PostgreSQL Automatic Failover

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

Quick Start CentOS 7

This quick start tutorial is based on CentOS 7.2, using the pcs command.

Network setup

The cluster we are about to build includes three servers called srv1, srv2 and srv3. Each of them have two network interfaces eth0 and eth1. IP addresses of these servers are 192.168.122.5x/24 on the first interface, 192.168.123.5x/24 on the second one.

The IP address 192.168.122.50, called pgsql-vip in this tutorial, will be set on the server hosting the master PostgreSQL instance.

Considering the firewall, 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 --add-service=high-availability
firewall-cmd --permanent --add-service=postgresql
firewall-cmd --add-service=postgresql

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:

192.168.122.50 pgsql-vip
192.168.122.51 srv1
192.168.122.52 srv2
192.168.122.53 srv3
192.168.123.51 srv1-alt
192.168.123.52 srv2-alt
192.168.123.53 srv3-alt

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 (192.168.122.51): icmp_seq=1 ttl=64 time=0.028 ms
64 bytes from srv2 (192.168.122.52): icmp_seq=1 ttl=64 time=0.296 ms
64 bytes from srv3 (192.168.122.53): icmp_seq=1 ttl=64 time=0.351 ms

PostgreSQL and Cluster stack installation

Run this whole chapter on ALL nodes.

We are using the PostgreSQL packages from the PGDG repository. Here is how to install and set up this repository on your system:

yum install -y http://yum.postgresql.org/9.6/redhat/rhel-7-x86_64/pgdg-centos96-9.6-3.noarch.rpm

Make sure to double adapt the previous command with the latest package available and the PostgreSQL version you need.

We can now install everything we need for our cluster:

yum install -y pacemaker postgresql96 postgresql96-contrib postgresql96-server resource-agents pcs fence-agents-all fence-agents-virsh

Finally, we need to install the “PostgreSQL Automatic Failover” (PAF) resource agent:

yum install -y https://github.com/dalibo/PAF/releases/download/v2.1.0/resource-agents-paf-2.1.0-1.noarch.rpm

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 slaves ready to replicate. Make sure to setup your PostgreSQL master on your preferred node to host the master: during the very first startup of the cluster, PAF detects the master based on its shutdown status.

Moreover, it requires a recovery.conf template ready to use. You can create a recovery.conf file suitable to your needs, the only requirements are:

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

WARNING: as recovery.conf.pcmk and pg_hba.conf files 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 to deal with this using the hba_file parameter in your postgresql.conf file and recovery_template parameter in PAF for the recovery.conf.pcmk file.

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

On the primary:

/usr/pgsql-9.6/bin/postgresql96-setup initdb

su - postgres

cd 9.6/data/
cat <<EOP >> postgresql.conf

listen_addresses = '*'
wal_level = replica
max_wal_senders = 10
hot_standby = on
hot_standby_feedback = on
EOP

cat <<EOP >> pg_hba.conf
# forbid self-replication
host replication postgres 192.168.122.50/32 reject
host replication postgres $(hostname -s) reject

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

cat <<EOP > recovery.conf.pcmk
standby_mode = on
primary_conninfo = 'host=192.168.122.50 application_name=$(hostname -s)'
recovery_target_timeline = 'latest'
EOP

exit

systemctl start postgresql-9.6
ip addr add 192.168.122.50/24 dev eth0

Now, on each standby, clone the primary. E.g.:

su - postgres

pg_basebackup -h pgsql-vip -D ~postgres/9.6/data/ -X stream -P

cd ~postgres/9.6/data/

sed -ri s/srv[0-9]+/$(hostname -s)/ pg_hba.conf
sed -ri s/srv[0-9]+/$(hostname -s)/ recovery.conf.pcmk

cp recovery.conf.pcmk recovery.conf

exit

systemctl start postgresql-9.6

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:

systemctl stop postgresql-9.6
systemctl disable postgresql-9.6

And remove the master IP address from srv1:

ip addr del 192.168.122.50/24 dev eth0

Cluster pre-requisites

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:

systemctl disable corosync # important!
systemctl disable pacemaker

This guide uses the cluster management tool pcsd provided by RHEL 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

Enable and start the pcsd daemon on all the nodes:

systemctl enable pcsd
systemctl start pcsd

Now, authenticate each node to the other ones using the following command:

pcs cluster auth srv1 srv2 srv3 -u hacluster

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 --name cluster_pgsql srv1,srv1-alt srv2,srv2-alt srv3,srv3-alt

NOTE: If you don’t have an alternative network available use the following syntax instead of the previous one:

pcs cluster setup --name cluster_pgsql srv1 srv2 srv3

Make sure you have a redundant network at system level. This is a **CRITICAL** part of your cluster.

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 now start the whole cluster from one node:

pcs cluster start --all

After some seconds of startup and cluster membership stuff, 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 -f cluster1.xml resource defaults migration-threshold=5
pcs -f cluster1.xml resource defaults resource-stickiness=10

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

Node fencing

One of 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 open docs/FENCING.md in the source code of PAF or read online: http://dalibo.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.

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 connexion 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-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:

pcs cluster cib cluster1.xml
pcs -f cluster1.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-c7"       \
  action="off" identity_file="/root/.ssh/id_rsa"

pcs -f cluster1.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-c7"       \
  action="off" identity_file="/root/.ssh/id_rsa"

pcs -f cluster1.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-c7"       \
  action="off" identity_file="/root/.ssh/id_rsa"

pcs -f cluster1.xml constraint location fence_vm_srv1 avoids srv1=INFINITY
pcs -f cluster1.xml constraint location fence_vm_srv2 avoids srv2=INFINITY
pcs -f cluster1.xml constraint location fence_vm_srv3 avoids srv3=INFINITY
pcs cluster cib-push cluster1.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, pgsql-ha, and pgsql-master-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 pgsql-ha resource controls all the PostgreSQL instances pgsqld in your cluster, decides where the primary is promoted and where the standbys are started.

The pgsql-master-ip resource controls the pgsql-vip IP address. It is started on the node hosting the PostgreSQL master 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 pgsql-ha 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-9.6/bin pgdata=/var/lib/pgsql/9.6/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

# pgsql-ha
pcs -f cluster1.xml resource master pgsql-ha pgsqld 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.

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

pcs -f cluster1.xml resource create pgsql-master-ip ocf:heartbeat:IPaddr2 \
    ip=192.168.122.50 cidr_netmask=24 op monitor interval=10s

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

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