PostgreSQL Automatic Failover

High-Availibility for Postgres, based on Pacemaker and Corosync.

Quick Start CentOS 8

This quick start purpose is to help you to build your first cluster to experiment with. It does not implement various good practices related to your system, Pacemaker or PostgreSQL. This quick start alone is not enough. During your journey in building a safe HA cluster, you must train about security, network, PostgreSQL, Pacemaker, PAF, etc. In regard with PAF, make sure to read carefully documentation from https://clusterlabs.github.io/PAF/documentation.html.

This tutorial is based on CentOS 8.1, using Pacemaker 2.0.1, the pcs command, PostgreSQL 12 and PAF 2.3.

WARNING: Do not test with Pacemaker 2.1.0.

Pacemaker 2.1.0 introduced some regressions that break the PAF resource agent. This has been fixed in Pacemaker 2.1.1.

Table of contents:

Repository setup

To install PAF and PostgreSQL, this tutorial uses the PGDG repository maintained by the PostgreSQL community. Here is how to add it:

dnf install -y https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm

We have to disable module stream for PostgreSQL to avoid conflicts with PGDG repository and enable the official CentOS “HighAvailability” repository:

dnf module disable -y postgresql
dnf config-manager --enable ha

NOTE: you can install PostgreSQL from official CentOS repository if you prefer.

Network setup

The cluster we are about to build includes three servers called srv1, srv2 and srv3. IP addresses of these servers are 192.168.122.4x/24.

NOTE: It is essential to setup network redundancy, either at system level using eg. bonding or teaming, or at cluster level.

The IP address 192.168.122.40, called pgsql-vip in this tutorial, will be set on the server hosting the primary PostgreSQL instance.

If the firewall is enabled, we have to allow the network traffic related to the cluster and PostgreSQL to go through:

firewall-cmd --permanent --add-service=high-availability
firewall-cmd --permanent --add-service=postgresql
firewall-cmd --reload

During the cluster setup, we use the node names in various places, make sure all your server hostnames can be resolved to the correct IPs. We usually set this in the /etc/hosts file:

192.168.122.40 pgsql-vip
192.168.122.41 srv1
192.168.122.42 srv2
192.168.122.43 srv3

Now, the three servers should be able to ping each others, eg.:

root@srv1:~# for s in srv1 srv2 srv3; do ping -W1 -c1 $s; done| grep icmp_seq
64 bytes from srv1 (192.168.122.41): icmp_seq=1 ttl=64 time=0.028 ms
64 bytes from srv2 (192.168.122.42): icmp_seq=1 ttl=64 time=0.296 ms
64 bytes from srv3 (192.168.122.43): icmp_seq=1 ttl=64 time=0.351 ms

Make sure hostnames are correctly set on each nodes, or use hostnamectl. Eg.:

hostnamectl set-hostname srv1

PostgreSQL and Cluster stack installation

Run this whole chapter on ALL nodes.

Let’s install everything we need: PostgreSQL, Pacemaker, cluster related packages and PAF:

dnf install -y postgresql12 postgresql12-contrib postgresql12-server \
               pacemaker resource-agents resource-agents-paf pcs     \
               fence-agents-all fence-agents-virsh

PostgreSQL setup

WARNING: building PostgreSQL standby is not the main subject here. The following steps are **quick and dirty, VERY DIRTY**. They lack of security, WAL retention and so on. Rely on the PostgreSQL documentation for a proper setup.

The resource agent requires the PostgreSQL instances to be already set up, ready to start and standbys ready to replicate. Make sure to setup your primary on your preferred node to host it: during the very first startup of the cluster, PAF detects the primary based on its shutdown status.

Moreover, on all nodes, it requires a primary_conninfo with an application_name set to the node name. This is mandatory. Note that it requires recovery_target_timeline = latest as well, but this is already the default with PostgreSQL 12.

Last but not least, make sure each instance is not able to replicate with itself! A scenario exists where the primary IP address pgsql-vip will be on the same node than a standby for a very short lap of time!

WARNING: primary_conninfo and pg_hba.conf are different on each node. It is best to keep them out of the $PGDATA so you do not have to deal with them (or worst: forget to edit them) each time you rebuild a standby! We advice you use hba_file and include or include_dir parameters in your postgresql.conf file to load your specific setup from outside of the $PGDATA.

Here are some quick steps to build your primary PostgreSQL instance and its standbys. This quick start considers srv1 is the preferred primary node.

