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Deploying PostgreSQL for high availability with Patroni on Debian or Ubuntu

This guide provides instructions on how to set up a highly available PostgreSQL cluster with Patroni on Debian or Ubuntu.

Considerations

  1. 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.

  2. For this setup, we will use the nodes running on Ubuntu 22.04 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.

  1. Run the following command on each node. Change the node name to node1, node2 and node3 respectively:

    $ sudo hostnamectl set-hostname node1
    
  2. 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:

    # 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 the software

Run the following commands on node1,node2andnode3`:

  1. Install Percona Distribution for PostgreSQL

    $ sudo percona-release setup ppg15
    
    * Install Percona Distribution for PostgreSQL packages.

  2. Install some Python and auxiliary packages to help with Patroni and ETCD

    {.bash data-prompt="$"}
    $ sudo apt install python3-pip python3-dev binutils
    
  3. Install ETCD, Patroni, pgBackRest packages:

    $ sudo apt install percona-patroni \
    etcd etcd-server etcd-client \
    percona-pgbackrest
    
  4. Stop and disable all installed services:

    $ sudo systemctl stop {etcd,patroni,postgresql}
    $ systemctl disable {etcd,patroni,postgresql}
    
  5. Even though Patroni can use an existing Postgres installation, remove the data directory to force it to initialize a new Postgres cluster instance.

$ sudo rm -rf /var/lib/postgresql/15/main

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.

This document provides configuration for ETCD version 3.5.x. For how to configure ETCD cluster with earlier versions of ETCD, read the blog post by Fernando Laudares Camargos and Jobin Augustine PostgreSQL HA with Patroni: Your Turn to Test Failure Scenarios

The etcd cluster is first started in one node and then the subsequent nodes are added to the first node using the addcommand.

Note

Users with deeper understanding of how ETCD works can configure and start all ETCD nodes at a time and bootstrap the cluster using one of the following methods:

  • 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.

See the How to configure ETCD nodes simultaneously section for details.

Configure node1

  1. Create the configuration file. You can edit the sample configuration file /etc/etcd/etcd.conf.yaml or create your own one. Replace the node name and IP address with the actual name and IP address of your node.

    /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
    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
    
  2. Enable and start the etcd service to apply the changes on node1.

    $ sudo systemctl enable --now etcd
    $ sudo systemctl status etcd
    
  3. Check the etcd cluster members on node1:

    $ sudo etcdctl member list
    
    Sample output
    21d50d7f768f153a: name=default peerURLs=http://10.104.0.1:2380 clientURLs=http:// 10.104.0.1:2379 isLeader=true
    
  4. Add the node2 to the cluster. Run the following command on node1:

    $ sudo etcdctl member add node2 http://10.104.0.2:2380
    
    Sample output
    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"
    

Configure node2

  1. Create the configuration file. 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.

    /etc/etcd/etcd.conf.yaml
    name: 'node2'
    initial-cluster-token: PostgreSQL_HA_Cluster_1
    initial-cluster-state: existing
    initial-cluster: node1=http://10.104.0.1:2380,node2=http://10.104.0.2:2380
    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
    
  2. Enable and start the etcd service to apply the changes on node2:

    $ sudo systemctl enable --now etcd
    $ sudo systemctl status etcd
    

Configure node3

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

    $ sudo etcdctl member add node3 http://10.104.0.3:2380
    
  2. On node3, create the configuration file. 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.

    /etc/etcd/etcd.conf.yaml
    name: 'node1'
    initial-cluster-token: PostgreSQL_HA_Cluster_1
    initial-cluster-state: existing
    initial-cluster: node1=http://10.104.0.1:2380,node2=http://10.104.0.2:238node3=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
    
  3. Enable and start the etcd service to apply the changes on node3.

    $ sudo systemctl enable --now etcd
    $ sudo systemctl status etcd
    
  4. Check the etcd cluster members.

    $ sudo etcdctl member list
    
    Sample output
    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

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

  1. 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/postgresql/15/main"
    PG_BIN_DIR="/usr/lib/postgresql/15/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"
    
  2. Create the /etc/patroni/patroni.yml configuration file. 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
          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: ${NODE_IP}:5432
        data_dir: ${DATADIR}
        bin_dir: ${PG_BIN_DIR}
        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
    " | sudo tee -a /etc/patroni/patroni.yml
    
    Patroni configuration file

    Let’s take a moment to understand the contents of the patroni.yml file.

    The first section provides the details of the node and its connection ports. After that, we have the etcd service and its port details.

    Following these, there is a bootstrap section that contains the PostgreSQL configurations and the steps to run once the database is initialized. The pg_hba.conf entries specify all the other nodes that can connect to this node and their authentication mechanism.

  3. Check that the systemd unit file 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:

```ini title="/etc/systemd/system/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
```
  1. Make systemd aware of the new service:

    $ sudo systemctl daemon-reload
    
  2. 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.

  1. 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.

  2. Check the cluster:

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

    The output on node1 resembles the following:

    + Cluster: cluster_1 --+---------+---------+----+-----------+
    | Member | Host        | Role    | State   | TL | Lag in MB |
    +--------+-------------+---------+---------+----+-----------+
    | node-1 | 10.0.100.1  | Leader  | running |  1 |           |
    +--------+-------------+---------+---------+----+-----------+
    

    On the remaining nodes:

    + Cluster: cluster_1 --+---------+---------+----+-----------+
    | Member | Host        | Role    | State   | TL | Lag in MB |
    +--------+-------------+---------+---------+----+-----------+
    | node-1 | 10.0.100.1  | Leader  | running |  1 |           |
    | node-2 | 10.0.100.2  | Replica | running |  1 |         0 |
    +--------+-------------+---------+---------+----+-----------+
    

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

$ sudo psql -U postgres

The command output is the following:

psql (15.4)
Type "help" for help.

postgres=#

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.

  1. Install HAProxy on the HAProxy-demo node:

    $ sudo apt install percona-haproxy
    
  2. 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.

  3. Restart HAProxy:

    $ sudo systemctl restart haproxy
    
  4. Check the HAProxy logs to see if there are any errors:

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

Next steps

Configure pgBackRest

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