The following is for dockerizing a simulated multi data center Postgres 16 on Ubuntu 20.04 using Patroni, etcd, pgbackrest and Docker.
You have the ability to generate docker-compose files for x number of data centers and x number of nodes per data center.
Below is a TL;DR section followed by a more detailed explanation of what is happening.
At a minimum, use docker-compose version 1.29.2. Using an older version may generate errors when you try to run the docker-compose.yaml file
From inside the etcd folder ...
docker build -t pgha-etcd-3.5 .
From inside the pgpatroni folder ...
docker build -t pgha-pg16-patroni .
From inside the pgbackrest folder ...
docker build -t pgha-pgbackrest .
From the main folder
We will generate containers for 2 data centers with 2 nodes each using the docker-compose file generator
./genCompose -n pg -d 2 -c 2 -v16
Usage:
./genCompose [OPTION]
-d Number of data centers to simulate. (default = 1)
-c number of nodes per data center. (default = 2)
-n Prefix name to use for db connertainer.
-v Postgres Major version number. i.e 16
-b Start patroni in background and keep container running even if it stops.
Good for upgrades and maintenance tasks.
Number of db nodes is capped at 9. So (-d * -c) should be <= 9
Number of etcd nodes are calculated on (-d * -c ) / 2 + 1
From the main folder
docker-compose create
Followed by
docker-compose start
You should now be able to access the containers
What we have installed ...
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Ubuntu 22.04.4 LTS"
PRETTY_NAME="Ubuntu 22.04.4 LTS"
NAME="Ubuntu"
VERSION_ID="22.04"
VERSION="22.04.4 LTS (Jammy Jellyfish)"
VERSION_CODENAME=jammy
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=jammy
patronictl version 3.3.0
pgBackRest 2.51
etcd Version: 3.5.13
Git SHA: c9063a0dc
Go Version: go1.21.8
Go OS/Arch: linux/amd64
in addition to the building block packages above, the following are also installed. Feel free to modify the Docker file and remove packages you don't feel you need for a lighter footprint.
&& apt-get install -y wget \
&& apt-get install -y curl \
&& apt-get install -y jq \
&& apt-get install -y vim \
&& apt-get install -y apt-utils \
&& apt-get install -y net-tools \
&& apt-get install -y iputils-ping \
&& apt-get install -y gnupg \
&& apt-get install -y openssh-server \
&& apt-get install -y less \
&& apt-get install -y python3 \
&& apt-get install -y python3-etcd \
&& apt-get install -y postgresql-common
In order for pgbackrest to work and interact with the nodes on the network we need sshd running. We need to be trusted across all the servers. Prior to building your images, generate an ssh key that you will copy into all the containers. Or, you can use the ones in this repo which reside in the main folder. They include the public, private and authorised_keys.
The ssh keys in this repo were generated inside a docker container and have no existence outside of them and have since been trashed. Feel free to use them.
sshd is started at the command line
/usr/sbin/sshd
rather than running under systemd.
The generated images listed below are what we will use.
docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
pgha-pg16-patroni latest bf7e8fa50dc4 2 hours ago 548MB
pgha-pgbackrest latest 23765655a0d5 2 hours ago 314MB
pgha-etcd-3.5 latest edab3b23c2c4 3 hours ago 344MB
The genCompose file lets you generate docker-compose.yaml files for your desired environment.
- etcd containers are created based on your choice below
- Specify the number of data centers to simulate (-d). Docker nertwork is assignbed based on dc number..
- specify the number of postgres containers per data center (-c). Default is 2.
- name your postgres containers.
- Specify the version of postgres. Used vor data directory. ( Will automate this eventually )
- Specify if the patroini containers will run in background and keep container up even if patroni is down.
When you run genCompose and specify -b, patroni will start with a nohup followed by a
tail -f /dev/null
This will allow you to make changes in patroni that require it to stop and restart manually by not stopping the container when the patroni process terminates.
It is also used for custom backup restores.
When naming a postgres container, it really means a prefix for the container name because genCompose will generate the name with a specific format.
