL_CLUSTER_STATE="new"
ETCD_INITIAL_CLUSTER_TOKEN="etcd-cluster"
ETCD_ADVERTISE_CLIENT_URLS="http://0.0.0.0:2379"
ETCD_ENABLE_V2="true"

Do not forget to replace highlighted text with yours. Save and close the editor when you are finished.

 

Edit /etc/etcd/etcd.conf configuration file on your third node (patroni3) in our case, to make the required changes:

sudo mv /etc/etcd/etcd.conf /etc/etcd/etcd.conf.orig

sudo nano /etc/etcd/etcd.conf

Add following configuration:

 

ETCD_NAME=patroni3
ETCD_DATA_DIR="/var/lib/etcd/patroni3"
ETCD_LISTEN_PEER_URLS="http://192.168.10.3:2380"
ETCD_LISTEN_CLIENT_URLS="http://0.0.0.0:2379"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://192.168.10.3:2380"
ETCD_INITIAL_CLUSTER="patroni1=http://192.168.10.1:2380,patroni2=http://192.168.10.2:2380,patroni3=http://192.168.10.3:2380"
ETCD_INITIAL_CLUSTER_STATE="new"
ETCD_INITIAL_CLUSTER_TOKEN="etcd-cluster"
ETCD_ADVERTISE_CLIENT_URLS="http://0.0.0.0:2379"
ETCD_ENABLE_V2="true"

Do not forget to replace highlighted text with yours. Save and close the editor when you are finished. 

 

Edit .bash_profile on each  node, and set environment variables like below:

 

cd ~
nano .bash_profile

Add etcd environment variables at the end of the file:

 

export PGDATA="/var/lib/pgsql/12/data"
export ETCDCTL_API="3"
export PATRONI_ETCD_URL="http://127.0.0.1:2379"
export PATRONI_SCOPE="pg_cluster"
patroni1=192.168.10.1
patroni2=192.168.10.2
patroni3=192.168.10.3
ENDPOINTS=$patroni1:2379,$patroni2:2379,$patroni3:2379

Do not forget to replace highlighted text with yours. Save and close the editor when you are finished.

 

With the etcd environment variable, .bash_profile will look similar to like as show in image below:

 

 

Make sure you repeat the same on each node before proceeding to next.

 

Type below command simultaneously on each node (patroni1, patroni2, patroni3) to start etcd cluster:

 

sudo systemctl start etcd

Check etcd status on each node:

 

sudo systemctl start etcd

Check etcd cluster from any of your nodes:

 

source ~/.bash_profile

etcdctl endpoint status --write-out=table --endpoints=$ENDPOINTS

You will see the output similar to like as shown in image below:

 

 

Install Patroni

Type below command to install patroni on your first node (patroni1) in our case:

 

sudo dnf -y install python3 python3-devel python3-pip gcc libpq-devel

sudo -H pip3 install --upgrade testresources

sudo -H pip3 install --upgrade setuptools

sudo -H pip3 install psycopg2

sudo dnf -y install python3-etcd

sudo dnf -y install patroni patroni-etcd watchdog

Make sure you repeat the same on each node before proceeding to next.

Configure Patroni

Create a configuration file for Patroni on your first node (patroni1) in our case, like below:

 

sudo mkdir -p /etc/patroni

sudo cp -p /usr/share/doc/patroni/postgres0.yml /etc/patroni/patroni.yml

Edit the patroni.yml file with any of your favorite text editor:

 

sudo nano /etc/patroni/patroni.yml

Remove everything from this file, and add the following configuration parameters:

 

scope: pg_cluster
namespace: /service/
name: patroni1

restapi:
    listen: 192.168.10.1:8008
    connect_address: 192.168.10.1:8008

etcd:
    hosts: 192.168.10.1:2379,192.168.10.2:2379,192.168.10.3:2379

bootstrap:
  dcs:
    ttl: 30
    loop_wait: 10
    retry_timeout: 10
    maximum_lag_on_failover: 1048576
    postgresql:
      use_pg_rewind: true
      use_slots: true
      parameters:

  initdb:
  - encoding: UTF8
  - data-checksums

  pg_hba:
  - host replication replicator 127.0.0.1/32 md5
  - host replication replicator 192.168.10.1/0 md5
  - host replication replicator 192.168.10.2/0 md5
  - host replication replicator 192.168.10.3/0 md5
  - host all all 0.0.0.0/0 md5

  users:
    admin:
      password: admin
      options:
        - createrole
        - createdb

postgresql:
  listen: 192.168.10.1:5432
  connect_address: 192.168.10.1:5432
  data_dir: /var/lib/pgsql/12/data
  bin_dir: /usr/pgsql-12/bin
  pgpass: /tmp/pgpass
  authentication:
    replication:
      username: replicator
      password: replicator
    superuser:
      username: postgres
      password: postgres

watchdog:
  mode: required
  device: /dev/watchdog
  safety_margin: 5

tags:
    nofailover: false
    noloadbalance: false
    clonefrom: false
    nosync: false

Make sure, you change namespace, listen and connect_address values to reflect yours. Save and close the editor when you are finished. 

