Rui TrigoI often get asked if it's possible to build a resilient system with PostgreSQL.
Considering that resilience should feature cluster high-availability, fault tolerance, and self-healing, it's not an easy answer. But there is a lot to be said about this.
As of today, we can't achieve that level of resilience with the same ease as MongoDB built-in features. But let's see what we can in fact do with the help of repmgr and some other tooling.
At the end of this exercise, we will have achieved some things that come in handy, such as:
- a few Ansible roles that can be reused for production
- a Vagrantfile for single-command cluster deployment
- a development environment that’s more realistic; being close to production state is good to foresee "production-exclusive issues"
Objectives
- build a local development environment PostgreSQL cluster with fault tolerance capabilities;
- develop configuration management code to reuse in production.
Pre-requisites
Install Vagrant, VirtualBox and Ansible
sudo apt install vagrant
sudo apt install virtualbox && sudo apt install virtualbox-dkms
sudo apt install ansible
Note: An alternative to installing Ansible on your host machine would be using the ansible-local Vagrant provider, which needs Ansible installed on the generated virtual machine instead.
Configuration
1. Write a Vagrantfile
You can use vagrant init to generate the file or simply create it and insert our first blocks.
Vagrant.configure("2") do |config|
(1..3).each do |n|
config.vm.define "node#{n}" do |define|
define.ssh.insert_key = false
define.vm.box = "ubuntu/bionic64"
define.vm.hostname = "node#{n}"
define.vm.network :private_network, ip: "172.16.1.1#{n}"
define.vm.provider :virtualbox do |v|
v.cpus = 2
v.memory = 1024
v.name = "node#{n}"
end
end
end
end
Let's go block by block:
- the 1st block is where we set up the Vagrant version;
- on the 2nd block, we iterate the following code so we reuse it to generate 3 equal VMs;
- OS, hostname and network settings are set in the 3rd block;
- the 4th block contains VirtualBox specific settings.
You can create the servers with:
# create all 3 VMs
vagrant up
# or create only a specific VM
vagrant up node1
2. Add a provisioner
Just by doing the first step alone, we can already launch 3 working virtual machines. A little exciting, but the best is yet to come.
Launching virtual machines is a nice feature of Vagrant, but we want these servers to have PostgreSQL and repmgr configured, so we will use configuration management software to help us. This is the moment Ansible walks in to amaze us.
Vagrant supports several providers, two of them being Ansible and Ansible Local. The difference between them is where Ansible runs, or in other words, where it must be installed. By Vagrant terms, the Ansible provider runs on a host machine (your computer) and the Ansible Local provider runs on guest machines (virtual machines). As we already installed Ansible in the prerequisites section, we'll go with the first option.
Let's add a block for this provisioner in our Vagrantfile.
Vagrant.configure("2") do |config|
(1..3).each do |n|
config.vm.define "node#{n}" do |define|
define.ssh.insert_key = false
define.vm.box = "ubuntu/bionic64"
define.vm.hostname = "node#{n}"
define.vm.network :private_network, ip: "172.16.1.1#{n}"
define.vm.provider :virtualbox do |v|
v.cpus = 2
v.memory = 1024
v.name = "node#{n}"
end
if n == 3
define.vm.provision :ansible do |ansible|
ansible.limit = "all"
ansible.playbook = "provisioning/playbook.yaml"
ansible.host_vars = {
"node1" => {:connection_host => "172.16.1.11",
:node_id => 1,
:role => "primary" },
"node2" => {:connection_host => "172.16.1.12",
:node_id => 2,
:role => "standby" },
"node3" => {:connection_host => "172.16.1.13",
:node_id => 3,
:role => "witness" }
}
end
end
end
end
end
Ansible allows us to configure several servers simultaneously. To take advantage of this feature on Vagrant, we add ansible.limit = "all" and must wait until all 3 VMs are up. Vagrant knows they are all created because of the condition if n == 3, which makes Ansible only run after Vagrant iterated 3 times.
ansible.playbook is the configuration entry point and ansible.host_vars contains the Ansible host variables to be used on the tasks and templates we are about to create.
