Deploying PostgreSQL for high availability with Patroni on RHEL and derivatives¶

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

Preconditions¶

This is an example deployment where etcd runs on the same host machines as the Patroni and PostgreSQL and there is a single dedicated HAProxy host. Alternatively etcd can run on different set of nodes.

If etcd is deployed on the same host machine as Patroni and PostgreSQL, separate disk system for etcd and PostgreSQL is recommended due to performance reasons.

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

We recommend not to expose the hosts / nodes where Patroni / etcd / PostgreSQL are running to public networks due to security risks. Use Firewalls, Virtual networks, subnets or the like to protect the database hosts from any kind of attack.

Initial setup¶

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.

node1node2node3HAproxy-demo

Set up the hostname for the node
```
$ sudo hostnamectl set-hostname node1
```
Modify the /etc/hosts file 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:
```
# Cluster IP and names 
10.104.0.1 node1 
10.104.0.2 node2 
10.104.0.3 node3
```

Set up the hostname for the node
```
$ sudo hostnamectl set-hostname node2
```
Modify the /etc/hosts file 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:
```
# Cluster IP and names 
10.104.0.1 node1 
10.104.0.2 node2 
10.104.0.3 node3
```

Set up the hostname for the node
```
$ sudo hostnamectl set-hostname node3
```
Modify the /etc/hosts file 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:
```
# Cluster IP and names 
10.104.0.1 node1 
10.104.0.2 node2 
10.104.0.3 node3
```

Set up the hostname for the node

$ sudo hostnamectl set-hostname HAProxy-demo

Modify the /etc/hosts file. 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 the software¶

Run the following commands on node1, node2 and node3:

Install Percona Distribution for PostgreSQL:
- Check the platform specific notes
- Install the percona-release repository management tool
  
  Run the following command as the root user or with sudo privileges:
```
$ sudo yum install -y https://repo.percona.com/yum/percona-release-latest.noarch.rpm
```
- Enable the repository:
```
$ sudo percona-release setup ppg13
```
- Install Percona Distribution for PostgreSQL package
```
$ sudo apt install percona-postgresql13-server
```
Important

Don’t initialize the cluster and start the postgresql service. The cluster initialization and setup are handled by Patroni during the bootsrapping stage.
Install some Python and auxiliary packages to help with Patroni and etcd
```
$ sudo yum install python3-pip python3-devel binutils
```

Install etcd, Patroni, pgBackRest packages. Check platform specific notes for Patroni:

$ sudo yum install percona-patroni \
etcd python3-python-etcd \
percona-pgbackrest

Stop and disable all installed services:

$ sudo systemctl stop {etcd,patroni,postgresql}
$ systemctl disable {etcd,patroni,postgresql}

Configure etcd distributed store¶

In our implementation we use etcd distributed configuration store. Refresh your knowledge about etcd.

Note

If you installed the software from tarballs, you must first enable etcd before configuring it.

To get started with etcd cluster, you need to bootstrap it. This means setting up the initial configuration and starting the etcd nodes so they can form a cluster. There are the following bootstrapping mechanisms:

Static in the case when the IP addresses of the cluster nodes are known
Discovery service - for cases when the IP addresses of the cluster are not known ahead of time.

Since we know the IP addresses of the nodes, we will use the static method. For using the discovery service, please refer to the etcd documentation :octicons-external-link-16:.

We will configure and start all etcd nodes in parallel. This can be done either by modifying each node’s configuration or using the command line options. Use the method that you prefer more.

