In my last blog post, I showed you how easy it is to set up a three node Percona XtraDB Cluster.  But how do we know if the cluster is working or not?  In this post I will go over the built-in variables that ship with Galera you can use to check on the status of your cluster.  Before we dive in, it is important to realize that Galera is a multi-master cluster.  This means that you can read and write to all the nodes.  One of the implications of the manic that makes Galera work is that each node can be independently configured. As such, it is important that you check all the nodes, not just one, to determine the overall health of your cluster.  For the rest of this post I will be only running the checks on one of the nodes for brevity.  But you should really run these checks on all the nodes to ensure a fully functioning cluster.

Probably the most important variable is wsrep_cluster_status. This variable will tell you if the node is up and fully part of the cluster.  If the node is fully part of the cluster this variable will return the status of Primary.  Any other response and the node is not fully part of the cluster.  You can check the status by running:

node1 mysql> SHOW GLOBAL STATUS LIKE 'wsrep_cluster_status';
+----------------------+---------+
| Variable_name        | Value   |
+----------------------+---------+
| wsrep_cluster_status | Primary |
+----------------------+---------+
1 row in set (0.00 sec)

But checking the variable alone is not enough, because you can bootstrap all three nodes and have three seperate clusters. In this case, all three would return Primary but you would still not have a three node cluster. In order to tell if you if the node is part of the cluster you can use wsrep_cluster_size. This will tell you the number of nodes in the cluster.

node1 mysql> SHOW GLOBAL STATUS LIKE 'wsrep_cluster_size';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 3     |
+--------------------+-------+
1 row in set (0.00 sec)

For our simple three node example, this is probably good enough. But what if you work in an environment with multiple clusters. How can you tell if the node is part of the right cluster? To be sure we can use wsrep_cluster_state_uuid. All the nodes fully participating in the cluster should return the same value.

node1 mysql> SHOW GLOBAL STATUS LIKE 'wsrep_cluster_state_uuid';
+--------------------------+--------------------------------------+
| Variable_name            | Value                                |
+--------------------------+--------------------------------------+
| wsrep_cluster_state_uuid | c0803766-24db-11e5-ac7a-abcded9c6987 |
+--------------------------+--------------------------------------+
1 row in set (0.00 sec)

For more information on how to use wsrep variables to check on the health of your cluster you can use the Galera documentation.  In future posts we will dive more into Galera.

In this post I will be walking you through installing a 3 node test XtraDB Cluster (PXC) environment using VirtualBox.  The version of VirtualBox I will be using is 4.3.26. And I will be using CentOS 6.6 for the operating system on the three nodes. To get started you will want to perform a minimal install of 6.6 on 3 virtual machines. I named mine pxc-node1, pxc-node2, and pxc-node3. I also set up port forwarding in VirtualBox to allow me to ssh to the machines via a local port on the host. I set up 22201 to point to pxc-node1, 22202 to point to pxc-node2, and 22203 to point to pxc-node3. The last thing that I did was to add a second host only network interface to each of the nodes and ensure the same name was set for all the nodes.  You can do all of this work yourselves, or simply download an already configured appliance from here.

Now that we have the images created, it is time to discuss the plan for installing our Percona XtraDB Cluster. The actual installation process is very similar to the process that I used for installing Percona Server. But once we have the binaries laid down the configuration of the instances will be different and we will need to start the first node of the cluster a special way. While this quick overview does not give a lot of detail, I think it best to learn by doing. With that in mind, here we go.

First ensure that all of the nodes are running. To keep things simple we will be installing as root. But I have done installs at work on RHEL 6 using sudo and the process is pretty much identical. First thing we will need to do is to enable the network interfaces on each machine and disable SELinux. Galera does not work with SELinux and since XtraDB Cluster is just Percona’s Galera implementation we will need to disable it. In the console for each of the nodes you will need to run the following commands.