3. Create an organized Ansible folder structure
If you're already familiar with Ansible, there's little to learn in this section. For those who aren't, it doesn't get too complicated.
First, we have a folder for all Ansible files, named provisioning. Inside this folder, we have our aforementioned entry point playbook.yaml, a group_vars folder for Ansible group variables, and a roles folder.
We could have all Ansible tasks within playbook.yaml, but role folder structure helps with organization. You can read the Ansible documentation to learn the best practices. Below, you will find the folder structure for this tutorial.
project_root
| provisioning
| | group_vars
| | | all.yaml
| | roles
| | | postgres_12
| | | registration
| | | repmgr
| | | ssh
| | playbook.yaml
| Vagrantfile
4. Ansible roles
4.1 PostgreSQL role
To configure repmgr on PostgreSQL, we need to edit two well-known PostgreSQL configuration files: postgresql.conf and pg_hba.conf. We will then write our tasks to apply the configurations on tasks/main.yaml. I named the PostgreSQL role folder as postgres_12 but you can easily use another version if you want to.
postgres_12
| tasks
| | main.yaml
| templates
| | pg_hba.conf.j2
| | pg_hba.conf.j2
You can reuse the default file which comes with PostgreSQL installation and add the following lines to whitelist repmgr database sessions from your trusted VMs. Create an Ansible template file (Jinja2 format) like so:
# default configuration (...)
# repmgr
local replication repmgr trust
host replication repmgr 127.0.0.1/32 trust
host replication repmgr {{ node1_ip }}/32 trust
host replication repmgr {{ node2_ip }}/32 trust
host replication repmgr {{ node3_ip }}/32 trust
local repmgr repmgr trust
host repmgr repmgr 127.0.0.1/32 trust
host repmgr repmgr {{ node1_ip }}/32 trust
host repmgr repmgr {{ node2_ip }}/32 trust
host repmgr repmgr {{ node3_ip }}/32 trust
In the same fashion as pg_hba.conf, you can reuse the postgresql.conf default file and add a few more replication related settings to the bottom of the file:
# default configuration (...)
# repmgr
listen_addresses = '*'
shared_preload_libraries = 'repmgr'
wal_level = replica
max_wal_senders = 5
wal_keep_segments = 64
max_replication_slots = 5
hot_standby = on
wal_log_hints = on
The tasks below will install PostgreSQL and apply our configurations. Their names are self-explanatory.
- name: Add PostgreSQL apt key
apt_key:
url: https://www.postgresql.org/media/keys/ACCC4CF8.asc
- name: Add PostgreSQL repository
apt_repository:
# ansible_distribution_release = xenial, bionic, focal
repo: deb http://apt.postgresql.org/pub/repos/apt/ {{ ansible_distribution_release }}-pgdg main
- name: Install PostgreSQL 12
apt:
name: postgresql-12
update_cache: yes
- name: Copy database configuration
template:
src: full_postgresql.conf.j2
dest: /etc/postgresql/12/main/postgresql.conf
group: postgres
mode: '0644'
owner: postgres
- name: Copy user access configuration
template:
src: pg_hba.conf.j2
dest: /etc/postgresql/12/main/pg_hba.conf
group: postgres
mode: '0640'
owner: postgres
4.2 SSH server configuration
ssh
| files
| | keys
| | | id_rsa
| | | id_rsa.pub
| tasks
| | main.yaml
Generate a key pair to use throughout our virtual machines to allow access to them. If you don't know how to do it, this link can help. Just make sure the keys file paths match the paths in the next step.
The tasks below will install the OpenSSH server and apply our configurations. Their names are self-explanatory.