Method 1. Modify the configuration file¶

Create the etcd configuration file on every node. You can edit the sample configuration file /etc/etcd/etcd.conf.yaml or create your own one. Replace the node names and IP addresses with the actual names and IP addresses of your nodes.

node1node2node3

/etc/etcd/etcd.conf.yaml

name: 'node1'
initial-cluster-token: PostgreSQL_HA_Cluster_1
initial-cluster-state: new
initial-cluster: node1=http://10.104.0.1:2380,node2=http://10.104.0.2:2380,node3=http://10.104.0.3:2380
data-dir: /var/lib/etcd
initial-advertise-peer-urls: http://10.104.0.1:2380 
listen-peer-urls: http://10.104.0.1:2380
advertise-client-urls: http://10.104.0.1:2379
listen-client-urls: http://10.104.0.1:2379

/etc/etcd/etcd.conf.yaml

name: 'node2'
initial-cluster-token: PostgreSQL_HA_Cluster_1
initial-cluster-state: new
initial-cluster: node1=http://10.104.0.1:2380,node2=http://10.104.0.2:2380,     node3=http://10.104.0.3:2380
data-dir: /var/lib/etcd
initial-advertise-peer-urls: http://10.104.0.2:2380 
listen-peer-urls: http://10.104.0.2:2380
advertise-client-urls: http://10.104.0.2:2379
listen-client-urls: http://10.104.0.2:2379

/etc/etcd/etcd.conf.yaml

name: 'node3'
initial-cluster-token: PostgreSQL_HA_Cluster_1
initial-cluster-state: new
initial-cluster: node1=http://10.104.0.1:2380,node2=http://10.104.0.2:2380,     node3=http://10.104.0.3:2380
data-dir: /var/lib/etcd
initial-advertise-peer-urls: http://10.104.0.3:2380 
listen-peer-urls: http://10.104.0.3:2380
advertise-client-urls: http://10.104.0.3:2379
listen-client-urls: http://10.104.0.3:2379

Enable and start the etcd service on all nodes:
```
$ sudo systemctl enable --now etcd
$ sudo systemctl start etcd
$ sudo systemctl status etcd
```
During the node start, etcd searches for other cluster nodes defined in the configuration. If the other nodes are not yet running, the start may fail by a quorum timeout. This is expected behavior. Try starting all nodes again at the same time for the etcd cluster to be created.
Check the etcd cluster members. Use etcdctl for this purpose. Ensure that etcdctl interacts with etcd using API version 3 and knows which nodes, or endpoints, to communicate with. For this, we will define the required information as environment variables. Run the following commands on one of the nodes:
```
export ETCDCTL_API=3
HOST_1=10.104.0.1
HOST_2=10.104.0.2
HOST_3=10.104.0.3
ENDPOINTS=$HOST_1:2379,$HOST_2:2379,$HOST_3:2379
```

Now, list the cluster members and output the result as a table as follows:

$ sudo etcdctl --endpoints=$ENDPOINTS -w table member list

Sample output

+------------------+---------+-------+------------------------+----------------------------+------------+
|        ID        | STATUS  | NAME  |         PEER ADDRS     |        CLIENT ADDRS        | IS LEARNER |
+------------------+---------+-------+------------------------+----------------------------+------------+
| 4788684035f976d3 | started | node2 | http://10.104.0.2:2380 | http://192.168.56.102:2379 |      false |
| 67684e355c833ffa | started | node3 | http://10.104.0.3:2380 | http://192.168.56.103:2379 |      false |
| 9d2e318af9306c67 | started | node1 | http://10.104.0.1:2380 | http://192.168.56.101:2379 |      false |
+------------------+---------+-------+------------------------+----------------------------+------------+

To check what node is currently the leader, use the following command

$ sudo etcdctl --endpoints=$ENDPOINTS -w table endpoint status

Sample output

+-----------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
|      ENDPOINT   |        ID        | VERSION | DB SIZE | IS LEADER | IS LEARNER | RAFT TERM | RAFT INDEX | RAFT APPLIED INDEX | ERRORS |
+-----------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
| 10.104.0.1:2379 | 9d2e318af9306c67 |  3.5.16 |   20 kB |      true |      false |         2 |         10 |                 10 |        |
| 10.104.0.2:2379 | 4788684035f976d3 |  3.5.16 |   20 kB |     false |      false |         2 |         10 |                 10 |        |
| 10.104.0.3:2379 | 67684e355c833ffa |  3.5.16 |   20 kB |     false |      false |         2 |         10 |                 10 |        |
+-----------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+