[root@localhost ~]$ vi /etc/sysconfig/network-scripts/ifcfg-eth0

You will want to modify the ONBOOT to be yes. The file should look like this when finished.

DEVICE=eth0
HWADDR=08:00:27:24:FD:19
TYPE=Ethernet
UUID=b3c128fe-335e-437d-bf3b-32f3706d9273
ONBOOT=yes
NM_CONTROLLED=yes
BOOTPROTO=dhcp

Now save the file.  We also need to add a config for the secondary NIC which we want to not set up for DHCP, but use static ip addresses.  We will use 192.168.70.11 for pxc-node1, 192.168.70.12 for pxc-node2, and 192.168.70.13 for pxc-node3.  The contents of /etc/sysconfig/network-scripts/ifcfg-eth1 for pxc-node1 would look like.

NM_CONTROLLED=no
BOOTPROTO=none
ONBOOT=yes
IPADDR=192.168.70.11
NETMASK=255.255.255.0
DEVICE=eth1

PEERDNS=no

To disable SELInux we need to run.

[root@localhost ~] vi /etc/selinux/config

You will want to set it to disabled. The file should look like this when finished.

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
#     targeted - Targeted processes are protected,
#     mls - Multi Level Security protection.
SELINUXTYPE=targeted

Now save the file. We also will be stopping the firewall and setting it not to restart.  In production we would want to add rules to allow traffic, but this is just a test environment so we’ll take the short cut.

[root@localhost ~]$ service iptables stop
[root@localhost ~]$ chkconfig iptables off

To make it easier to know what system you are on I would modify the shell to show the node name. To do that edit the bash configuration file using.

vi /etc/bashrc

Find the line that says

[ "$PS1" = "\\s-\\v\\\$ " ] && PS1="[\u@\h \W]\\$ "

Comment that line out and add the following

PS1='\u@pxc-node1:\w\$ '

The section should look like this when finished then save the file.

# Turn on checkwinsize
  shopt -s checkwinsize
#  [ "$PS1" = "\\s-\\v\\\$ " ] && PS1="[\u@\h \W]\\$ "
  PS1='\u@pxc-node1:\w\$ '

After making these changes you will want to restart the node using the following command and then perform the same steps on the other two nodes.

[root@localhost ~] shutdown -r 0

Once we are finished with the prep of the three nodes we are ready to start the installation of the PXC cluster. First we want to make sure the the MySQL libraries are not already on the box.

root@pxc-node1:~# yum -y remove mysql-libs

Then we want to import the EPEL and the Percona Repositories.

root@pxc-node1:~# rpm -Uvh http://download.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
root@pxc-node1:~# rpm -Uvh http://www.percona.com/downloads/percona-release/redhat/0.1-3/percona-release-0.1-3.noarch.rpm

Now that we have the repository registered it is time to lay down the binaries.

root@pxc-node1:~# yum install -y socat
root@pxc-node1:~# yum install -y Percona-XtraDB-Cluster-server-56 Percona-XtraDB-Cluster-client-56 Percona-XtraDB-Cluster-shared-56 percona-toolkit percona-xtrabackup
root@pxc-node1:~# touch /etc/my.cnf
root@pxc-node1:~# /usr/bin/mysql_install_db --defaults-file=/etc/my.cnf --force --datadir=/var/lib/mysql --basedir=/usr/ --user=mysql

You will need to run these same steps on all three nodes to install the binaries. This will also create the data directory and populate it with the system tables.
Now that we have the binaries on each machine it is time to set up the cluster. This involves editing the /etc/my.cnf file. To start with, let’s configure the first node in our cluster. On pxc-node1 edit the /etc/my.cnf file so that it looks like this.