For example, If you were to specify the name pg and 2 data centers with 2 nodes per data center, genCompose would create the following nodes.
- pg1-node1
- pg1-node2
- pg2-node1
- pg2-node2
You create a node name of "dude" you would get
dude1-node1, dude1-node2 and so on ...
the number after the name prefix is synonymous with a data center. So, pg1 would reside in data center 1.
genCompose builds 3 networks within the docker environment.
- net1
- net2
- net3
As each node gets added to the docker-compose file, priority is given to the network based on the node name.
- pg1 nodes would be prioritized to net1
- pg2 nodes would be prioritized to net2
- pg3 nodes would be prioritized to net3
Additional data centers woud loop around net1 through 3.
Feel free to modify the genCompose script and add additional networks and change the priority logic.
The script adds depends_on sections to the postgres and pgbackrest service. Postgres depends on etcd services to be running prior to starting. Postgres, (except for first node in first dc) depends on pg1 postgres to be running as well.
Keep in mind, depends_on is not the best way to specify startup order. Health checks should be used.
Additionally, genCompose assigns static port mapping to postgres containers so you can consistently access them with the same connection string. genCompose trys to identify the highest port number already mapped to 5432 in your environment and create maps higher than that.
In this example, there is one data center with 9 nodes
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
c83187335a3d 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50558->5432/tcp pgha-pg1-node8
e6ab34c49d96 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50559->5432/tcp pgha-pg1-node9
da696d20bde7 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50553->5432/tcp pgha-pg1-node3
d055362db7aa 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50552->5432/tcp pgha-pg1-node2
968ceb990f4f 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50556->5432/tcp pgha-pg1-node6
dc7fc7aa2519 pgha-pgbackrest "/entrypoint.sh" About an hour ago Up About an hour pgha-pgbackrest
a1f85f977850 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50554->5432/tcp pgha-pg1-node4
e80e26167b0c 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50557->5432/tcp pgha-pg1-node7
8596dfc62104 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50555->5432/tcp pgha-pg1-node5
b80a73f9bb4a 3bef0a1c14a4 "/entrypoint.sh" About an hour ago Up About an hour 0.0.0.0:50551->5432/tcp pgha-pg1-node1
9728a711dee1 pgha-etcd-3.5 "/usr/bin/etcd --nam…" About an hour ago Up About an hour 2380/tcp, 0.0.0.0:52635->2379/tcp pgha-etcd3
1a33819a0c11 pgha-etcd-3.5 "/usr/bin/etcd --nam…" About an hour ago Up About an hour 2380/tcp, 0.0.0.0:52637->2379/tcp pgha-etcd1
db8dcedb06a6 pgha-etcd-3.5 "/usr/bin/etcd --nam…" About an hour ago Up About an hour 2380/tcp, 0.0.0.0:52634->2379/tcp pgha-etcd2
afb30ebb7d4b pgha-etcd-3.5 "/usr/bin/etcd --nam…" About an hour ago Up About an hour 2380/tcp, 0.0.0.0:52638->2379/tcp pgha-etcd4
58f7baa8c01c pgha-etcd-3.5 "/usr/bin/etcd --nam…" About an hour ago Up About an hour 2380/tcp, 0.0.0.0:52636->2379/tcp pgha-etcd5
Some notes about the entrypoint scripts.
This script will
- create patroni.conf
- create pgbackrest.conf
- Check if it has to restore itself from backup
- Remove strict host key checking from ssh config so no answering prompts is required
If the file /pgha/config/restoreme exists, the container will try to initialize itself from a restored backup. Additional action is required and detailed below.
This script will
- create pgbackrest.conf
- Check for the presence of a required stanza and attempt to create it.
If the stanza does not exist, pgbackrest will attempt to create it 10 times with a 15 second sleep between each attempt.
You could manually create it with the following command. Remember all pgbackrest commands must be run as user postgres.
docker exec -it pgha-pgbackrest sudo -u postgres pgbackrest --stanza=${STANZA_NAME} stanza-create
There are two ways of restoring backups for these docker containers.
