Deploying PostgreSQL for high availability with Patroni on RHEL or CentOS¶

This guide provides instructions on how to set up a highly available PostgreSQL cluster with Patroni on Red Hat Enterprise Linux or CentOS.

Considerations¶

This is the example deployment suitable to be used for testing purposes in non-production environments.
In this setup ETCD resides on the same hosts as Patroni. In production, consider deploying ETCD cluster on dedicated hosts because ETCD writes every request from the cluster to disk which requires significant amount of disk space. See hardware recommendations for details.

For this setup, we use the nodes running on Red Hat Enterprise Linux 8 as the base operating system:

Node name	Application	IP address
node1	Patroni, PostgreSQL, ETCD	10.104.0.1
node2	Patroni, PostgreSQL, ETCD	10.104.0.2
node3	Patroni, PostgreSQL, ETCD	10.104.0.3
HAProxy-demo	HAProxy	10.104.0.6

Note

Ideally, in a production (or even non-production) setup, the PostgreSQL and ETCD nodes will be within a private subnet without any public connectivity to the Internet, and the HAProxy will be in a different subnet that allows client traffic coming only from a selected IP range. To keep things simple, we have implemented this architecture in a private environment, and each node can access the other by its internal, private IP.

Preparation¶

Set up hostnames in the `/etc/hosts` file¶

It’s not necessary to have name resolution, but it makes the whole setup more readable and less error prone. Here, instead of configuring a DNS, we use a local name resolution by updating the file /etc/hosts. By resolving their hostnames to their IP addresses, we make the nodes aware of each other’s names and allow their seamless communication.

Modify the /etc/hosts file of each PostgreSQL node to include the hostnames and IP addresses of the remaining nodes. Add the following at the end of the /etc/hosts file on all nodes:

node1node2node3HAproxy-demo

# Cluster IP and names 
10.104.0.1 node1 
10.104.0.2 node2 
10.104.0.3 node3

# Cluster IP and names 
10.104.0.1 node1 
10.104.0.2 node2 
10.104.0.3 node3

# Cluster IP and names 
10.104.0.1 node1 
10.104.0.2 node2 
10.104.0.3 node3

The HAProxy instance should have the name resolution for all the three nodes in its /etc/hosts file. Add the following lines at the end of the file:

# Cluster IP and names
10.104.0.6 HAProxy-demo
10.104.0.1 node1
10.104.0.2 node2
10.104.0.3 node3

Install Percona Distribution for PostgreSQL¶

Install Percona Distribution for PostgreSQL on node1, node2 and node3 from Percona repository:

Install percona-release.
Enable the repository:
```
$ sudo percona-release setup ppg11
```
Install Percona Distribution for PostgreSQL packages.

Important

Don’t initialize the cluster and start the postgresql service. The cluster initialization and setup are handled by Patroni during the bootsrapping stage.

Configure ETCD distributed store¶

The distributed configuration store provides a reliable way to store data that needs to be accessed by large scale distributed systems. The most popular implementation of the distributed configuration store is ETCD. ETCD is deployed as a cluster for fault-tolerance and requires an odd number of members (n/2+1) to agree on updates to the cluster state. An ETCD cluster helps establish a consensus among nodes during a failover and manages the configuration for the three PostgreSQL instances.

The etcd cluster is first started in one node and then the subsequent nodes are added to the first node using the addcommand. The configuration is stored in the /etc/etcd/etcd.conf configuration file.

Install etcd on every PostgreSQL node. For CentOS 8, the etcd packages are available from Percona repository:
Install percona-release.
Enable the repository:
```
$ sudo percona-release setup ppg12
```
Install the etcd packages using the following command:
```
$ sudo yum install etcd python3-python-etcd
```

Configure ETCD on node1.