[mysqld]
datadir                         = /var/lib/mysql
log_error                       = error.log

log-bin
server-id = 1

query_cache_size=0
query_cache_type=0

innodb_buffer_pool_size                 = 48M
innodb_log_file_size            = 24M
innodb_flush_method                             = O_DIRECT
innodb_file_per_table
innodb_flush_log_at_trx_commit  = 0

performance_schema=OFF

binlog_format = ROW

# galera settings
wsrep_provider = /usr/lib64/libgalera_smm.so
wsrep_cluster_name = mycluster
wsrep_cluster_address = gcomm://192.168.70.11,192.168.70.12,192.168.11.13
wsrep_node_name = node1
wsrep_node_address = 192.168.70.11
wsrep_sst_auth = sst:secret
innodb_autoinc_lock_mode = 2
innodb_locks_unsafe_for_binlog = ON


[mysql]
prompt                          = "node1 mysql> "

[client]
user                            = root​

If we try to start the cluster now using the same method for a normal MySQL instance it will fail.

root@pxc-node1:~# service mysql start
Starting MySQL (Percona XtraDB Cluster)................................... ERROR! The server quit without updating PID file (/var/lib/mysql/localhost.localdomain.pid).
 ERROR! MySQL (Percona XtraDB Cluster) server startup failed!

This is because the first node in a cluster has to be forced online as a safety feature since you cannot have quorum with only one node. To force it online run the following.

root@pxc-node1:~# service mysql bootstrap-pxc
Bootstrapping PXC (Percona XtraDB Cluster)Starting MySQL (Percona XtraDB Cluster)...... SUCCESS!

You may see a message similar to the following since we just tried to start the cluster without bootstrapping first.

root@pxc-node1:~# service mysql bootstrap-pxc
Bootstrapping PXC (Percona XtraDB Cluster) ERROR! MySQL (Percona XtraDB Cluster) is not running, but lock file (/var/lock/subsys/mysql) exists
Starting MySQL (Percona XtraDB Cluster)......... SUCCESS!

Either way you now have a cluster up and running with one node. Before we add any more nodes, let’s put some things in place to verify the cluster once it is up. We will be installing sysbench to allow us to drive a load at the machine, and we will be installing myq_tools to check the flow of transactions in the cluster. Run this on all three nodes to set up sysbench and myq_tools and get them ready for our use.

root@pxc-node1:~# yum install -y sysbench
root@pxc-node1:~# mysql
node1 mysql> CREATE USER 'test'@'localhost' IDENTIFIED BY 'test';
node1 mysql> GRANT ALL PRIVILEGES ON test.* TO 'test'@'localhost';
node1 mysql> \q
root@pxc-node1:~# yum install -y wget
root@pxc-node1:~# cd /usr/local/bin​ ​
root@pxc-node1:~# ​wget https://github.com/jayjanssen/myq-tools/releases/download/v0.5/myq_tools.tgz ​
​root@pxc-node1:~# tar -xzvf myq_tools.tgz ​
root@pxc-node1:~# cd bin ​
root@pxc-node1:~# mv * ../ ​
root@pxc-node1:~# cd .. ​
root@pxc-node1:~# rm -rf​ bin​ ​
​root@pxc-node1:~# ln -s /usr/local/bin/myq_status.linux-amd64 myq_status​​
​root@pxc-node1:~# vi /usr/local/bin/run_sysbench_oltp.sh

sysbench --db-driver=mysql --test=/usr/share/doc/sysbench/tests/db/oltp.lua --mysql-user=test --mysql-password=test --mysql-db=test --mysql-host=localhost --mysql-ignore-errors=all --oltp-tables-count=1 --oltp-table-size=250000 --oltp-auto-inc=off --num-threads=1 --report-interval=1 --max-requests=0 --tx-rate=10 run | grep tps

root@pxc-node1:~# chmod +x run_sysbench_oltp.sh
root@pxc-node1:~# sysbench --db-driver=mysql --test=/usr/share/doc/sysbench/tests/db/oltp.lua --mysql-user=test --mysql-password=test --mysql-db=test --mysql-host=localhost --mysql-ignore-errors=all --oltp-table-size=250000 --num-threads=1 prepare