Method 2. Start etcd nodes with command line options¶

On each etcd node, set the environment variables for the cluster members, the cluster token and state:

TOKEN=PostgreSQL_HA_Cluster_1
CLUSTER_STATE=new
NAME_1=node1
NAME_2=node2
NAME_3=node3
HOST_1=10.104.0.1
HOST_2=10.104.0.2
HOST_3=10.104.0.3
CLUSTER=${NAME_1}=http://${HOST_1}:2380,${NAME_2}=http://${HOST_2}:2380,${NAME_3}=http://${HOST_3}:2380

Start each etcd node in parallel using the following command:

node1node2node3

THIS_NAME=${NAME_1}
THIS_IP=${HOST_1}
etcd --data-dir=data.etcd --name ${THIS_NAME} \
    --initial-advertise-peer-urls http://${THIS_IP}:2380 --listen-peer-urls http://${THIS_IP}:2380 \
    --advertise-client-urls http://${THIS_IP}:2379 --listen-client-urls http://${THIS_IP}:2379 \
    --initial-cluster ${CLUSTER} \
    --initial-cluster-state ${CLUSTER_STATE} --initial-cluster-token ${TOKEN}

THIS_NAME=${NAME_2}
THIS_IP=${HOST_2}
etcd --data-dir=data.etcd --name ${THIS_NAME} \
    --initial-advertise-peer-urls http://${THIS_IP}:2380 --listen-peer-urls http://${THIS_IP}:2380 \
    --advertise-client-urls http://${THIS_IP}:2379 --listen-client-urls http://${THIS_IP}:2379 \
    --initial-cluster ${CLUSTER} \
    --initial-cluster-state ${CLUSTER_STATE} --initial-cluster-token ${TOKEN}

THIS_NAME=${NAME_3}
THIS_IP=${HOST_3}
etcd --data-dir=data.etcd --name ${THIS_NAME} \
    --initial-advertise-peer-urls http://${THIS_IP}:2380 --listen-peer-urls http://${THIS_IP}:2380 \
    --advertise-client-urls http://${THIS_IP}:2379 --listen-client-urls http://${THIS_IP}:2379 \
    --initial-cluster ${CLUSTER} \
    --initial-cluster-state ${CLUSTER_STATE} --initial-cluster-token ${TOKEN}

Check the etcd cluster members. Use etcdctl for this purpose. Ensure that etcdctl interacts with etcd using API version 3 and knows which nodes, or endpoints, to communicate with. For this, we will define the required information as environment variables. Run the following commands on one of the nodes:
```
export ETCDCTL_API=3
HOST_1=10.104.0.1
HOST_2=10.104.0.2
HOST_3=10.104.0.3
ENDPOINTS=$HOST_1:2379,$HOST_2:2379,$HOST_3:2379
```

Now, list the cluster members and output the result as a table as follows:

$ sudo etcdctl --endpoints=$ENDPOINTS -w table member list

Sample output

+------------------+---------+-------+------------------------+----------------------------+------------+
|        ID        | STATUS  | NAME  |         PEER ADDRS     |        CLIENT ADDRS        | IS LEARNER |
+------------------+---------+-------+------------------------+----------------------------+------------+
| 4788684035f976d3 | started | node2 | http://10.104.0.2:2380 | http://192.168.56.102:2379 |      false |
| 67684e355c833ffa | started | node3 | http://10.104.0.3:2380 | http://192.168.56.103:2379 |      false |
| 9d2e318af9306c67 | started | node1 | http://10.104.0.1:2380 | http://192.168.56.101:2379 |      false |
+------------------+---------+-------+------------------------+----------------------------+------------+