First things first. Make sure you have backups.
Simply create the trigger file /pgha/config/restoreme by running
/pgha/config/restoremeOnStartup
and following the process instructions generated by the restoremeOnStartup script. This will restore from the latest backup in your repo.
Here is a sample output from the command
************* READ THIS *****************
Copy these commands and shut down the replica containers before restarting pgha-pg1-node1
docker stop pgha-pg1-node2
docker stop pgha-pg1-node3
Validate all replicas are down and pg1-node1 is running as the Leader
patronictl -c /pgha/config/patroni.conf list
Before restarting pgha-pg1-node1.
docker restart pgha-pg1-node1
When pg1-node1 is back on line and running as a Leader again, restart the replicas.
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf list
Restart the replicas
docker start pgha-pg1-node2
docker start pgha-pg1-node3
If there are error and the replica does not come online, you can reinitilaze them with the following commands.
- Note:
- Depending on the size of the database, this could take time.
- Consider logging into the conatiner and checking the logs before reinitilizing.
- You must run these from inside one of the containers
curl -s http://pg1-node2:8008/reinitialize -XPOST -d '{"force":"true"}'
curl -s http://pg1-node3:8008/reinitialize -XPOST -d '{"force":"true"}'
Or you can use the patronictl commands below.
patronictl -c /pgha/config/patroni.conf reinit pgha_cluster pg1-node2 --force
patronictl -c /pgha/config/patroni.conf reinit pgha_cluster pg1-node3 --force
If you change your mind, remove the trigger file /pgha/config/restoreme *****
The other way to restore is using custom pgbackrest restore commands. This requires that the container stays running even if patroni is shut down. You can accomplish this by passing passing the option -b when you generate your docker-compose file using genCompose.
The -b will create the compose-file and indicate that when patroni starts, it is done so using nohup and then followed by a
tail -f /dev/null
which keeps the container up even when patroni is not running.
- make sure docker-compose was generated with -b option
- as root inside the Leader node validate you have backups with the following command:
sudo -u postgres pgbackrest --config=${CFG_DIR}/pgbackrest.conf --stanza=${STANZA_NAME} info- Bring down all replica containers
docker stop pgha-pg1-node2docker stop pgha-pg1-node3- Validate the primary is the only service running
patronictl -c /pgha/config/patroni.conf list
+ Cluster: pgha_cluster (7366718977191354411) --+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+--------+---------+----+-----------+
| pg1-node1 | pg1-node1 | Leader | running | 1 | |
+-----------+-----------+--------+---------+----+-----------+
- log into the primary container
- look for the patroni process
root@pg1-node1:/# ps -ef | grep patroni
root 13 1 0 19:51 ? 00:00:00 sudo -u postgres nohup /usr/bin/patroni /pgha/config/patroni.conf
postgres 15 13 0 19:51 ? 00:00:00 /usr/bin/python3 /usr/bin/patroni /pgha/config/patroni.conf
root 331 96 0 20:04 pts/0 00:00:00 grep --color=auto patroni
- kill the pid running the sudo command
kill 13- Make sure patroni is not running anymore with another ps command
- Empty out the data directory
rm -rf /pgdata/16/*- Run you custom pgbackrest restore command:
- For this example, We are just restoring the latest backup. However, you can get specific with sets, times, db's etc ..