 

Next, edit patroni.yml file on your second node (patroni2) in our case, and add the following configuration parameters:

 

scope: pg_cluster
namespace: /service/
name: patroni2

restapi:
    listen: 192.168.10.2:8008
    connect_address: 192.168.10.2:8008

etcd:
    hosts: 192.168.10.1:2379,192.168.10.2:2379,192.168.10.3:2379

bootstrap:
  dcs:
    ttl: 30
    loop_wait: 10
    retry_timeout: 10
    maximum_lag_on_failover: 1048576
    postgresql:
      use_pg_rewind: true
      use_slots: true
      parameters:

  initdb:
  - encoding: UTF8
  - data-checksums

  pg_hba:
  - host replication replicator 127.0.0.1/32 md5
  - host replication replicator 192.168.10.1/0 md5
  - host replication replicator 192.168.10.2/0 md5
  - host replication replicator 192.168.10.3/0 md5
  - host all all 0.0.0.0/0 md5

  users:
    admin:
      password: admin
      options:
        - createrole
        - createdb

postgresql:
  listen: 192.168.10.2:5432
  connect_address: 192.168.10.2:5432
  data_dir: /var/lib/pgsql/12/data
  bin_dir: /usr/pgsql-12/bin
  pgpass: /tmp/pgpass
  authentication:
    replication:
      username: replicator
      password: replicator
    superuser:
      username: postgres
      password: postgres

watchdog:
  mode: required
  device: /dev/watchdog
  safety_margin: 5

tags:
    nofailover: false
    noloadbalance: false
    clonefrom: false
    nosync: false

Make sure, you change namespace, listen and connect_address values to reflect yours. Save and close the editor when you are finished.

 

Next, edit patroni.yml file on your third node (patroni3) in our case, and add the following configuration parameters:

 

scope: pg_cluster
namespace: /service/
name: patroni3

restapi:
    listen: 192.168.10.3:8008
    connect_address: 192.168.10.3:8008

etcd:
    hosts: 192.168.10.1:2379,192.168.10.2:2379,192.168.10.3:2379

bootstrap:
  dcs:
    ttl: 30
    loop_wait: 10
    retry_timeout: 10
    maximum_lag_on_failover: 1048576
    postgresql:
      use_pg_rewind: true
      use_slots: true
      parameters:

  initdb:
  - encoding: UTF8
  - data-checksums

  pg_hba:
  - host replication replicator 127.0.0.1/32 md5
  - host replication replicator 192.168.10.1/0 md5
  - host replication replicator 192.168.10.2/0 md5
  - host replication replicator 192.168.10.3/0 md5
  - host all all 0.0.0.0/0 md5

  users:
    admin:
      password: admin
      options:
        - createrole
        - createdb

postgresql:
  listen: 192.168.10.3:5432
  connect_address: 192.168.10.3:5432
  data_dir: /var/lib/pgsql/12/data
  bin_dir: /usr/pgsql-12/bin
  pgpass: /tmp/pgpass
  authentication:
    replication:
      username: replicator
      password: replicator
    superuser:
      username: postgres
      password: postgres

watchdog:
  mode: required
  device: /dev/watchdog
  safety_margin: 5

tags:
    nofailover: false
    noloadbalance: false
    clonefrom: false
    nosync: false

Make sure, you change namespace, listen and connect_address values to reflect yours. Save and close the editor when you are finished.

 

Enable Watchdog

Patroni configuration will try to use /dev/watchdog on Linux if it is accessible to Patroni. For most use cases using software watchdog built into the Linux kernel is secure enough.

Edit /etc/watchdog.conf to enable software watchdog:

 

sudo nano /etc/watchdog.conf

Uncomment following line:

 

watchdog-device = /dev/watchdog

Save and close the editor when you are finished.

