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.
For this setup, we will use the nodes running on Ubuntu 20.04 as the base operating system and having the following IP addresses:
|Node name||Public IP address||Internal IP address|
In a production (or even non-production) setup, the PostgreSQL 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
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 node as follows:
|node 1||node 2||node 3|
/etc/hosts file of the HAProxy-demo node looks like the following:
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
Install Percona Distribution for PostgreSQL¶
Follow the installation instructions to install Percona Distribution for PostgreSQL on
Remove the data directory. Patroni requires a clean environment to initialize a new cluster. Use the following commands to stop the PostgreSQL service and then remove the data directory:
$ sudo systemctl stop postgresql $ sudo rm -rf /var/lib/postgresql/11/main
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 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
etcdon every PostgreSQL node using the following command:
$ sudo apt install etcd
/etc/default/etcdconfiguration file on each node.
node1, add the IP address of
ETCD_INITIAL_CLUSTERparameter. The configuration file looks as follows:
ETCD_NAME=node1 ETCD_INITIAL_CLUSTER="node1=http://10.104.0.7:2380" ETCD_INITIAL_CLUSTER_TOKEN="devops_token" ETCD_INITIAL_CLUSTER_STATE="new" ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.7:2380" ETCD_DATA_DIR="/var/lib/etcd/postgresql" ETCD_LISTEN_PEER_URLS="http://10.104.0.7:2380" ETCD_LISTEN_CLIENT_URLS="http://10.104.0.7:2379,http://localhost:2379" ETCD_ADVERTISE_CLIENT_URLS="http://10.104.0.7:2379" …
node2, add the IP addresses of both
ETCD_NAME=node2 ETCD_INITIAL_CLUSTER="node1=http://10.104.0.7:2380,node2=http://10.104.0.2:2380" ETCD_INITIAL_CLUSTER_TOKEN="devops_token" ETCD_INITIAL_CLUSTER_STATE="existing" ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.2:2380" ETCD_DATA_DIR="/var/lib/etcd/postgresql" 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" …
ETCD_INITIAL_CLUSTERparameter includes the IP addresses of all three nodes:
ETCD_NAME=node3 ETCD_INITIAL_CLUSTER="node1=http://10.104.0.7:2380,node2=http://10.104.0.2:2380,node3=http://10.104.0.8:2380" ETCD_INITIAL_CLUSTER_TOKEN="devops_token" ETCD_INITIAL_CLUSTER_STATE="existing" ETCD_INITIAL_ADVERTISE_PEER_URLS="http://10.104.0.8:2380" ETCD_DATA_DIR="/var/lib/etcd/postgresql" ETCD_LISTEN_PEER_URLS="http://10.104.0.8:2380" ETCD_LISTEN_CLIENT_URLS="http://10.104.0.8:2379,http://localhost:2379" ETCD_ADVERTISE_CLIENT_URLS="http://10.104.0.8:2379" …
node3to the cluster using the
$ sudo etcdctl member add node2 http://10.104.0.2:2380 $ sudo etcdctl member add node3 http://10.104.0.8:2380
$ sudo systemctl restart etcd
Check the etcd cluster members.
$ sudo etcdctl member list
The output resembles the following:
21d50d7f768f153a: name=node1 peerURLs=http://10.104.0.7:2380 clientURLs=http://10.104.0.7:2379 isLeader=true af4661d829a39112: name=node2 peerURLs=http://10.104.0.2:2380 clientURLs=http://10.104.0.2:2379 isLeader=false e3f3c0c1d12e9097: name=node3 peerURLs=http://10.104.0.8:2380 clientURLs=http://10.104.0.8:2379 isLeader=false
Set up the watchdog service¶
The Linux kernel uses the utility called a watchdog to protect against an unresponsive system. The watchdog monitors a system for unrecoverable application errors, depleted system resources, etc., and initiates a reboot to safely return the system to a working state. The watchdog functionality is useful for servers that are intended to run without human intervention for a long time. Instead of users finding a hung server, the watchdog functionality can help maintain the service.
In this example, we will configure Softdog - a standard software implementation for watchdog that is shipped with Ubuntu 20.04.
Complete the following steps on all three PostgreSQL nodes to load and configure Softdog.
$ sudo sh -c 'echo "softdog" >> /etc/modules'
Patroni will be interacting with the watchdog service. Since Patroni is run by the
postgresuser, this user must have access to Softdog. To make this happen, change the ownership of the
watchdog.rulesfile to the
$ sudo sh -c 'echo "KERNEL==\"watchdog\", OWNER=\"postgres\", GROUP=\"postgres\"" >> /etc/udev/rules.d/61-watchdog.rules'
Remove Softdog from the blacklist.
- Find out the files where Softdog is blacklisted:
$ grep blacklist /lib/modprobe.d/* /etc/modprobe.d/* |grep softdog
In our case,
modprobeis blacklisting the Softdog:
- Remove the
blacklist softdogline from the
- Restart the service
$ sudo modprobe softdog
- Verify the
modprobeis working correctly by running the
$ sudo lsmod | grep softdog
The output will show a process identifier if it’s running.
softdog 16384 0
Check that the Softdog files under the
/dev/folder are owned by the
$ ls -l /dev/watchdog* crw-rw---- 1 postgres postgres 10, 130 Sep 11 12:53 /dev/watchdog crw------- 1 root root 245, 0 Sep 11 12:53 /dev/watchdog0
If the ownership has not been changed for any reason, run the following command to manually change it:
$ sudo chown postgres:postgres /dev/watchdog*
Install Patroni on every PostgreSQL node:
$ sudo apt install percona-patroni
patroni.ymlconfiguration file under the
/etc/patronidirectory. The file holds the default configuration values for a PostgreSQL cluster and will reflect the current cluster setup.