sudo -u postgres pgbackrest --config=${CFG_DIR}/pgbackrest.conf --log-path=${CFG_DIR} --stanza=${STANZA_NAME} --pg1-path=${DATADIR} --log-level-console=info --log-level-file=detail restore- Sample output
sudo -u postgres pgbackrest --config=/pgha/config/pgbackrest.conf --log-path=/pgha/config --stanza=pgha_db --pg1-path=/pgdata/16 --log-level-console=info --log-level-file=detail restore
root@pg1-node1:/pgdata/16# sudo -u postgres pgbackrest --config=${CFG_DIR}/pgbackrest.conf --log-path=${CFG_DIR} --stanza=${STANZA_NAME} --pg1-path=${DATADIR} --log-level-console=info --log-level-file=detail restore
2024-05-08 20:07:36.398 P00 INFO: restore command begin 2.51: --config=/pgha/config/pgbackrest.conf --exec-id=347-3f5dbef2 --log-level-console=info --log-level-file=detail --log-path=/pgha/config --pg1-path=/pgdata/16 --process-max=16 --repo1-host=pgbackrest --repo1-host-user=postgres --stanza=pgha_db
2024-05-08 20:07:36.652 P00 INFO: repo1: restore backup set 20240508-195548F, recovery will start at 2024-05-08 19:55:48
2024-05-08 20:07:52.204 P00 INFO: write updated /pgdata/16/postgresql.auto.conf
2024-05-08 20:07:52.257 P00 INFO: restore global/pg_control (performed last to ensure aborted restores cannot be started)
2024-05-08 20:07:52.259 P00 INFO: restore size = 202.4MB, file total = 1274
2024-05-08 20:07:52.259 P00 INFO: restore command end: completed successfully (15862ms)
- For fresh logs, remove the log files from the log directory
rm /pgdata/16/log/*- Restart the container
- Validate the node is running as a Leader
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf list
+ Cluster: pgha_cluster (7366718977191354411) --+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+--------+---------+----+-----------+
| pg1-node1 | pg1-node1 | Leader | running | 2 | |
+-----------+-----------+--------+---------+----+-----------+
- If yes,
- Restart the replicas
docker restart pgha-pg1-node2- Validate it comes up and resyncs
Depending on your db size and backup, this could take some time.
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf list
+ Cluster: pgha_cluster (7366718977191354411) +----+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+---------+-----------+----+-----------+
| pg1-node1 | pg1-node1 | Leader | running | 2 | |
| pg1-node2 | pg1-node2 | Replica | streaming | 2 | 0 |
+-----------+-----------+---------+-----------+----+-----------+
- Repeat replica restarts for remaining replicas
docker restart pgha-pg1-node3docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf list- Be patient
+ Cluster: pgha_cluster (7366718977191354411) +----+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+---------+-----------+----+-----------+
| pg1-node1 | pg1-node1 | Leader | running | 2 | |
| pg1-node2 | pg1-node2 | Replica | streaming | 2 | 0 |
| pg1-node3 | pg1-node3 | Replica | stopped | | unknown |
+-----------+-----------+---------+-----------+----+-----------+
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf list
+ Cluster: pgha_cluster (7366718977191354411) +----+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+---------+-----------+----+-----------+
| pg1-node1 | pg1-node1 | Leader | running | 2 | |
| pg1-node2 | pg1-node2 | Replica | streaming | 2 | 0 |
| pg1-node3 | pg1-node3 | Replica | streaming | 2 | 0 |
+-----------+-----------+---------+-----------+----+-----------+
docker exec -it pgha-pgbackrest sudo -u postgres pgbackrest --stanza=${STANZA_NAME} --type=full backup
.
.
.