To check what node is currently the leader, use the following command

$ sudo etcdctl --endpoints=$ENDPOINTS -w table endpoint status

Sample output

+-----------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
|      ENDPOINT   |        ID        | VERSION | DB SIZE | IS LEADER | IS LEARNER | RAFT TERM | RAFT INDEX | RAFT APPLIED INDEX | ERRORS |
+-----------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
| 10.104.0.1:2379 | 9d2e318af9306c67 |  3.5.16 |   20 kB |      true |      false |         2 |         10 |                 10 |        |
| 10.104.0.2:2379 | 4788684035f976d3 |  3.5.16 |   20 kB |     false |      false |         2 |         10 |                 10 |        |
| 10.104.0.3:2379 | 67684e355c833ffa |  3.5.16 |   20 kB |     false |      false |         2 |         10 |                 10 |        |
+-----------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+

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¶

Run the following commands on all nodes. You can do this in parallel:

Export and create environment variables to simplify the config file creation:
- Node name:
```
$ export NODE_NAME=`hostname -f`
```
- Node IP:
```
$ export NODE_IP=`hostname -i | awk '{print $1}'`
```
- Create variables to store the PATH:
```
DATA_DIR="/var/lib/pgsql/data/"
PG_BIN_DIR="/usr/pgsql-12/bin"
```
NOTE: Check the path to the data and bin folders on your operating system and change it for the variables accordingly.
- Patroni information:
```
NAMESPACE="percona_lab"
SCOPE="cluster_1
```
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/
```
- We won’t use the default RHEL to store PostgreSQL data, but will create a data directory for PostgreSQL. We also need to 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
```

Use the following command to create the /etc/patroni/patroni.yml configuration file and add the following configuration for node1:

echo "
namespace: ${NAMESPACE}
scope: ${SCOPE}
name: ${NODE_NAME}

restapi:
    listen: 0.0.0.0:8008
    connect_address: ${NODE_IP}:8008

etcd3:
    host: ${NODE_IP}:2379

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"
              wal_keep_segments: 10
              max_wal_senders: 5
              max_replication_slots: 10
              wal_log_hints: "on"
              logging_collector: 'on'
              max_wal_size: '10GB'
              archive_mode: "on"
              archive_timeout: 600s
              archive_command: "cp -f %p /home/postgres/archived/%f"

  # 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: ${NODE_IP}:5432
    data_dir: ${DATA_DIR}
    bin_dir: ${PG_BIN_DIR}
    pgpass: /tmp/pgpass0
    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
" | sudo tee -a /etc/patroni/patroni.yml

Check that the systemd unit file percona-patroni.service is created in /etc/systemd/system. If it is created, skip this step.

If it’s not created, create it manually and specify the following contents within:

/etc/systemd/system/percona-patroni.service

[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
```
Repeat steps 1-5 on the remaining nodes. In the end you must have the configuration file and the systemd unit file created on every node.
Now it’s time to start Patroni. You need the following commands on all nodes but not in parallel. Start with the node1 first, wait for the service to come to live, and then proceed with the other nodes one-by-one, always waiting for them to sync with the primary node:
```
$ sudo systemctl enable --now patroni
$ sudo systemctl restart patroni
```
When Patroni starts, it initializes PostgreSQL (because the service is not currently running and the data directory is empty) following the directives in the bootstrap section of the configuration file.
Check the service to see if there are errors:
```
$ sudo journalctl -fu patroni
```
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 (13.12)
Type "help" for help.

postgres=#
```

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

The output resembles the following:

+ Cluster: cluster_1 (7440127629342136675) -----+----+-------+
| Member | Host       | Role    | State     | TL | Lag in MB |
+--------+------------+---------+-----------+----+-----------+
| node1  | 10.0.100.1 | Leader  | running   |  1 |           |
| node2  | 10.0.100.2 | Replica | streaming |  1 |         0 |
| node3  | 10.0.100.3 | Replica | streaming |  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 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

Next steps¶

Configure pgBackRest

Get expert help¶

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Deploying PostgreSQL for high availability with Patroni on RHEL and derivatives¶

Preconditions¶

Initial setup¶

Set up hostnames in the /etc/hosts file¶

Install the software¶

Configure etcd distributed store¶

Method 1. Modify the configuration file¶

Method 2. Start etcd nodes with command line options¶

Configure Patroni¶

Configure HAProxy¶

Next steps¶

Get expert help¶

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