Backup the etcd.conf file:

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

Modify the /etc/etcd/etcd.conf configuration file:

[Member]
ETCD_DATA_DIR="/var/lib/etcd/default.etcd"
ETCD_LISTEN_PEER_URLS="http://10.104.0.1:2380,http://localhost:2380" 
ETCD_LISTEN_CLIENT_URLS="http://10.104.0.1:2379,http://localhost:2379"  

ETCD_NAME="node1"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.1:2380"
ETCD_ADVERTISE_CLIENT_URLS="http://10.104.0.1:2379"
ETCD_INITIAL_CLUSTER="node1=http://10.104.0.1:2380"
ETCD_INITIAL_CLUSTER_TOKEN="percona-etcd-cluster"
ETCD_INITIAL_CLUSTER_STATE="new"

Start the etcd to apply the changes on node1:

$ sudo systemctl enable etcd
$ sudo systemctl start etcd
$ sudo systemctl status etcd

Check the etcd cluster members on node1.

$ sudo etcdctl member list

The output resembles the following:

21d50d7f768f153a: name=default peerURLs=http://10.104.0.5:2380 clientURLs=http://10.104.0.5:2379 isLeader=true

Add node2 to the cluster. Run the following command on node1:

$ sudo etcdctl member add node2 http://10.104.0.2:2380

The output resembles the following one:

Added member named node2 with ID 10042578c504d052 to cluster

ETCD_NAME="node2"
ETCD_INITIAL_CLUSTER="node2=http://10.104.0.2:2380,node1=http://10.104.0.1:2380"
ETCD_INITIAL_CLUSTER_STATE="existing"

Edit the /etc/etcd/etcd.conf configuration file on node2 and add the output from the add command:

[Member]
ETCD_NAME="node2"
ETCD_INITIAL_CLUSTER="node2=http://10.104.0.2:2380,node1=http://10.104.0.1:2380"
ETCD_INITIAL_CLUSTER_STATE="existing"
ETCD_DATA_DIR="/var/lib/etcd/default.etcd"
ETCD_INITIAL_CLUSTER_TOKEN="percona-etcd-cluster"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.2:2380"
ETCD_LISTEN_PEER_URLS="http://10.104.0.2:2380"
ETCD_LISTEN_CLIENT_URLS="http://10.104.0.2:2379,http://localhost:2379"
ETCD_ADVERTISE_CLIENT_URLS="http://10.104.0.2:2379"

Start the etcd to apply the changes on node2:

$ sudo systemctl enable etcd
$ sudo systemctl start etcd
$ sudo systemctl status etcd

Add node3 to the cluster. Run the following command on node1:

$ sudo etcdctl member add node3 http://10.104.0.3:2380

Configure etcd on node3. Edit the /etc/etcd/etcd.conf configuration file on node3 and add the output from the add command as follows:

ETCD_NAME=node3
ETCD_INITIAL_CLUSTER="node1=http://10.104.0.1:2380,node2=http://10.104.0.2:2380,node3=http://10.104.0.3:2380"
ETCD_INITIAL_CLUSTER_STATE="existing"

ETCD_INITIAL_CLUSTER_TOKEN="percona-etcd-cluster"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.3:2380"
ETCD_DATA_DIR="/var/lib/etcd/postgresql"
ETCD_LISTEN_PEER_URLS="http://10.104.0.3:2380"
ETCD_LISTEN_CLIENT_URLS="http://10.104.0.3:2379,http://localhost:2379"
ETCD_ADVERTISE_CLIENT_URLS="http://10.104.0.3:2379"
…

Start the etcd service on node3:

$ sudo systemctl enable etcd
$ sudo systemctl start etcd
$ sudo systemctl status etcd

Check the etcd cluster members.

