Linux High Availability Software with Synchronous Replication and Failover

Evidian SafeKit brings high availability to Linux applications. This article explains how to implement quickly a Linux cluster without shared disk and without specific skills. A free trial is offered.

How the Evidian SafeKit software simply implements Linux high availability with real-time synchronous replication and failover without shared disk

How the Evidian SafeKit mirror cluster implements Linux high availability with synchronous replication and failover?

On the previous figure, the server 1/PRIM (Linux) runs a critical application. Users are connected to the virtual IP address of the mirror cluster. SafeKit replicates files opened by the Linux application in real time. Only changes in the files are replicated across the network, thus limiting traffic (byte-level file replication). Names of file directories containing the application data are simply configured in SafeKit. There are no pre-requisites on disk organization for the two servers. Directories to replicate may be located in the system disk. SafeKit implements synchronous replication with no data loss on failure contrary to asynchronous replication.

In case of server 1 failure, there is an automatic failover on server 2 with restart of the Linux application. Then, when server 1 is restarted, SafeKit implements failback with reintegration of replicated data without stopping the application on server 2. Finally, the system returns to synchronous replication between server 2 and server 1. The administrator can decide to swap the role of primary and secondary and return to a server 1 running the application. The swap can also be done automatically by configuration.

Configuration overview of Linux high availability with synchronous replication and failover

With SafeKit, you can configure either a farm application module or a mirror application module according the high availability architecture suited for an application. For synchronous replication and failover, the right module is the mirror module.

Configuration overview of Linux high availability with synchronous replication and failover

The configuration files of the generic mirror module for Linux are given here.

They include:

1. the Linux application stop and start scripts,

2. the configuration file userconfig.xml which contains:

SafeKit configuration files for Linux high availability with synchronous replication and failover

Installation instructions

Note: Examples of application modules built from this generic module are given here.

userconfig.xml

<!DOCTYPE safe>
<safe>
   <service mode="mirror" defaultprim="alone" maxloop="3" loop_interval="24" failover="on">
      <!-- Server Configuration -->
      <!-- Names or IP addresses on the default network are set during initialization in the console -->
      <heart pulse="700" timeout="30000">
         <heartbeat name=”default” ident=”flow”/>
      </heart>
      <!-- Virtual IP Configuration -->
      <!-- Replace
         * VIRTUAL_TO_BE_DEFINED by the name/IP of your virtual server 
      -->
      <vip>
         <interface_list>
            <interface check="on" arpreroute="on">
               <real_interface>
                  <virtual_addr addr="VIRTUAL_TO_BE_DEFINED" where="one_side_alias"/>
               </real_interface>
            </interface>
         </interface_list>
      </vip>
      <!-- Software Error Detection Configuration -->
      <!-- Replace
         * PROCESS_NAME by the name of the process to monitor
      -->
      <errd polltimer="10">
        <proc name="PROCESS_NAME" atleast="1" action="restart" class="prim" />
      </errd>
      <!-- File Replication Configuration -->
      <rfs mountover="off" async="second" acl="off" nbrei="3" >
         <replicated dir="/test1replicated" mode="read_only"/>
         <replicated dir="/test2replicated" mode="read_only"/>
      </rfs>
      <!-- User scripts activation -->
      <user nicestoptimeout="300" forcestoptimeout="300" logging="userlog"/>
   </service>
</safe>

start_prim

#!/bin/sh
# Script called on the primary server for starting application

# For logging into SafeKit log use:
# $SAFE/safekit printi | printe "message" 

# stdout goes into Application log
echo "Running start_prim $*" 

res=0

# Fill with your application start call

if [ $res -ne 0 ] ; then
  $SAFE/safekit printe "start_prim failed"

  # uncomment to stop SafeKit when critical
  # $SAFE/safekit stop -i "start_prim"
fi

stop_prim

#!/bin/sh
# Script called on the primary server for stopping application

# For logging into SafeKit log use:
# $SAFE/safekit printi | printe "message" 

#----------------------------------------------------------
#
# 2 stop modes:
#
# - graceful stop
#   call standard application stop
#
# - force stop ($1=force)
#   kill application's processes
#
#----------------------------------------------------------

# stdout goes into Application log
echo "Running stop_prim $*" 

res=0

# default: no action on forcestop
[ "$1" = "force" ] && exit 0

# Fill with your application stop call

[ $res -ne 0 ] && $SAFE/safekit printe "stop_prim failed"

Demonstration

This demonstration is made with Microsoft SQL Server Express but the operating mode is the same as with a Linux application.

More on SafeKit

Other examples of high availability modules:

Mirror modules

Windows

Linux

Microsoft SQL ServerWindows module-
OracleWindows moduleLinux module
MySQLWindows moduleLinux module
PostgreSQLWindows moduleLinux module
FirebirdWindows moduleLinux module
Hyper-VWindows module-
Hanwha SSMWindows module-
Milestone XProtectWindows module-
Generic moduleWindows moduleLinux module

Farm modules

Windows

Linux

IIS moduleWindows module-
Apache moduleWindows moduleLinux module
Generic moduleWindows moduleLinux module
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