On the primary:

/usr/pgsql-12/bin/postgresql-12-setup initdb

su - postgres

cat <<EOF >> ~postgres/12/data/postgresql.conf
listen_addresses = '*'
wal_keep_segments = 32
hba_file = '/var/lib/pgsql/12/pg_hba.conf'
include = '../standby.conf'
EOF

cat <<EOF > ~postgres/12/standby.conf
primary_conninfo = 'host=192.168.122.40 application_name=$(hostname -s)'
EOF

rm ~postgres/12/data/pg_hba.conf

cat <<EOF > ~postgres/12/pg_hba.conf
local all         all                     trust
host  all         all      0.0.0.0/0      trust
host  all         all      ::/0           trust

# forbid self-replication from vIP
host  replication postgres 192.168.122.40/32  reject
# forbid self-replication its own IP
local replication all                     reject
host  replication all      $(hostname -s) reject
host  replication all      127.0.0.0/8    reject
host  replication all      ::1/128        reject

# allow any standby connection
host  replication postgres 0.0.0.0/0      trust
EOF

exit

systemctl start postgresql-12
ip addr add 192.168.122.40/24 dev eth1

Now, on other nodes, clone the primary as standby and configure it:

su - postgres

pg_basebackup -h pgsql-vip -D ~postgres/12/data/ -X stream -P

cat <<EOF > ~postgres/12/standby.conf
primary_conninfo = 'host=192.168.122.40 application_name=$(hostname -s)'
EOF

cat <<EOF > ~postgres/12/pg_hba.conf
local all         all                     trust
host  all         all      0.0.0.0/0      trust
host  all         all      ::/0           trust

# forbid self-replication from vIP
host  replication postgres 192.168.122.40/32  reject
# forbid self-replication its own IP
local replication all                     reject
host  replication all      $(hostname -s) reject
host  replication all      127.0.0.0/8    reject
host  replication all      ::1/128        reject

# allow any standby connection
host  replication postgres 0.0.0.0/0      trust
EOF

touch ~postgres/12/data/standby.signal

exit

systemctl start postgresql-12

Check your three instances are replicating as expected (in processes, logs, pg_stat_replication, etc).

Finally, make sure to stop the PostgreSQL services everywhere and to disable them, as Pacemaker will take care of starting/stopping everything for you during cluster normal cluster operations. Start with the primary:

systemctl disable --now postgresql-12

And remove the vIP from srv1:

ip addr del 192.168.122.40/24 dev eth1

Cluster pre-requisites

This guide uses the cluster management tool pcsd to ease the creation and setup of a cluster. It allows to create the cluster from command line, without editing configuration files or XML by hands.

pcsd uses the hacluster system user to work and communicate with other members of the cluster. We need to set a password to this user so it can authenticate to other nodes easily. As cluster management commands can be run on any member of the cluster, it is recommended to set the same password everywhere to avoid confusions:

passwd hacluster

Make sure the pcsd daemon is enabled and started on all nodes:

systemctl enable --now pcsd

Now, authenticate each node to the other ones using the following command, on each nodes. The command ask for the hacluster password:

pcs host auth -u hacluster srv1 srv2 srv3

Cluster creation

The pcs cli tool is able to create and start the whole cluster for us. From one of the nodes, run the following command:

pcs cluster setup cluster_pgsql srv1 srv2 srv3

NOTE: Make sure you have a redundant network at system level. This is a **CRITICAL** part of your cluster. If you have second interfaces not in bonding or teaming already (prefered method), you can add them to the cluster setup using eg.:

pcs cluster setup cluster_pgsql srv1,srv1-alt srv2,srv2-alt srv3,srv3-alt

This command creates the /etc/corosync/corosync.conf file and propagate it everywhere. For more information about it, read the corosync.conf(5) manual page.

WARNING: whatever you edit in your /etc/corosync/corosync.conf file, **ALWAYS** make sure all the nodes in your cluster has the exact same copy of the file. You can use pcs cluster sync.

It is advised to keep Pacemaker off on server boot. It helps the administrator to investigate after a node fencing before Pacemaker starts and potentially enters in a death match with the other nodes. Make sure to disable Corosync as well to avoid unexpected behaviors. Run this on all nodes:

pcs cluster disable --all

You can now start the whole cluster from one node:

pcs cluster start --all

After some seconds of startup and cluster membership stuffs, you should be able to see your three nodes up in crm_mon (or pcs status):

root@srv1:~# crm_mon -n1D

Node srv1: online
Node srv2: online
Node srv3: online

Now the cluster run, let’s start with some basic setup of the cluster. Run the following command from one node only (the cluster takes care of broadcasting the configuration on all nodes):

pcs resource defaults migration-threshold=5
pcs resource defaults resource-stickiness=10

This sets two default values for resources we create in the next chapter:

Node fencing

The most important resource in your cluster is the one able to fence a node. Please, stop reading this quick start and read our fencing documentation page before building your cluster. Take a deep breath, and read: http://clusterlabs.github.com/PAF/fencing.html.

WARNING: I really mean it. You need fencing. PAF is expecting fencing to work in your cluster. Without fencing, you will experience cluster refusing to move anything, even with stonith disabled, or worst, a split brain if you bend it hard enough to make it work anyway. If you don’t mind taking time rebuilding a database with corrupt and/or incoherent data and constraints, that’s fine though.

NOTE: if you can’t have active fencing, look as storage base death or watchdog methods. They are both described in the fencing documentation.

In this tutorial, we choose to create one fencing resource per node to fence. They are called fence_vm_xxxand use the fencing agent fence_virsh, allowing to power on or off a virtual machine using the virsh command through a ssh connection to the hypervisor.