$ sudo etcdctl member list

The output resembles the following:

2d346bd3ae7f07c4: name=node2 peerURLs=http://10.104.0.2:2380 clientURLs=http://10.104.0.2:2379 isLeader=false
8bacb519ebdee8db: name=node3 peerURLs=http://10.104.0.3:2380 clientURLs=http://10.104.0.3:2379 isLeader=false
c5f52ea2ade25e1b: name=node1 peerURLs=http://10.104.0.1:2380 clientURLs=http://10.104.0.1:2379 isLeader=true

Configure Patroni¶

Install Patroni on every PostgreSQL node:
```
$ sudo yum install percona-patroni
```
Install the Python module that enables Patroni to communicate with ETCD.
```
$ sudo python3 -m pip install patroni[etcd]
```
Create the directories required by Patroni
- Create the directory to store the configuration file and make it owned by the postgres user.
```
$ sudo mkdir -p /etc/patroni/
$ sudo chown -R  postgres:postgres /etc/patroni/
```
- Create the data directory to store PostgreSQL data. Change its ownership to the postgres user and restrict the access to it
```
$ sudo mkdir /data/pgsql -p
$ sudo chown -R postgres:postgres /data/pgsql
$ sudo chmod 700 /data/pgsql
```

Create the /etc/patroni/patroni.yml with the following configuration:

namespace: percona_lab
scope: cluster_1
name: node1

restapi:
    listen: 0.0.0.0:8008
    connect_address: 10.104.0.1:8008   # PostgreSQL node IP address

etcd:
    host: 10.104.0.1:2379  # ETCD node IP address

bootstrap:
  # this section will be written into Etcd:/<namespace>/<scope>/config after initializing new cluster
  dcs:
     ttl: 30
     loop_wait: 10
     retry_timeout: 10
     maximum_lag_on_failover: 1048576
     slots:
         percona_cluster_1:
         type: physical
     postgresql:
         use_pg_rewind: true
         use_slots: true
         parameters:
             wal_level: replica
             hot_standby: "on"
             wal_keep_segments: 10
             max_wal_senders: 5
             max_replication_slots: 10
             wal_log_hints: "on"
             logging_collector: 'on'
 # some desired options for 'initdb'
 initdb: # Note: It needs to be a list (some options need values, others are switches)
     - encoding: UTF8
     - data-checksums
 pg_hba: # Add following lines to pg_hba.conf after running 'initdb'
     - host replication replicator 127.0.0.1/32 trust
     - host replication replicator 0.0.0.0/0 md5
     - host all all 0.0.0.0/0 md5
     - host all all ::0/0 md5
 # Some additional users which needs to be created after initializing new cluster
 users:
     admin:
         password: qaz123
         options:
             - createrole
             - createdb
     percona:
         password: qaz123
         options:
             - createrole
             - createdb

postgresql:
  cluster_name: cluster_1
  listen: 0.0.0.0:5432
  connect_address: 10.104.0.1:5432
  data_dir: /data/pgsql
  bin_dir: /usr/pgsql-12/bin
  pgpass: /tmp/pgpass
  authentication:
      replication:
          username: replicator
          password: replPasswd
      superuser:
          username: postgres
          password: qaz123
  parameters:
      unix_socket_directories: "/var/run/postgresql/"
  create_replica_methods:
      - basebackup
  basebackup:
      checkpoint: 'fast'

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

Create the configuration files for node2 and node3. Replace the node name and IP address of node1 to those of node2 and node3, respectively.

Create the systemd unit file patroni.service in /etc/systemd/system.

$ sudo vim /etc/systemd/system/patroni.service

Add the following contents in the file:

[Unit]
Description=Runners to orchestrate a high-availability PostgreSQL
After=syslog.target network.target

[Service]
Type=simple

User=postgres
Group=postgres

# Start the patroni process
ExecStart=/bin/patroni /etc/patroni/patroni.yml

# Send HUP to reload from patroni.yml
ExecReload=/bin/kill -s HUP $MAINPID

# only kill the patroni process, not its children, so it will gracefully stop postgres
KillMode=process

# Give a reasonable amount of time for the server to start up/shut down
TimeoutSec=30

# Do not restart the service if it crashes, we want to manually inspect database on failure
Restart=no

[Install]
WantedBy=multi-user.target

Make systemd aware of the new service:

$ sudo systemctl daemon-reload
$ sudo systemctl enable patroni
$ sudo systemctl start patroni

Troubleshooting Patroni

To ensure that Patroni has started properly, check the logs using the following command:

