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.

Preconditions¶

For this setup, we will use the nodes running on CentOS 8 as the base operating system and having the following IP addresses:

Hostname	Public IP address	Internal IP address
node1	157.230.42.174	10.104.0.7
node2	68.183.177.183	10.104.0.2
node3	165.22.62.167	10.104.0.8
etcd	159.102.29.166	10.104.0.5
HAProxy-demo	134.209.111.138	10.104.0.6

Note

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 DigitalOcean VPS environment, and each node can access the other by its internal, private IP.

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

To make the nodes aware of each other and allow their seamless communication, resolve their hostnames to their public IP addresses. Modify the /etc/hosts file of each PostgreSQL node to include the hostnames and IP addresses of the remaining nodes. The following is the /etc/hosts file for node1:

127.0.0.1 localhost node1
10.104.0.7 node1 
10.104.0.2 node2 
10.104.0.8 node3

The /etc/hosts file of the HAProxy-demo node hostnames and IP addresses of all PostgreSQL nodes:

127.0.1.1 HAProxy-demo HAProxy-demo
127.0.0.1 localhost
10.104.0.6 HAProxy-demo
10.104.0.7 node1
10.104.0.2 node2
10.104.0.8 node3

Keep the /etc/hosts file of the etcd node unchanged.

Configure ETCD distributed store¶

The distributed configuration store helps establish a consensus among nodes during a failover and will manage the configuration for the three PostgreSQL instances. Although Patroni can work with other distributed consensus stores (i.e., Zookeeper, Consul, etc.), the most commonly used one is etcd.

In this setup we will configure ETCD on a dedicated node.

Install etcd on the ETCD 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
```
Modify the /etc/etcd/etcd.conf configuration file:

[Member]
ETCD_DATA_DIR="/var/lib/etcd/default.etcd"
ETCD_LISTEN_PEER_URLS="http://10.104.0.5:2380,http://localhost:2380" 
ETCD_LISTEN_CLIENT_URLS="http://10.104.0.5:2379,http://localhost:2379"


ETCD_NAME="default"
ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.5:2380"
ETCD_ADVERTISE_CLIENT_URLS="http://10.104.0.5:2379"
ETCD_INITIAL_CLUSTER="default=http://10.104.0.5:2380"
ETCD_INITIAL_CLUSTER_TOKEN="etcd-cluster"
ETCD_INITIAL_CLUSTER_STATE="new"

Start the etcd to apply the changes:

$ 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:

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

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 ppg12
```
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 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 for Patroni. Change its ownership to the postgres user and restrict the access to it
```
$ sudo mkdir /data/patroni -p
$ sudo chown -R postgres:postgres /data/patroni
$ sudo chmod 700 /data/patroni
```

Create the patroni.yml configuration file.

$ su postgres
$ vim /etc/patroni/patroni.yml

Specify the following configuration:

scope: postgres
namespace: /pg_cluster/
name: node1

restapi:
  listen: 10.104.0.7:8008            # PostgreSQL node IP address
  connect_address: 10.104.0.7:8008   # PostgreSQL node IP address

etcd:
  host: 10.104.0.5:2379  # 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
    postgresql:
      use_pg_rewind: true
      use_slots: true
      parameters:
        wal_level: replica
        hot_standby: "on"
        logging_collector: 'on'
        max_wal_senders: 5
        max_replication_slots: 5
        wal_log_hints: "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 md5
  - host replication replicator 10.104.0.2/32 md5  
  - host replication replicator 10.104.0.8/32 md5 
  - host replication replicator 10.104.0.7/32 md5
  - host all all 0.0.0.0/0 md5
#  - hostssl all all 0.0.0.0/0 md5

  # Some additional users users which needs to be created after initializing new cluster
  users:
    admin:
      password: admin
      options:
        - createrole
        - createdb

postgresql:
  listen: 10.104.0.7:5432            # PostgreSQL node IP address
  connect_address: 10.104.0.7:5432   # PostgreSQL node IP address
  data_dir: /data/patroni            # The datadir you created
  bin_dir: /usr/pgsql-12/bin
  pgpass: /tmp/pgpass0
  authentication:
    replication:
      username: replicator
      password: replicator
    superuser:
      username: postgres
      password: postgres
  parameters:
    unix_socket_directories: '.'

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

Create the configuration files for node2 and node3. Replace the node 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 node will accept client connection requests and route those to the active node of the PostgreSQL cluster. This way, a client application doesn’t have to know what node in the underlying cluster is the current primary. All it needs to do is to access a single HAProxy URL and send its read/write requests there. Behind-the-scene, HAProxy routes the connection to a healthy node (as long as there is at least one healthy node available) and ensures that client application requests are never rejected.

HAProxy is capable of routing 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.

Install HAProxy on the HAProxy-demo node:
```
$ sudo yum install 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: 2022-04-26