WARNING: unless you build your PoC cluster using libvirt for VM management, there’s great chances you will need to use a different STONITH agent. The stonith setup is provided as a simple example, be prepared to adjust it.

Now you’ve been warned again and again, let’s populating the cluster with some sample STONITH resources using virsh over ssh. First, we need to allow ssh password-less authentication to <user>@192.168.122.1 so these fencing resource can work. Again, this is specific to this setup. Depending on your fencing topology, you might not need this step. Run on all node:

ssh-keygen
ssh-copy-id <user>@192.168.122.1

Check the ssh connections are working as expected.

We can now create one STONITH resource for each node. Each fencing resource will not be allowed to run on the node it is supposed to fence. Note that in the port argument of the following commands, srv[1-3]-c8 are the names of the virutal machines as known by libvirtd side. See manpage fence_virsh(8) for more infos.

pcs cluster cib fencing.xml
pcs -f fencing.xml stonith create fence_vm_srv1 fence_virsh \
  pcmk_host_check="static-list" pcmk_host_list="srv1"        \
  ipaddr="192.168.122.1" login="<user>" port="srv1-c8"       \
  identity_file="/root/.ssh/id_rsa"

pcs -f fencing.xml stonith create fence_vm_srv2 fence_virsh \
  pcmk_host_check="static-list" pcmk_host_list="srv2"        \
  ipaddr="192.168.122.1" login="<user>" port="srv2-c8"       \
  identity_file="/root/.ssh/id_rsa"

pcs -f fencing.xml stonith create fence_vm_srv3 fence_virsh \
  pcmk_host_check="static-list" pcmk_host_list="srv3"        \
  ipaddr="192.168.122.1" login="<user>" port="srv3-c8"       \
  identity_file="/root/.ssh/id_rsa"

pcs -f fencing.xml constraint location fence_vm_srv1 avoids srv1=INFINITY
pcs -f fencing.xml constraint location fence_vm_srv2 avoids srv2=INFINITY
pcs -f fencing.xml constraint location fence_vm_srv3 avoids srv3=INFINITY
pcs cluster cib-push scope=configuration fencing.xml

Using crm_mon You should see the three resources appearing in your cluster and being dispatched on nodes.

Cluster resources

In this last chapter we create three resources: pgsqld, pgsqld-clone and pgsql-pri-ip.

The pgsqld defines the properties of a PostgreSQL instance: where it is located, where are its binaries, its configuration files, how to montor it, and so on.

The pgsqld-clone resource controls all the PostgreSQL instances pgsqld in your cluster, decides where the primary is promoted and where the standbys are started.

The pgsql-pri-ip resource controls the pgsql-vip IP address. It is started on the node hosting the PostgreSQL primary resource.

Now the fencing is working, we can add all other resources and constraints all together in the same time. Create a new offline CIB:

pcs cluster cib cluster1.xml

We add the PostgreSQL pgsqld resource and the multistate pgsqld-clone responsible to clone it everywhere and define the roles (Master/Slave) of each clone:

# pgsqld
pcs -f cluster1.xml resource create pgsqld ocf:heartbeat:pgsqlms \
    bindir=/usr/pgsql-12/bin                                     \
    pgdata=/var/lib/pgsql/12/data                                \
    op start timeout=60s                                         \
    op stop timeout=60s                                          \
    op promote timeout=30s                                       \
    op demote timeout=120s                                       \
    op monitor interval=15s timeout=10s role="Master"            \
    op monitor interval=16s timeout=10s role="Slave"             \
    op notify timeout=60s                                        \
    promotable notify=true

Note that the values for timeout and interval on each operation are based on the minimum suggested value for PAF Resource Agent. These values should be adapted depending on the context.

The last line of this command declare the resource pgsqld as promotable. This is handled by pcs which creates the pgsqld-clone resource automatically.

We add the IP address which should be started on the primary node:

pcs -f cluster1.xml resource create pgsql-pri-ip ocf:heartbeat:IPaddr2 \
    ip=192.168.122.40 cidr_netmask=24 op monitor interval=10s

We now define the collocation between pgsqld-clone and pgsql-pri-ip. The start/stop and promote/demote order for these resources must be asymetrical: we MUST keep the vIP on the primary during its demote process so the standbies receive everything during the primary shutdown.

pcs -f cluster1.xml constraint colocation add pgsql-pri-ip with master pgsqld-clone INFINITY
pcs -f cluster1.xml constraint order promote pgsqld-clone then start pgsql-pri-ip symmetrical=false kind=Mandatory
pcs -f cluster1.xml constraint order demote pgsqld-clone then stop pgsql-pri-ip symmetrical=false kind=Mandatory

We can now push our CIB to the cluster, which will start all the magic stuff:

pcs cluster cib-push scope=configuration cluster1.xml

Conclusion

Now you know the basics to build a Pacemaker cluster hosting some PostgreSQL instances replicating with each others, you should probably check: