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Galera-ClusterCheck

Script to make a proxy (ie HAProxy) capable of monitoring Galera Cluster nodes properly.

Usage

Below is a sample configuration for HAProxy on the client. The point of this is that the application will be able to connect to localhost port 3307, so although we are using Galera Cluster with several nodes, the application will see this as a single MySQL server running on localhost.

/etc/haproxy/haproxy.cfg

...
listen galera-cluster 0.0.0.0:3307
  balance leastconn
  option httpchk
  mode tcp
    server node1 1.2.3.4:3306 check port 9200 inter 5000 fastinter 2000 rise 2 fall 2
    server node2 1.2.3.5:3306 check port 9200 inter 5000 fastinter 2000 rise 2 fall 2
    server node3 1.2.3.6:3306 check port 9200 inter 5000 fastinter 2000 rise 2 fall 2 backup

MySQL connectivity is checked via HTTP on port 9200. The clustercheck script is a simple shell script which accepts HTTP requests and checks MySQL on an incoming request. If the Galera Cluster node is ready to accept requests, it will respond with HTTP code 200 (OK), otherwise a HTTP error 503 (Service Unavailable) is returned.

Setup with xinetd

This setup will create a process that listens on TCP port 9200 using xinetd. This process uses the clustercheck script from this repository to report the status of the node.

First, create a clustercheckuser that will be doing the checks.

mysql> GRANT PROCESS ON *.* TO 'clustercheckuser'@'localhost' IDENTIFIED BY 'clustercheckpassword!'

Copy the clustercheck from the repository to a location (/usr/bin in the example below) and make it executable. Then add the following service to xinetd (make sure to match your location of the script with the 'server'-entry).

/etc/xinetd.d/mysqlchk:

# default: on
# description: mysqlchk
service mysqlchk
{
        disable = no
        flags = REUSE
        socket_type = stream
        port = 9200
        wait = no
        user = nobody
        server = /usr/bin/clustercheck
        log_on_failure += USERID
        only_from = 0.0.0.0/0
        per_source = UNLIMITED
}

Also, you should add the mysqlchk service to /etc/services before restarting xinetd.

xinetd      9098/tcp    # ...
mysqlchk    9200/tcp    # MySQL check  <--- Add this line
git         9418/tcp    # Git Version Control System
zope        9673/tcp    # ...

Clustercheck will now listen on port 9200 after xinetd restart, and HAproxy is ready to check MySQL via HTTP poort 9200.

Setup with shell return values

If you do not want to use the setup with xinetd, you can also execute clustercheck on the commandline and check for the return value.

First, create a clustercheckuser that will be doing the checks.

mysql> GRANT PROCESS ON *.* TO 'clustercheckuser'@'localhost' IDENTIFIED BY 'clustercheckpassword!'

Then, you can execute the script. In case of a synced node:

# /usr/bin/clustercheck > /dev/null
# echo $?
0

In case of an un-synced node:

# /usr/bin/clustercheck > /dev/null
# echo $?
1

You can use this return value with monitoring tools like Zabbix or Zenoss.

Configuration options

The clustercheck script accepts several arguments:

clustercheck <user> <pass> <available_when_donor=0|1> <log_file>
  • user and pass (default clustercheckuser and clustercheckpassword!): defines the username and password for the check. You can pass an empty username and/or password by supplying ""
  • available_when_donor (default 0): By default, the node is reported unavailable if it’s a donor for SST. If you want to allow queries on a node which is a donor for SST, you can set this variable to 1. Note: when you set this variable to 1, you will also need to use a non-blocking SST method like xtrabackup
  • log_file (default "/dev/null"): defines where logs and errors from the checks should go
  • available_when_readonly (default 1): Depending on this setting and the MySQL status variable 'read_only', the node is reported available
  • defaults_extra_file (default /etc/my.cnf): This file (if exists) will be passed to the mysql-command with the commandline option --defaults-extra-file

Note: You can also specify the username and password for the check in the defaults_extra_file and pass empty values for user and pass. That way, nobody can see the username and password (which can otherwise be seen, since they are passed to the MySQL CLI)

Manually removing a node from the cluster

By touching /var/tmp/clustercheck.disabled, an admin may force clustercheck to return 503, regardless as to the actual state of the node. This is useful when the node is being put into maintenance mode.

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Script to make a proxy (ie HAProxy) capable of monitoring Galera Cluster nodes properly.

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