Execute following commands to activate watchdog:

 

sudo mknod /dev/watchdog c 10 130

sudo modprobe softdog

sudo chown postgres /dev/watchdog

Make sure you repeat the same on each node before proceeding to next.

 

Start Patroni Cluster

Type below command on your first node (patroni1) to start your patroni cluster:

 

sudo systemctl start patroni

Check the patroni status with below command:

 

sudo systemctl status patroni

If you look carefully at the bottom of the patroni status output, you will see that the (patroni1) is acting as leader node in the cluster:

 

Next, start patroni on subsequent nodes, (patroni2) for example, you will see (patroni2) is acting as secondary node in the cluster:

 

Start patroni on (patroni3), and it will also act as the secondary node in the cluster:

 

Install PgBouncer

Type below command on your first node (patroni1) in our case to install PgBouncer:

 

sudo dnf -y install pgbouncer

Make sure you repeat the same on each node before proceeding to next.

 

Configure PgBouncer

Edit /etc/pgbouncer/pgbouncer.ini file on your first node (patroni1) in our case, to make the required changes:

 

sudo cp -p /etc/pgbouncer/pgbouncer.ini /etc/pgbouncer/pgbouncer.ini.orig

sudo nano /etc/pgbouncer/pgbouncer.ini

Add your database in [databases] section like below:

 

* = host=192.168.10.1 port=5432 dbname=postgres

and change listen_addr=localhost to listen_addr=*

 

listen_addr = *

Do not forget to replace the highlighted text with yours. Save and close the editor when you are finished.

Since clients connect to pgbouncer, it will have to be able to authenticate them. For that, we need to create a userlist.txt file in the pgbouncer configuration directory. This userlist.txt file contains the database users and their encrypted passwords.

You can write the file by hand using the information from the pg_shadow catalog table, or you can create it automatically like below:

sudo su

psql -Atq -h patroni1 -p 5432 -U postgres -d postgres -c "SELECT concat('\"', usename, '\" \"', passwd, '\"') FROM pg_shadow" >> /etc/pgbouncer/userlist.txt

exit

All the database username, and their encrypted password will be stored in /etc/pgbouncer/userlist.txt file.

This method is useful if the number of database users is small and passwords don’t change frequently. The disadvantage is whenever new user is added to PostgreSQL, the user’s username and password has to be added to userlist.txt.

Make sure you repeat the same on each node before proceeding to next.

Type below command to start PgBouncer on each node:

sudo systemctl start pgbouncer

Install Keepalived

Type below command on your first node (patroni1) in our case, to install keepalived:

 

sudo dnf -y install keepalived

Make sure you repeat the same on each node before proceeding to next.

 

Configure Keepalived

We will create a floating IP (192.168.10.200) to share across the nodes. For example, if primary node (HAProxy) goes down, keepalived will automatically configure shared IP to secondary available node in order to keep the connectivity available and to avoid any single point of failure.

 

Edit the /etc/sysctl.conf file to allow the server to bind to the virtual IP address.

 

sudo nano /etc/sysctl.conf

Add the net.ipv4_ip_nonlocal_bind=1 directive, which allows the server to accept connections for IP addresses that are not bound to any of its interfaces, enabling the use of a floating, virtual IP:

 

net.ipv4.ip_nonlocal_bind = 1
net.ipv4.ip_forward = 1

Save and close the editor when you are finished.

 

Type the following command to reload settings from config file without rebooting:

sudo sysctl --system
sudo sysctl -p

Make sure you repeat the same on each node before proceeding to next.

 

Create /etc/keepalived/keepalived.conf file on your first node (patroni1) in our case, to make the required changes:

 

sudo nano /etc/keepalived/keepalived.conf 

Add following configuration:

 

vrrp_script chk_haproxy {
        script "pkill -0 haproxy"
        interval 5
        weight -4
        fall 2
        rise 1
}

vrrp_script chk_lb {
        script "pkill -0 keepalived"
        interval 1
        weight 6
        fall 2
        rise 1
}

vrrp_script chk_servers {
        script "echo 'GET /are-you-ok' | nc 127.0.0.1 7000 | grep -q '200 OK'"
        interval 2
        weight 2
        fall 2
        rise 2
}

vrrp_instance vrrp_1 {
        interface enp0s3
        state MASTER
        virtual_router_id 51
        priority 101
        virtual_ipaddress_excluded {
                192.168.10.200
        }
        track_interface {
                enp0s3 weight -2
        }
        track_script {
                chk_haproxy
                chk_lb
        }
}

Do not forget to replace the highlighted text with yours. Save and close the editor when you are finished. 