$ sudo journalctl -u patroni.service -n 100 -f

The output shouldn’t show any errors:

…

Sep 23 12:50:21 node01 systemd[1]: Started PostgreSQL high-availability manager.
Sep 23 12:50:22 node01 patroni[10119]: 2021-09-23 12:50:22,022 INFO: Selected new etcd server http://10.104.0.2:2379
Sep 23 12:50:22 node01 patroni[10119]: 2021-09-23 12:50:22,029 INFO: No PostgreSQL configuration items changed, nothing to reload.
Sep 23 12:50:22 node01 patroni[10119]: 2021-09-23 12:50:22,168 INFO: Lock owner: None; I am node1
Sep 23 12:50:22 node01 patroni[10119]: 2021-09-23 12:50:22,177 INFO: trying to bootstrap a new cluster
Sep 23 12:50:22 node01 patroni[10140]: The files belonging to this database system will be owned by user "postgres".
Sep 23 12:50:22 node01 patroni[10140]: This user must also own the server process.
Sep 23 12:50:22 node01 patroni[10140]: The database cluster will be initialized with locale "C.UTF-8".
Sep 23 12:50:22 node01 patroni[10140]: The default text search configuration will be set to "english".
Sep 23 12:50:22 node01 patroni[10140]: Data page checksums are enabled.
Sep 23 12:50:22 node01 patroni[10140]: creating directory /var/lib/postgresql/12/main ... ok
Sep 23 12:50:22 node01 patroni[10140]: creating subdirectories ... ok
Sep 23 12:50:22 node01 patroni[10140]: selecting dynamic shared memory implementation ... posix
Sep 23 12:50:22 node01 patroni[10140]: selecting default max_connections ... 100
Sep 23 12:50:22 node01 patroni[10140]: selecting default shared_buffers ... 128MB
Sep 23 12:50:22 node01 patroni[10140]: selecting default time zone ... Etc/UTC
Sep 23 12:50:22 node01 patroni[10140]: creating configuration files ... ok
Sep 23 12:50:22 node01 patroni[10140]: running bootstrap script ... ok
Sep 23 12:50:23 node01 patroni[10140]: performing post-bootstrap initialization ... ok
Sep 23 12:50:23 node01 patroni[10140]: syncing data to disk ... ok
Sep 23 12:50:23 node01 patroni[10140]: initdb: warning: enabling "trust" authentication for local connections
Sep 23 12:50:23 node01 patroni[10140]: You can change this by editing pg_hba.conf or using the option -A, or
Sep 23 12:50:23 node01 patroni[10140]: --auth-local and --auth-host, the next time you run initdb.
Sep 23 12:50:23 node01 patroni[10140]: Success. You can now start the database server using:
Sep 23 12:50:23 node01 patroni[10140]:     /usr/lib/postgresql/12/bin/pg_ctl -D /var/lib/postgresql/12/main -l logfile start
Sep 23 12:50:23 node01 patroni[10156]: 2021-09-23 12:50:23.672 UTC [10156] LOG:  redirecting log output to logging collector process
Sep 23 12:50:23 node01 patroni[10156]: 2021-09-23 12:50:23.672 UTC [10156] HINT:  Future log output will appear in directory "log".
Sep 23 12:50:23 node01 patroni[10119]: 2021-09-23 12:50:23,694 INFO: postprimary pid=10156
Sep 23 12:50:23 node01 patroni[10165]: localhost:5432 - accepting connections
Sep 23 12:50:23 node01 patroni[10167]: localhost:5432 - accepting connections
Sep 23 12:50:23 node01 patroni[10119]: 2021-09-23 12:50:23,743 INFO: establishing a new patroni connection to the postgres cluster
Sep 23 12:50:23 node01 patroni[10119]: 2021-09-23 12:50:23,757 INFO: running post_bootstrap
Sep 23 12:50:23 node01 patroni[10119]: 2021-09-23 12:50:23,767 INFO: Software Watchdog activated with 25 second timeout, timing slack 15 seconds
Sep 23 12:50:23 node01 patroni[10119]: 2021-09-23 12:50:23,793 INFO: initialized a new cluster
Sep 23 12:50:33 node01 patroni[10119]: 2021-09-23 12:50:33,810 INFO: no action. I am (node1) the leader with the lock
Sep 23 12:50:33 node01 patroni[10119]: 2021-09-23 12:50:33,899 INFO: no action. I am (node1) the leader with the lock
Sep 23 12:50:43 node01 patroni[10119]: 2021-09-23 12:50:43,898 INFO: no action. I am (node1) the leader with the lock
Sep 23 12:50:53 node01 patroni[10119]: 2021-09-23 12:50:53,894 INFO: no action. I am (node1) the leader with the