2024-05-04 01:32:39.073 P00 INFO: check archive for segment(s) 000000050000000000000013:000000050000000000000013
2024-05-04 01:32:39.283 P00 INFO: new backup label = 20240504-013221F
2024-05-04 01:32:39.320 P00 INFO: full backup size = 29.5MB, file total = 1275
2024-05-04 01:32:39.320 P00 INFO: backup command end: completed successfully (24286ms)
2024-05-04 01:32:39.320 P00 INFO: expire command begin 2.51: --exec-id=3240-c9b97e1e --log-level-console=detail --log-level-file=detail --repo1-path=/pgha/pgbackrest --repo1-retention-archive-type=full --repo1-retention-full=2 --stanza=pgha_db
2024-05-04 01:32:39.326 P00 DETAIL: repo1: 16-1 archive retention on backup 20240503-231613F, start = 000000010000000000000006
2024-05-04 01:32:39.329 P00 INFO: repo1: 16-1 remove archive, start = 000000010000000000000001, stop = 000000010000000000000005
2024-05-04 01:32:40.131 P00 INFO: expire command end: completed successfully (811ms)
docker exec -it pgha-pgbackrest sudo -u postgres pgbackrest --stanza=${STANZA_NAME} info
stanza: pgha_db
status: ok
cipher: none
db (current)
wal archive min/max (16): 000000010000000000000006/000000050000000000000013
full backup: 20240503-231613F
timestamp start/stop: 2024-05-03 23:16:13+00 / 2024-05-03 23:16:18+00
wal start/stop: 000000010000000000000006 / 000000010000000000000006
database size: 29.4MB, database backup size: 29.4MB
repo1: backup set size: 3.9MB, backup size: 3.9MB
full backup: 20240504-013221F
timestamp start/stop: 2024-05-04 01:32:21+00 / 2024-05-04 01:32:38+00
wal start/stop: 000000050000000000000013 / 000000050000000000000013
database size: 29.5MB, database backup size: 29.5MB
repo1: backup set size: 3.9MB, backup size: 3.9MB
The patroni config file is in /pgha/config/patroni.conf
Pick any pg container and run
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf list
+ Cluster: pgha_cluster (7364915463012110378) +----+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+---------+-----------+----+-----------+
| pg1-node1 | pg1-node1 | Leader | running | 3 | |
| pg1-node2 | pg1-node2 | Replica | streaming | 3 | 0 |
| pg1-node3 | pg1-node3 | Replica | streaming | 3 | 0 |
| pg1-node4 | pg1-node4 | Replica | streaming | 3 | 0 |
| pg1-node5 | pg1-node5 | Replica | streaming | 3 | 0 |
| pg1-node6 | pg1-node6 | Replica | streaming | 3 | 0 |
| pg1-node7 | pg1-node7 | Replica | streaming | 3 | 0 |
| pg1-node8 | pg1-node8 | Replica | streaming | 3 | 0 |
| pg1-node9 | pg1-node9 | Replica | streaming | 3 | 0 |
+-----------+-----------+---------+-----------+----+-----------+
Pick any running pg container and run ...
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf switchover --leader=pg1-node1 --candidate=pg1-node4 --force
+ Cluster: pgha_cluster (7364915463012110378) +----+-----------+
| Member | Host | Role | State | TL | Lag in MB |
+-----------+-----------+---------+-----------+----+-----------+
| pg1-node1 | pg1-node1 | Replica | streaming | 5 | 0 |
| pg1-node2 | pg1-node2 | Replica | streaming | 5 | 0 |
| pg1-node3 | pg1-node3 | Replica | streaming | 5 | 0 |
| pg1-node4 | pg1-node4 | Leader | running | 5 | |
| pg1-node5 | pg1-node5 | Replica | streaming | 5 | 0 |
| pg1-node6 | pg1-node6 | Replica | streaming | 5 | 0 |
| pg1-node7 | pg1-node7 | Replica | streaming | 5 | 0 |
| pg1-node8 | pg1-node8 | Replica | streaming | 5 | 0 |
| pg1-node9 | pg1-node9 | Replica | streaming | 5 | 0 |
+-----------+-----------+---------+-----------+----+-----------+
Pick any running pg container and run ...
docker exec -it pgha-pg1-node1 patronictl -c /pgha/config/patroni.conf show-config
loop_wait: 10
maximum_lag_on_failover: 1048576
postgresql:
parameters:
archive_command: pgbackrest --config=/pgha/config/pgbackrest.conf --stanza=pgha_db archive-push "/pgdata/16/pg_wal/%f"
archive_mode: true
archive_timeout: 1800s
hot_standby: true
log_filename: postgresql-%Y-%m-%d-%a.log
log_line_prefix: '%m [%r] [%p]: [%l-1] user=%u,db=%d,host=%h '
log_lock_waits: 'on'
log_min_duration_statement: 1000
logging_collector: 'on'
max_replication_slots: 10
max_wal_senders: 10
max_wal_size: 1GB
wal_keep_size: 4096
wal_level: logical
wal_log_hints: true
recovery_conf:
recovery_target_timeline: latest
restore_command: pgbackrest --config=/pgha/config/pgbackrest.conf --stanza=pgha_db archive-get %f %p
use_pg_rewind: true
use_slots: true
retry_timeout: 10
ttl: 30
As mentioned earlier, the etcd containers are created based on the number of nodes in your cluster.