- name: Install OpenSSH
apt:
name: openssh-server
update_cache: yes
state: present
- name: Create postgres SSH directory
file:
mode: '0755'
owner: postgres
group: postgres
path: /var/lib/postgresql/.ssh/
state: directory
- name: Copy SSH private key
copy:
src: "keys/id_rsa"
dest: /var/lib/postgresql/.ssh/id_rsa
owner: postgres
group: postgres
mode: '0600'
- name: Copy SSH public key
copy:
src: "keys/id_rsa.pub"
dest: /var/lib/postgresql/.ssh/id_rsa.pub
owner: postgres
group: postgres
mode: '0644'
- name: Add key to authorized keys file
authorized_key:
user: postgres
state: present
key: "{{ lookup('file', 'keys/id_rsa.pub') }}"
- name: Restart SSH service
service:
name: sshd
enabled: yes
state: restarted
4.3 repmgr installation
repmgr
| tasks
| | main.yaml
| templates
| | repmgr.conf.j2
We configure settings like promote command, follow command, timeouts and retry count on failure scenarios inside repmgr.conf. We will copy this file to its default directory /etc to avoid passing the -f argument on the repmgr command all the time.
The tasks below will install repmgr and apply our configurations. Their names are self-explanatory.
- name: Download repmgr repository installer
get_url:
dest: /tmp/repmgr-installer.sh
mode: 0700
url: https://dl.2ndquadrant.com/default/release/get/deb
- name: Execute repmgr repository installer
shell: /tmp/repmgr-installer.sh
- name: Install repmgr for PostgreSQL {{ pg_version }}
apt:
name: postgresql-{{ pg_version }}-repmgr
update_cache: yes
- name: Setup repmgr user and database
become_user: postgres
ignore_errors: yes
shell: |
createuser --replication --createdb --createrole --superuser repmgr &&
psql -c 'ALTER USER repmgr SET search_path TO repmgr_test, "$user", public;' &&
createdb repmgr --owner=repmgr
- name: Copy repmgr configuration
template:
src: repmgr.conf.j2
dest: /etc/repmgr.conf
- name: Restart PostgreSQL
systemd:
name: postgresql
enabled: yes
state: restarted
4.4 repmgr node registration
Finally, we reach the moment where fault tolerance is established.
registration
| tasks
| | main.yaml
node_id = {{ node_id }}
node_name = 'node{{ node_id }}'
conninfo = 'host={{ connection_host }} user=repmgr dbname=repmgr'
data_directory = '/var/lib/postgresql/{{ pg_version }}/main'
use_replication_slots = yes
reconnect_attempts = 5
reconnect_interval = 1
failover = automatic
pg_bindir = '/usr/lib/postgresql/{{ pg_version }}/bin'
promote_command = 'repmgr standby promote -f /etc/repmgr.conf'
follow_command = 'repmgr standby follow -f /etc/repmgr.conf'
log_level = INFO
log_file = '/var/log/postgresql/repmgr.log'
This role was built according to the repmgr documentation and it might be the most complex role, as it needs to:
- run some commands as root and others as Postgres;
- stop services between reconfigurations;
- have different tasks for primary, standby, and support witness role configuration (in case you want node3 to also be a standby node, just assign
role: standbyin Vagrantfileansible.host_vars)
- name: Register primary node
become_user: postgres
shell: repmgr primary register
ignore_errors: yes
when: role == "primary"
- name: Stop PostgreSQL
systemd:
name: postgresql
state: stopped
when: role == "standby"
- name: Clean up PostgreSQL data directory
become_user: postgres
file:
path: /var/lib/postgresql/{{ pg_version }}/main
force: yes
state: absent
when: role == "standby"
- name: Clone primary node data
become_user: postgres
shell: repmgr -h {{ node1_ip }} -U repmgr -d repmgr standby clone
ignore_errors: yes
when: role == "standby"
- name: Start PostgreSQL
systemd:
name: postgresql
state: started
when: role == "standby"
- name: Register {{ role }} node
become_user: postgres
shell: repmgr {{ role }} register -F
ignore_errors: yes
when: role != "primary"
- name: Start repmgrd
become_user: postgres
shell: repmgrd
ignore_errors: yes
5. Set group variables
Create a file group_vars/all.yaml to set your VMs IP addresses and the PostgreSQL version you would like to use. Like host_vars set on Vagrantfile, these variables will be placed in the templates placeholders.