Create /etc/keepalived/keepalived.conf file on your second node (patroni2) in our case, to make the required changes:

 

sudo nano /etc/keepalived/keepalived.conf 

Add following configuration:

 

vrrp_script chk_haproxy {
        script "pkill -0 haproxy"
        interval 5
        weight -4
        fall 2
        rise 1
}

vrrp_script chk_lb {
        script "pkill -0 keepalived"
        interval 1
        weight 6
        fall 2
        rise 1
}

vrrp_script chk_servers {
        script "echo 'GET /are-you-ok' | nc 127.0.0.1 7000 | grep -q '200 OK'"
        interval 2
        weight 2
        fall 2
        rise 2
}

vrrp_instance vrrp_1 {
        interface enp0s3
        state BACKUP
        virtual_router_id 51
        priority 100
        virtual_ipaddress_excluded {
                192.168.10.200
        }
        track_interface {
                enp0s3 weight -2
        }
        track_script {
                chk_haproxy
                chk_lb
        }
}

Do not forget to replace the highlighted text with yours. Save and close the editor when you are finished.

 

Create /etc/keepalived/keepalived.conf file on your third node (patroni3) in our case, to make the required changes:

 

sudo nano /etc/keepalived/keepalived.conf 

Add following configuration:

 

vrrp_script chk_haproxy {
        script "pkill -0 haproxy"
        interval 5
        weight -4
        fall 2
        rise 1
}

vrrp_script chk_lb {
        script "pkill -0 keepalived"
        interval 1
        weight 6
        fall 2
        rise 1
}

vrrp_script chk_servers {
        script "echo 'GET /are-you-ok' | nc 127.0.0.1 7000 | grep -q '200 OK'"
        interval 2
        weight 2
        fall 2
        rise 2
}

vrrp_instance vrrp_1 {
        interface enp0s3
        state BACKUP
        virtual_router_id 51
        priority 99
        virtual_ipaddress_excluded {
                192.168.10.200
        }
        track_interface {
                enp0s3 weight -2
        }
        track_script {
                chk_haproxy
                chk_lb
        }
}

Do not forget to replace the highlighted text with yours. Save and close the editor when you are finished.

 

Type below command on your first node (patroni1) to start keepalived:

 

sudo systemctl start keepalived

Check on your (MASTER) node to see if your (enp0s3) network interface has configured with an additional shared IP (192.168.10.200):

 

ip addr show enp0s3

You will see the output similar to like below:

 

With this configuration, if haproxy goes down on your MASTER node, keepalived will automatically failover to BACKUP node, and connectivity will remain available to your clients.

 

Install HAProxy

With patroni, you need a method to connect to the leader node regardless of which of the node in the cluster is the leader. HAProxy forwards the connection to whichever node is currently the leader. It does this using a REST endpoint that Patroni provides. Patroni ensures that, at any given time, only the leader node will appear as online, forcing HAProxy to connect to the correct node. Users or applications, (psql) for example, will connect to haproxy, and haproxy will make sure connecting to the leader node in the cluster.

 

Type below command to install haproxy on your first node (patroni1) in our case: 

 

sudo dnf -y install haproxy

Make sure you repeat the same on each node before proceeding to next.

 

Configure HAProxy

Edit haproxy.cfg file on your first node (patroni1) in our case, to make required changes:

 

sudo mv /etc/haproxy/haproxy.cfg /etc/haproxy/haproxy.cfg.orig

sudo nano /etc/haproxy/haproxy.cfg

Add following configuration:

global
    log         127.0.0.1 local2
    chroot      /var/lib/haproxy
    pidfile     /var/run/haproxy.pid
    maxconn     1000
    user        haproxy
    group       haproxy
    daemon
    stats socket /var/lib/haproxy/stats

defaults
    mode                    tcp
    log                     global
    option                  tcplog
    retries                 3
    timeout queue           1m
    timeout connect         10s
    timeout client          1m
    timeout server          1m
    timeout check           10s
    maxconn                 900

listen stats
    mode http
    bind *:7000
    stats enable
    stats uri /

listen primary
    bind 192.168.10.200:5000
    option httpchk OPTIONS /master
    http-check expect status 200
    default-server inter 3s fall 3 rise 2 on-marked-down shutdown-sessions
    server patroni1 192.168.10.1:6432 maxconn 100 check port 8008
    server patroni2 192.168.10.2:6432 maxconn 100 check port 8008
    server patroni3 192.168.10.3:6432 maxconn 100 check port 8008

listen standby
    bind 192.168.10.200:5001
    balance roundrobin
    option httpchk OPTIONS /replica
    http-check expect status 200
    default-server inter 3s fall 3 rise 2 on-marked-down shutdown-sessions
    server patroni1 192.168.10.1:6432 maxconn 100 check port 8008
    server patroni2 192.168.10.2:6432 maxconn 100 check port 8008
    server patroni3 192.168.10.3:6432 maxconn 100 check port 8008

Make sure you replace the highlighted text with yours. Save and close the editor when you are finished.

