MariaDB MaxScale HA with Lsyncd

This guide was written for lsyncd 2.1.5.

This document guides you in setting up multiple MariaDB MaxScale instances and synchronizing the configuration files with lsyncd. Lsyncd is a rsync wrapper which can synchronize files across the network. The lsyncd daemon uses a configuration file to control the files to synchronize and the remote targets where these files are synchronized to.

Copying the configuration file and running the lsyncd daemon on all the hosts keeps all the configuration files in sync. Modifications in the configuration file on one of the hosts will be copied on the other hosts. This allows administrators to easily provide a highly available, disaster resistant MariaDB MaxScale installation with up-to-date configuration files on all the hosts.

Requirements

You will need:

  • Access to the remote hosts.
  • MariaDB MaxScale installed on all systems
  • Configured maxscale.cnf file in /etc
  • SSH daemon and clients installed on all hosts

The installation and configuration of MariaDB MaxScale is covered in other documents.

Creating SSH keys

For lsyncd to work, we will need to either use an existing set of SSH keys or to create a new set of keys. The creation and copying of keys needs to be repeated on all of the hosts.

If you already have a SSH key generated, you can skip this next step and go to the Copying Keys part.

Generating keys

To generate a new set of SSH keys, we will use ssh-keygen.

[root@localhost ~]# ssh-keygen 
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
f4:99:0a:cc:d4:ac:ea:ed:ff:0d:bb:e5:87:3e:38:df root@localhost.localdomain
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|       o         |
|      . +        |
|     + o . o     |
|      = S +      |
|     . . .       |
|    .   . ....   |
|   . .    o*o..  |
|    ..o...+==oE  |
+-----------------+

The keys will be generated in the .ssh folder and will automatically be used by ssh.

Copying keys

To copy the SSH keys to the remote host we will use ssh-copy-id.

Use the username and host of the remote server you wish to synchronize MariaDB MaxScale's configuration files to. For example, if the server's address is 192.168.122.100 and the user we use for synchronization us user we can use the following command.

ssh-copy-id user@192.168.122.100

Repeat the last command with the usernames and addresses of all the remote hosts you want to synchronize the configuration files to.

Installing lsyncd

You will need to install lsyncd on all of the hosts for changes in the configuration file on one of the nodes to be synchronized to the other nodes.

You can install lsyncd with either a package manager or by building from source code. This guide demonstrates installation using a package manager and those looking to build lsyncd from source should refer to its documentation: https://github.com/axkibe/lsyncd/wiki/Manual-to-Lsyncd-2.1.x

Installing with Yum:

yum install lsyncd

Installing with Apt:

apt-get install lsyncd

Creating the Lsyncd configuration file

Lsyncd uses a configuration file to determine where to read files from and where to synchronize them if changes in them occur. Lsyncd is written in Lua and the configuration file is also valid Lua code.

Here is an example configuration file with descriptions on the meaning of the values in it.

-- Lsyncd will log to these two files.
settings{
         logfile         = "/var/log/maxscale/maxscale-ha.log",
         statusFile      = "/var/log/maxscale/maxscale-ha-status.log"
}

-- Copy and paste the sync section and change the host value to add new remote targets.
sync{
default.rsyncssh,

-- This is where the maxscale.cnf file is copied from.
source="/etc", 

-- This is the user and host where the maxscale.cnf is copied to.
-- Change this to the user and destination host where you want maxscale.cnf to be synchronized to.
host="user@192.168.122.100", 

-- This is where the maxscale.cnf is copied to on the remote host.
targetdir="/etc", 

-- This is an optional section which defines a custom SSH port. Uncomment to enable.
-- ssh={port=2222}, 

-- These are values passed to rsync. Only change these if you know what you are doing.
rsync={
      compress=true,
      _extra = {[[--filter=+ *maxscale.cnf]],
                [[--filter=- **]]
               }
      }
}

The most important part is the sync section which defines a target for synchronization. The default.rsyncssh tells lsyncd to synchronize files using SSH.

The source parameter tells lsyncd where to read the files from. This should be the location of the maxscale.cnf file. The host parameter defines the host where the files should be synchronized to and the user account lsyncd should use when synchronizing the files. The targetdir parameter defines the local directory on the remote target where the files should be synchronized to. This value should be the location on the remote host where the maxscale.cnf file is searched from. By default, this is the /etc folder.

The optional ssh parameter and its sub-parameter portdefine a custom port for the SSH connection. Most users do not need this parameter. The rsycn parameter contains an array of options that are passed to the rsycn executable. These should not be changed unless you specifically know what you are doing. For more information on the options passed to rsync read the rsync(1) manpage.

You can add multiple remote targets by defining multiple sync sections. Here is an example with two sync sections defining different hosts that have MariaDB MaxScale installed and whose configuration files should be kept in sync.

settings{
         logfile         = "/var/log/maxscale/maxscale-ha.log",
         statusFile      = "/var/log/maxscale/maxscale-ha-status.log"
}

sync{
default.rsyncssh,
source="/etc", 
host="maxuser@192.168.0.50", 
targetdir="/etc", 
rsync={
      compress=true,
      _extra = {[[--filter=+ *maxscale.cnf]],
                [[--filter=- **]]
               }
      }
}


sync{
default.rsyncssh,
source="/etc", 
host="syncuser@192.168.122.105", 
targetdir="/etc", 
rsync={
      compress=true,
      _extra = {[[--filter=+ *maxscale.cnf]],
                [[--filter=- **]]
               }
      }
}

Starting Lsyncd

Starting lsyncd can be done from the command line or through a init script. To start lsyncd from the command like, execute the lsyncd command and pass the configuration file as the only parameter.

By default lsyncd will search for the configuration file in /etc/lsyncd.conf. By placing the configuration file we created in the /etc folder, we can start lsyncd with the following command.

service lsyncd start

Here is an example which start lsyncd and reads the configuration options from the lsyncd.cnf file.

lsyncd lsyncd.cnf

For more information on the lsyncd executable and its options, please see the --help output of lsyncd or the lsyncd(1) manpage.