client_ip: "172.16.1.1"
node1_ip: "172.16.1.11"
node2_ip: "172.16.1.12"
node3_ip: "172.16.1.13"
pg_version: "12"
6. Put all pieces together with a playbook
The only thing missing is the playbook itself. Create a file named playbook.yaml and invoke the roles we have been developing. gather_facts is an Ansible property to fetch operative system data like distribution (ansible_distribution_release) among other useful variables. You can also read these variables with the Ansible setup module.
- hosts: all
gather_facts: yes
become: yes
roles:
- postgres_12
- ssh
- repmgr
- registration
7. Start cluster
It's finished. You can now start your cluster with vagrant up and then perform your connections and failover tests.
Testing cluster failover
Now that our cluster is up and configured, you can start by shutting down your standby node:
# save standby state and shut it down ungracefully
vagrant suspend node2
You will see that the cluster is operating normally. Bring the standby node back and it will stay that way.
# bring standby back online after suspension
vagrant resume node1
How about taking down the primary node?
# save primary state and shut it down ungracefully
vagrant suspend node1
At this point, as repmgrd is enabled, the standby node will retry connecting to the primary node the configured number of times (reconnect_attempts = 5) and, if it obtains no response, will promote itself to primary and take over write operations on the PostgreSQL cluster. Success!
To join the cluster again, the old primary node will have to lose its current data, clone the new primary data, and register as a new standby.
vagrant resume node1
vagrant ssh node1
service postgresql stop
rm -r /var/lib/postgresql/12/main
repmgr -h 172.16.1.12 -U -d repmgr standby clone
service postgresql start
repmgr standby register -F
repmgrd
repmgr service status
This last command shows us that the cluster is working properly, but with inverted roles.
postgres@node1:~$ repmgr service status
ID | Name | Role | Status | Upstream | repmgrd | PID | Paused? | Upstream last seen
----+-------+---------+-----------+----------+---------+-------+---------+--------------------
1 | node1 | standby | running | node2 | running | 22490 | no | n/a
2 | node2 | primary | * running | | running | 22548 | no | 0 second(s) ago
3 | node3 | witness | * running | node2 | running | 22535 | no | 0 second(s) ago
Nothing wrong with this, but let's make these nodes switch their roles.
# ssh in and out just to add host key to known_hosts file
ssh <current_primary_ip_address> -o StrictHostKeyChecking=no
exit
# trigger switchover on current standby
repmgr standby switchover --siblings-follow
And we're back to the initial state.
postgres@node1:~$ repmgr service status
ID | Name | Role | Status | Upstream | repmgrd | PID | Paused? | Upstream last seen
----+-------+---------+-----------+----------+---------+-------+---------+--------------------
1 | node1 | primary | * running | | running | 22490 | no | n/a
2 | node2 | standby | running | node1 | running | 22548 | no | 0 second(s) ago
3 | node3 | witness | * running | node1 | running | 22535 | no | 0 second(s) ago
Conclusion
We managed to build a fault-tolerant PostgreSQL cluster using Vagrant and Ansible.
High availability is a big challenge. Much like life’s own matters, we are only prepared for the biggest challenges when we fit that challenges’ conditions.
Production environment unique problems are natural and tough to guess. Bridging the gap between development and production is a way to prevent deployment/production issues. We can make some efforts toward that objective, and that is precisely what we just achieved with this high availability database setup.
You can find the source code of this tutorial here.