Add the following configuration for
scope: stampede1 name: node1 restapi: listen: 0.0.0.0:8008 connect_address: node1:8008 etcd: host: node1:2379 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 # primary_start_timeout: 300 # synchronous_mode: false 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" #archive_mode: "on" #archive_timeout: 600 #archive_command: "cp -f %p /home/postgres/archived/%f" #recovery_conf: #restore_command: cp /home/postgres/archived/%f %p # 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 all all 10.104.0.7/32 md5 - host replication replicator 127.0.0.1/32 trust - host all all 10.104.0.2/32 md5 - host all all 10.104.0.8/32 md5 - host all all 10.104.0.6/32 trust # - hostssl all all 0.0.0.0/0 md5 # Additional script to be launched after initial cluster creation (will be passed the connection URL as parameter) # post_init: /usr/local/bin/setup_cluster.sh # Some additional users users which needs to be created after initializing new cluster users: admin: password: admin options: - createrole - createdb replicator: password: password options: - replication postgresql: listen: 0.0.0.0:5432 connect_address: node1:5432 data_dir: "/var/lib/postgresql/11/main" bin_dir: "/usr/lib/postgresql/11/bin" # config_dir: pgpass: /tmp/pgpass0 authentication: replication: username: replicator password: password superuser: username: postgres password: password parameters: unix_socket_directories: '/var/run/postgresql' watchdog: mode: required # Allowed values: off, automatic, required device: /dev/watchdog safety_margin: 5 tags: nofailover: false noloadbalance: false clonefrom: false nosync: false
Patroni configuration file
Let’s take a moment to understand the contents of the
The first section provides the details of the first node (
node1) and its connection ports. After that, we have the
etcdservice and its port details.
Following these, there is a
bootstrapsection that contains the PostgreSQL configurations and the steps to run once the database is initialized. The
pg_hba.confentries specify all the other nodes that can connect to this node and their authentication mechanism.
Create the configuration files for
node3. Replace the reference to
Enable and restart the patroni service on every node. Use the following commands:
$ sudo systemctl enable 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.
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: Started PostgreSQL high-availability manager. Sep 23 12:50:22 node01 patroni: 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: 2021-09-23 12:50:22,029 INFO: No PostgreSQL configuration items changed, nothing to reload. Sep 23 12:50:22 node01 patroni: 2021-09-23 12:50:22,168 INFO: Lock owner: None; I am node1 Sep 23 12:50:22 node01 patroni: 2021-09-23 12:50:22,177 INFO: trying to bootstrap a new cluster Sep 23 12:50:22 node01 patroni: The files belonging to this database system will be owned by user "postgres". Sep 23 12:50:22 node01 patroni: This user must also own the server process. Sep 23 12:50:22 node01 patroni: The database cluster will be initialized with locale "C.UTF-8". Sep 23 12:50:22 node01 patroni: The default text search configuration will be set to "english". Sep 23 12:50:22 node01 patroni: Data page checksums are enabled. Sep 23 12:50:22 node01 patroni: creating directory /var/lib/postgresql/12/main ... ok Sep 23 12:50:22 node01 patroni: creating subdirectories ... ok Sep 23 12:50:22 node01 patroni: selecting dynamic shared memory implementation ... posix Sep 23 12:50:22 node01 patroni: selecting default max_connections ... 100 Sep 23 12:50:22 node01 patroni: selecting default shared_buffers ... 128MB Sep 23 12:50:22 node01 patroni: selecting default time zone ... Etc/UTC Sep 23 12:50:22 node01 patroni: creating configuration files ... ok Sep 23 12:50:22 node01 patroni: running bootstrap script ... ok Sep 23 12:50:23 node01 patroni: performing post-bootstrap initialization ... ok Sep 23 12:50:23 node01 patroni: syncing data to disk ... ok Sep 23 12:50:23 node01 patroni: initdb: warning: enabling "trust" authentication for local connections Sep 23 12:50:23 node01 patroni: You can change this by editing pg_hba.conf or using the option -A, or Sep 23 12:50:23 node01 patroni: --auth-local and --auth-host, the next time you run initdb. Sep 23 12:50:23 node01 patroni: Success. You can now start the database server using: Sep 23 12:50:23 node01 patroni: /usr/lib/postgresql/11/bin/pg_ctl -D /var/lib/postgresql/11/main -l logfile start Sep 23 12:50:23 node01 patroni: 2021-09-23 12:50:23.672 UTC  LOG: redirecting log output to logging collector process Sep 23 12:50:23 node01 patroni: 2021-09-23 12:50:23.672 UTC  HINT: Future log output will appear in directory "log". Sep 23 12:50:23 node01 patroni: 2021-09-23 12:50:23,694 INFO: postprimary pid=10156 Sep 23 12:50:23 node01 patroni: localhost:5432 - accepting connections Sep 23 12:50:23 node01 patroni: localhost:5432 - accepting connections Sep 23 12:50:23 node01 patroni: 2021-09-23 12:50:23,743 INFO: establishing a new patroni connection to the postgres cluster Sep 23 12:50:23 node01 patroni: 2021-09-23 12:50:23,757 INFO: running post_bootstrap Sep 23 12:50:23 node01 patroni: 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: 2021-09-23 12:50:23,793 INFO: initialized a new cluster Sep 23 12:50:33 node01 patroni: 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: 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: 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: 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
The command output is the following
psql (11.13) Type "help" for help. postgres=#
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
$ sudo apt 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.
$ sudo systemctl restart haproxy
Check the HAProxy logs to see if there are any errors:
$ sudo journalctl -u haproxy.service -n 100 -f
See the Testing PostgreSQL cluster for the guidelines on how to test your PostgreSQL cluster for replication, failure, switchover.
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Created: December 15, 2021