 

There are two sections in haproxy configuration: one is primary, using port 5000, and other is standby, using port 5001.

All three nodes are included in both sections: that is because all the nodes are potential candidates to be either primary or secondary. Patroni provides a built-in REST API support for health check monitoring that works perfectly with HAproxy. HAProxy will send an HTTP request to port 8008 of patroni to know which role each node currently has.

 

The haproxy configuration will remain same on each node so make sure you repeat the same on your remaining nodes before proceeding to next.

 

Type below command to start HAProxy on each node: 

 

sudo systemctl start haproxy

Check HAProxy status:

 

sudo systemctl status haproxy

You can manually test keepalived failover scenario by killing haproxy on your MASTER  node with sudo systemctl stop haproxy command, and within few seconds of delay  192.168.10.200 will automatically be configured on your BACKUP node.

Test Patroni Cluster

You can test your Patroni cluster by initiating a connection request from any of your applications (psql) for example to your (shared_ip:port), and see if this successfully establish connection back to the database leader node in the cluster.

 

psql -h 192.168.10.200 -p 5000 -U postgres

As you can see in the image below, the (psql) successfully connected to database via haproxy using shared ip:

 

Execute two read-request to verify HAProxy round-robin mechanism is working as expected:

 

psql -h 192.168.10.200 -p 5001 -U postgres -t -c "select inet_server_addr()"

This should return your replica node IP as shown in image below:

 

 

Execute the same read-request second time:

 

psql -h 192.168.10.200 -p 5001 -U postgres -t -c "select inet_server_addr()"

This should return your other replica node IP as shown in image below:

 

You can test write-request by executing same command but using port 5000:

 

psql -h 192.168.10.200 -p 5000 -U postgres -t -c "select inet_server_addr()"

This should return your primary (leader) node IP as shown in image below:

You can also check the state of HAproxy by visiting http://192.168.10.200:7000/ on your browser.

 

As you can see, in the primary section (patroni1) row is highlighted in green. This indicates that 192.168.10.1 is currently a leader node in the cluster.

 

In the standby section, the (patroni2, patroni3) row is highlighted as green. This indicates that both nodes are replica node in the cluster.

 

If you kill the leader node using (sudo systemctl stop patroni) or by completely shutting down the server, the dashboard will look similar to like below:

As you can see, in the primary section (patroni2) row is now highlighted in green. This indicates that 192.168.10.2 is currently a leader node in the cluster.

Please note that, in this particular scenario, it just so happens that the second node in the cluster is promoted to leader. This might not always be the case and it is equally likely that the 3rd node may be promoted to leader.

 

Test Database Replication

We will create a test database to see if it is replicated to other nodes in the cluster. For this guide, we will use (psql) to connect to database via haproxy like below: 

psql -h 192.168.10.200 -p 5000 -U postgres

From the Postgres prompt, create a test database like below:

 

create database testdb;
create user testuser with encrypted password 'mystrongpass';
grant all privileges on database testdb to testuser;

\q

Update your userlist.txt file on each node for testuser as explained in PgBouncer section.

 

Stop patroni on leader node (patroni1) in our case with below command:

 

sudo systemctl stop patroni

Connect to database using psql, and this time haproxy will automatically make connection to  whichever node is currently leader in the cluster: 

psql -h 192.168.10.200 -p 5000 testuser -d testdb

As you can see in the output below, connection to testdb was successful via haproxy:

 

Now bring up your first node with (sudo systemctl start patroni), and it will automatically rejoin the cluster as secondary and automatically synchronize with the leader.

 

Patroni Cluster Failover

With patronictl, you can administer, manage and troubleshoot your PostgreSQL cluster. Type below command to list the options and commands you can use with patronictl: 

sudo patronictl --help

This will show you the options and commands you can use with patronictl.