We are using a newer version of etcd which means some changes to commands. To make things a little easier to do, the docker-compose file creates an ENDPOINTS environment variable inside the container so you don't have to create one every time you want to check etcd.
For example check the status like this using the ENDPOINTS env
Log onto any etcd container
docker exec -it pgha-etcd2 /bin/bash
and run
etcdctl --write-out=table --endpoints=$ENDPOINTS endpoint status
+------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
| ENDPOINT | ID | VERSION | DB SIZE | IS LEADER | IS LEARNER | RAFT TERM | RAFT INDEX | RAFT APPLIED INDEX | ERRORS |
+------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
| etcd1:2380 | 3881244e074acb1d | 3.5.13 | 82 kB | false | false | 2 | 193 | 193 | |
| etcd2:2380 | 328bbe88afec63c5 | 3.5.13 | 82 kB | false | false | 2 | 193 | 193 | |
| etcd3:2380 | db322f4bdb3697fb | 3.5.13 | 82 kB | true | false | 2 | 193 | 193 | |
| etcd4:2380 | f61ed83a70a5bdbc | 3.5.13 | 82 kB | false | false | 2 | 193 | 193 | |
| etcd5:2380 | c0d9e8e50aa266f2 | 3.5.13 | 82 kB | false | false | 2 | 193 | 193 | |
+------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+
Rather than using a third party tool such as haproxy to manage and load balance our connections to the database, we can use Postgreges libpq to achieve similar results without the overhead. By customizing our connection string we can do the following.
Connect to a primary (r/w) node from the list of hosts specified in the string. In our docker environment, we are running this command from inside one of the containers.
psql 'host=pg1-node1,pg1-node2,pg1-node3,pg1-node4,pg1-node5,pg1-node6,pg1-node7,pg1-node8,pg1-node9 user=postgres password=postgres target_session_attrs=primary'
psql 'host=pg1-node1,pg1-node2,pg1-node3,pg1-node4,pg1-node5,pg1-node6,pg1-node7,pg1-node8,pg1-node9 user=postgres password=postgres target_session_attrs=primary'
psql (16.2 (Ubuntu 16.2-1.pgdg22.04+1))
Type "help" for help.
postgres=# select pg_is_in_recovery();
pg_is_in_recovery
-------------------
f
(1 row)
To perform a similar connection from outside the container, we specify localhosts and the mapped port for the container. In the below example, we also set the option of load_balance_hosts=random which will pick any of the hosts specified at random and specify target_session_attrs=standby which will connect to a standby / replica.
psql 'host=localhost,localhost,localhost,localhost,localhost,localhost,localhost,localhost,localhost port=50551,50552,50553,50554,50555,50556,50557,50558,50559 user=postgres password=postgres load_balance_hosts=random target_session_attrs=standby'
psql (16.2 (Ubuntu 16.2-1.pgdg22.04+1))
Type "help" for help.
postgres=# select pg_is_in_recovery();
pg_is_in_recovery
-------------------
t
(1 row)
Additionally, you can select a host from the connection list of hosts at random with the following option in your connection string.
load_balance_hosts=random
In future versions, a weight option is expected.
The following are the target session attribute options currently available in Postgres 16. For the online documentation, visit https://www.postgresql.org/docs/current/libpq-connect.html
This option determines whether the session must have certain properties to be acceptable. It's typically used in combination with multiple host names to select the first acceptable alternative among several hosts. There are six modes:
any successful connection is acceptable
session must accept read-write transactions by default (that is, the server must not be in hot standby mode and the default_transaction_read_only parameter must be off)
session must not accept read-write transactions by default (the converse)
server must not be in hot standby mode
server must be in hot standby mode
first try to find a standby server, but if none of the listed hosts is a standby server, try again in any mode
More updates to follow ...