SafeKit quick installation guide for a new Linux application (mirror.safe module for real-time replication and failover)

SafeKit quick installation guide for a new application on Linux (mirror.safe)

Step by step configuration

2. Configure node addresses

Enter the node IP addresses, press the Tab key to check connectivity and fill node names.
Then, click on Save and apply to save the configuration.

If either node1 or node2 has a red color, check connectivity of the browser to both nodes and check firewall on both nodes for troubleshooting.

If you want, you can add a new LAN for a second heartbeat and for a dedicated replication network.

This operation will place the IP addresses in the cluster.xml file on both nodes (more information in the training with the command line).

4. Configure the module

Choose an Automatic start of the module at boot without delay.
Normally, you have a single Heartbeat network on which the replication is made. But, you can define a private network if necessary (by adding a LAN at step 2).
Enter a Virtual IP address. A virtual IP address is a standard IP address in the same IP network (same subnet) as the IP addresses of both nodes.
Application clients must be configured with the virtual IP address (or the DNS name associated with the virtual IP address).
The virtual IP address is automatically switched in the event of a failure.
Check that the Replicated directories are installed on both nodes and contain the application data.
Data and log replication are essential for a database..
You can create additional replicated directories as required.
Note that if a process name is displayed in Monitored processes/services, it will be monitored with a restart action in case of failure. Configuring a wrong process name will cause the module to stop right after its start.

If you click on Advanced configuration, the userconfig.xml file is displayed (example with Microsoft SQL Server).

5. Edit scripts (optional)

start_prim and stop_prim must contain starting and stopping of the Linux application (example provided for Microsoft SQL Server on the right).
You can add new services in these scripts.
Check that the names of the services started in these scripts are those installed on both nodes, otherwise modify them in the scripts.
On Windows and on both nodes, with the Windows services manager, set Boot Startup Type = Manual for all the services registered in start_prim (SafeKit controls the start of services in start_prim).

9. Start the node with up-to-date data

If node 1 has the up-to-date replicated directories, select it and start it As primary.

When node 2 will be started, all data will be copied from node 1 to node 2.

If you make the wrong choice, you run the risk of synchronizing outdated data on both nodes.

It is also assumed that the Linux application is stopped on node 1 so that SafeKit installs the replication mechanisms and then starts the application in the start_prim script.

Use Start for subsequent starts: SafeKit retains the most up-to-date server. Starting As primary is a special start-up the first time or during exceptional operations.

10. Wait for the transition to ALONE (green)

Node 1 should reach the ALONE (green) state, which means that the virtual IP is set and that the start_prim script has been executed on node 1.

If ALONE (green) is not reached or if the application is not started, analyze why with the module log of node 1.

click the "log" icon of node1 to open the module log and look for error messages such as a checker detecting an error and stopping the module.
click on start_prim in the log: output messages of the script are displayed on the right and errors can be detected such as a service incorrectly started.

If the cluster is in WAIT (red) not uptodate, STOP (red) not uptodate state, stop the WAIT node and force its start as primary.

13. Testing

Stop the PRIM node by scrolling down its contextual menu and clicking Stop.
Verify that there is a failover on the SECOND node which should become ALONE (green).
And with Microsoft Management Console (MMC) on Windows or with command lines on Linux, check the failover of Linux services (stopped on node 1 in the stop_prim script and started on node 2 in the start_prim script).

If ALONE (green) is not reached on node2 or if the application is not started, analyze why with the module log of node 2.

click the "log" icon of node2 to open the module log and look for error messages such as a checker detecting an error and stopping the module.
click on start_prim in the log: output messages of the script are displayed on the right and errors can be detected such as a service incorrectly started.

If everything is okay, initiate a start on node1, which will resynchronize the replicated directories from node2.

If things go wrong, stop node2 and force the start as primary of node1, which will restart with its locally healthy data at the time of the stop.

More information on tests in the User's Guide.

15. If necessary, configure a splitbrain checker

See below "What are the different scenarios in case of network isolation in a cluster?" to know if you need to configure a splitbrain checker.
Go to the module configuration and click on Checkers / Splitbrain (see image) to edit the splitbrain parameters.
Save and apply the new configuration to redeploy it on both nodes (module must be stopped on both nodes to save and apply).

Parameters:

Resource name identifies the witness with a resource name: splitbrain.witness. You can change this value to identify the witness.
Witness address is the argument for a ping when a node goes from PRIM to ALONE or from SECOND to ALONE. Change this value with the IP of the witness (a robust element, typically a router).
Note: you can set several IPs separated by white spaces. Pay attention that the IP addresses must be accessible from one node but not from the other in the event of network isolation.

When there is a network isolation, the behavior with a split-brain checker is:

a split-brain checker has been configured with the IP address of a witness (typically a router),
the split-brain checker operates when a server goes from PRIM to ALONE or from SECOND to ALONE,
in case of network isolation, before going to ALONE, both nodes test the IP address,
the node which can access the IP address goes to ALONE, the other one goes to WAIT,
when the isolation is repaired, the WAIT node resynchronizes its data and becomes SECOND.

Note: If the witness is down or disconnected, both nodes go to WAIT and the application is no more running. That's why you must choose a robust witness like a router.

Internals

Internals of a SafeKit / Linux high availability cluster with synchronous replication and failover

Go to the Advanced Configuration tab in the console, for editing these files

Internal files of the Linux mirror.safe module

userconfig.xml (description in the User's Guide)

<!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"