A common error is Patroni complaining about the lack of proper entries in the pg_hba.conf file. If you see such errors, you must manually add or fix the entries in that file and then restart the service.

Changing the patroni.yml file and restarting the service will not have any effect here because the bootstrap section specifies the configuration to apply when PostgreSQL is first started in the node. It will not repeat the process even if the Patroni configuration file is modified and the service is restarted.

If Patroni has started properly, you should be able to locally connect to a PostgreSQL node using the following command:

$ sudo psql -U postgres

psql (12.3)
Type "help" for help.

postgres=#

Configure, enable and start Patroni on the remaining nodes.
When all nodes are up and running, you can check the cluster status using the following command:

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


+ Cluster: postgres (7011110722654005156) -----------+
| Member | Host  | Role    | State   | TL | Lag in MB |
+--------+-------+---------+---------+----+-----------+
| node1  | node1 | Leader  | running |  1 |           |
| node2  | node2 | Replica | running |  1 |         0 |
| node3  | node3 | Replica | running |  1 |         0 |
+--------+-------+---------+---------+----+-----------+

Configure HAProxy¶

HAproxy is the load balancer and the single point of entry to your PostgreSQL cluster for client applications. A client application accesses the HAPpoxy URL and sends its read/write requests there. Behind-the-scene, HAProxy routes write requests to the primary node and read requests - to the secondaries in a round-robin fashion so that no secondary instance is unnecessarily loaded. To make this happen, provide different ports in the HAProxy configuration file. In this deployment, writes are routed to port 5000 and reads - to port 5001

This way, a client application doesn’t know what node in the underlying cluster is the current primary. HAProxy sends connections to a healthy node (as long as there is at least one healthy node available) and ensures that client application requests are never rejected.

Install HAProxy on the HAProxy-demo node:
```
$ sudo yum install percona-haproxy
```

The HAProxy configuration file path is: /etc/haproxy/haproxy.cfg. Specify the following configuration in this file.

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 *:7000
    stats enable
    stats uri /

listen primary
    bind *:5000
    option httpchk /primary 
    http-check expect status 200
    default-server inter 3s fall 3 rise 2 on-marked-down shutdown-sessions
    server node1 node1:5432 maxconn 100 check port 8008
    server node2 node2:5432 maxconn 100 check port 8008
    server node3 node3:5432 maxconn 100 check port 8008

listen standbys
    balance roundrobin
    bind *:5001
    option httpchk /replica 
    http-check expect status 200
    default-server inter 3s fall 3 rise 2 on-marked-down shutdown-sessions
    server node1 node1:5432 maxconn 100 check port 8008
    server node2 node2:5432 maxconn 100 check port 8008
    server node3 node3:5432 maxconn 100 check port 8008

HAProxy will use the REST APIs hosted by Patroni to check the health status of each PostgreSQL node and route the requests appropriately.

Enable a SELinux boolean to allow HAProxy to bind to non standard ports:
```
$ sudo setsebool -P haproxy_connect_any on
```
Restart HAProxy:
```
$ sudo systemctl restart haproxy
```

Check the HAProxy logs to see if there are any errors:

$ sudo journalctl -u haproxy.service -n 100 -f

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Last update: August 1, 2023
Created: December 15, 2021