Options:
  -c, --config-file TEXT  Configuration file
  -d, --dcs TEXT          Use this DCS
  -k, --insecure          Allow connections to SSL sites without certs
  --help                  Show this message and exit.

Commands:
  configure    Create configuration file
  dsn          Generate a dsn for the provided member, defaults to a dsn of...
  edit-config  Edit cluster configuration
  failover     Failover to a replica
  flush        Discard scheduled events (restarts only currently)
  history      Show the history of failovers/switchovers
  list         List the Patroni members for a given Patroni
  pause        Disable auto failover
  query        Query a Patroni PostgreSQL member
  reinit       Reinitialize cluster member
  reload       Reload cluster member configuration
  remove       Remove cluster from DCS
  restart      Restart cluster member
  resume       Resume auto failover
  scaffold     Create a structure for the cluster in DCS
  show-config  Show cluster configuration
  switchover   Switchover to a replica
  version      Output version of patronictl command or a running Patroni 

Check patroni member nodes:

 

patronictl -c /etc/patroni/patroni.yml list

 

 

 

 

 

 

 

 

 

Check failover/switchover history:

patronictl -c /etc/patroni/patroni.yml history

Remember: failover is executed automatically, when the Leader node is getting unavailable for unplanned reason. If you wish to test failover across the nodes in the cluster, you can manually initiate failover to a replica node with below command:

 

patronictl -c /etc/patroni/patroni.yml failover

In some cases it is necessary to perform maintenance task on a single node such as applying patches or release updates. When you manually disable auto failover, patroni won’t change the state of the PostgreSQL cluster.

You can disable auto failover with below command:

patronictl -c /etc/patroni/patroni.yml pause

 

Patroni Cluster Switchover

There are two possibilities to run a switchover, either in scheduled mode or immediately. At the given time, the switchover will take place, and you will see in the logfile an entry of switchover activity.

 

patronictl -c /etc/patroni/patroni.yml switchover --master your_leader_node --candidate your_replica_node

If you go with [now] option, switchover will take place immediately.

 

Simulate Patroni Cluster Failure Scenarios

To simulate failure scenarios in production environment, we will execute continuous reads and writes to the database using a simple Python script as we are interested in observing the state of the cluster upon a server failure.

You should have a Linux workstation with PostgreSQL client, and PostgreSQL driver for Python installed:

sudo dnf -y install postgresql python3-psycopg2

Download HAtester.py script on your Linux workstation:

cd ~

curl -LO https://raw.githubusercontent.com/jobinau/pgscripts/main/patroni/HAtester.py

chmod +x HAtester.py

Edit HAtester.py and replace database credentials with yours:

nano HAtester.py

Replace following credentials with yours:

 

host = "192.168.10.200"
dbname = "postgres"
user = "postgres"
password = "your_postgres_user_password"

Save and close the editor when you are finished.

 

You need to create a target table "HATEST" in your database before executing HAtester.py script:

 

psql -h 192.168.10.200 -p 5000 -U postgres -c "CREATE TABLE HATEST (TM TIMESTAMP);"

psql -h 192.168.10.200 -p 5000 -U postgres -c "CREATE UNIQUE INDEX idx_hatext ON hatest (tm desc);"

To get the best result, we will use tmux terminal for multiple tabs in order run HAtester and to monitor state of the patroni cluster all together in real time:

 

 

On the left side of the screen, we have one ssh session open for each of the 3 nodes, continuously monitoring patroni cluster state:

On the right side of the screen, we are running the HAtester.py script sending writes through port 5000, and reads through port 5001 from our workstation:

 

 

To observe what happens with database traffic when the environment experiences a failure, we will manually stop patroni on our leader node using the following command:

 

sudo systemctl stop patroni

When we stopped patroni on leader node, within few seconds a replica node become leader, and continues writing to the database stopped for few seconds, then reconnected automatically.

 

There is no disconnection happens to continues reading to the database because replica node was available throughout.

 

 

Let's bring back our patroni node using following command:

 

sudo systemctl start patroni

The node has automatically rejoined the cluster as replica:

 

 

Test Environment Failure Scenarios

We leave it up to you to test and experiment with your patroni cluster to see what happens when environment experiences a failure such as:

• Loss of network connectivity
• Power breakdown

When simulating these tests, you should continuously monitor how the patroni cluster re-adjusts itself and how it affects read and write traffic for each failure scenario.

 

Conclusion

I hope this guide was helpful to set up a highly available PostgreSQL release 12 cluster on CentOS 8 for your production use.