You can verify that you have sysbench set up correctly but running the following.  You should not see any errors.  There should also be numbers in the reads and writes columns.

root@pxc-node1:/usr/local/bin# run_sysbench_oltp.sh
[   1s] threads: 1, tps: 10.99, reads: 153.85, writes: 43.96, response time: 12.64ms (95%), errors: 0.00, reconnects:  0.00
[   2s] threads: 1, tps: 17.02, reads: 238.28, writes: 68.08, response time: 12.84ms (95%), errors: 0.00, reconnects:  0.00
[   3s] threads: 1, tps: 10.00, reads: 153.02, writes: 40.01, response time: 10.10ms (95%), errors: 0.00, reconnects:  0.00
[   4s] threads: 1, tps: 19.00, reads: 253.00, writes: 76.00, response time: 10.26ms (95%), errors: 0.00, reconnects:  0.00
[   5s] threads: 1, tps: 9.00, reads: 125.99, writes: 36.00, response time: 11.42ms (95%), errors: 0.00, reconnects:  0.00
[   6s] threads: 1, tps: 6.00, reads: 83.98, writes: 23.99, response time: 12.19ms (95%), errors: 0.00, reconnects:  0.00
[   7s] threads: 1, tps: 6.00, reads: 84.01, writes: 24.00, response time: 63.33ms (95%), errors: 0.00, reconnects:  0.00

Now that we have the cluster running we need to add the other two nodes. But before we do that, we need to create a user with permissions for the SST process. We’ll talk more about SST in the future, but for now its important to know this process allows the other nodes to get a copy of the database when they join the cluster. To do that we need to run the following on node 1.

root@pxc-node1:/usr/local/bin# mysql
node1 mysql> GRANT RELOAD, LOCK TABLES, REPLICATION CLIENT ON *.* TO 'sst'@'localhost' IDENTIFIED BY 'secret';​​
node1 mysql> \q

Since we already have the binaries installed we just need to configure the other two instances and start mysql on the nodes. The instructions for node 2 are below. Notice that we don’t need to bootstrap these instances. In fact, if we do bootstrap them we would end up with 3 clusters instead of just one.

root@pxc-node2:~#vi /etc/my.cnf

[mysqld]
datadir                         = /var/lib/mysql
log_error                       = error.log

log-bin
server-id = 2

query_cache_size=0
query_cache_type=0

innodb_buffer_pool_size                 = 48M
innodb_log_file_size            = 24M
innodb_flush_method                             = O_DIRECT
innodb_file_per_table
innodb_flush_log_at_trx_commit  = 0

performance_schema=OFF

binlog_format = ROW

# galera settings
wsrep_provider = /usr/lib64/libgalera_smm.so
wsrep_cluster_name = mycluster
wsrep_cluster_address = gcomm://192.168.70.11,192.168.70.12,192.168.70.13
wsrep_node_name = node2
wsrep_node_address = 192.168.70.12
wsrep_sst_auth = sst:secret
innodb_autoinc_lock_mode = 2
innodb_locks_unsafe_for_binlog = ON


[mysql]
prompt                          = "node2 mysql> "

[client]
user                            = root​

root@pxc-node2:~# service mysql restart
Shutting down MySQL (Percona XtraDB Cluster).. SUCCESS!
Starting MySQL (Percona XtraDB Cluster).....State transfer in progress, setting sleep higher
... SUCCESS!

At this point we now have a two node cluster. To add the last node you need to modify /etc/my.cnf on that node and then start the mysql service. You can use the configuration file for node 2 as an example. You will need to modify the server-id, wsrep_node_name, wsrep_node_address, and prompt to the values for pxc-node3. If you have any issues let me know.

In a future post, I will show you how to set up HAProxy to provide a single connection (actually we will use 2 connections but more on that later) for your applications. We will also play around with some of the more interesting features of Galera and Percona XtraDB Cluster.