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Network isolation case and power outage case in a cluster

Evidian SafeKit

What are the different scenarios in case of network isolation in a cluster?

A single network

When there is a network isolation, the default behavior is:

  • as heartbeats are lost for each node, each node goes to ALONE and runs the application with its virtual IP address (double execution of the application modifying its local data),
  • when the isolation is repaired, one ALONE node is forced to stop and to resynchronize its data from the other node,
  • at the end the cluster is PRIM-SECOND (or SECOND-PRIM according the duplicate virtual IP address detection made by Windows).

Two networks with a dedicated replication network

When there is a network isolation, the behavior with a dedicated replication network is:

  • a dedicated replication network is implemented on a private network,
  • heartbeats on the production network are lost (isolated network),
  • heartbeats on the replication network are working (not isolated network),
  • the cluster stays in PRIM/SECOND state.

A single network and a splitbrain checker

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.

What are the different scenarios in case of power outage in a cluster?

Primary node power outage

When a power outage stops only the primary node:

  • there is an automatic failover on the secondary node, which becomes ALONE and restarts the application,
  • when node 1 is rebooted, it becomes SEDOND after resynchronization of replicated data,
  • the roles of primary and secondary can be swapped by an adminsitrator if needed.

Secondary node power outage

When a power outage stops only the secondary node:

  • there is no failover, the primary becomes ALONE and the application continues its execution on node 1,
  • when node 2 is rebooted, it becomes SEDOND after resynchronization of replicated data.

General power outage - case 1

When a power outage stops both nodes, the default behavior is:

  • both nodes goes to STOP,
  • when node 1 is rebooted, it does not go into ALONE state and does not restart the application because it doesn't know if it has the up-to-date data. So it goes to the WAIT state waiting for the restart of the other node,
  • when node 2 is rebooted, both nodes return to their previous PRIM/SECOND states.

General power outage - case 2

When a power outage stops both nodes, the behavior with syncdelta is :

  • syncdelta is set for example to 10 minutes in the configuration meaning that start of a node is accepted even if its data is 10 minutes behind the last sync,
  • when node 1 is rebooted, it goes to ALONE and restarts the application assuming that the restart is done within 10 minutes after the power failure,
  • when node 2 is rebooted, it becomes SECOND after resynchronization of replicated data,
  • Note: if node 2 is rebooted the first, then it becomes ALONE and node 1 will become SECOND at its start.

How the SafeKit mirror cluster works with Windows or Linux?

Step 1. Real-time replication

Server 1 (PRIM) runs the Windows or Linux application. Clients are connected to a virtual IP address. SafeKit replicates in real time modifications made inside files through the network. 

File replication at byte level in a mirror Windows or Linux cluster

The replication is synchronous with no data loss on failure contrary to asynchronous replication.

You just have to configure the names of directories to replicate in SafeKit. There are no pre-requisites on disk organization. Directories may be located in the system disk.

Step 2. Automatic failover

When Server 1 fails, Server 2 takes over. SafeKit switches the virtual IP address and restarts the Windows or Linux application automatically on Server 2.

The application finds the files replicated by SafeKit uptodate on Server 2. The application continues to run on Server 2 by locally modifying its files that are no longer replicated to Server 1.

Failover of Windows or Linux in a mirror cluster

The failover time is equal to the fault-detection time (30 seconds by default) plus the application start-up time.

Step 3. Automatic failback

Failback involves restarting Server 1 after fixing the problem that caused it to fail.

SafeKit automatically resynchronizes the files, updating only the files modified on Server 2 while Server 1 was halted.

Failback in a mirror Windows or Linux cluster

Failback takes place without disturbing the Windows or Linux application, which can continue running on Server 2.

Step 4. Back to normal

After reintegration, the files are once again in mirror mode, as in step 1. The system is back in high-availability mode, with the Windows or Linux application running on Server 2 and SafeKit replicating file updates to Server 1.

Return to normal operation in a mirror Windows or Linux cluster

If the administrator wishes the application to run on Server 1, he/she can execute a "swap" command either manually at an appropriate time, or automatically through configuration.

Choose between redundancy at the application level or at the virtual machine level

Redundancy at the application level

In this type of solution, only application data are replicated. And only the application is restared in case of failure.

Application HA - redundancy at the application level

This solution requires a technical understanding of the application. You need to define which services to restart, specify the application folders to replicate, and configure a virtual IP address for failover.

This solution is platform agnostic and works with applications inside physical machines, virtual machines, in the Cloud. Any hypervisor is supported (VMware, Hyper-V...).

Redundancy at the virtual machine level

In this type of solution, the full Virtual Machine (VM) is replicated (Application + OS). And the full VM is restarted in case of failure.

VM HA - redundancy at the virtual machine level

The advantage of this solution is that it does not require a technical understanding of the application; you only need to define the location of the VM files where the application is installed. If you do not know how the application works, this is the best solution.

This solution works with Windows/Hyper-V and Linux/KVM but not with VMware. This is an active/active solution with several virtual machines replicated and restarted between two nodes.

Typical usage with SafeKit

Why a replication of a few Tera-bytes?

Resynchronization time after a failure (step 3)

  • 1 Gb/s network ≈ 3 Hours for 1 Tera-bytes.
  • 10 Gb/s network ≈ 1 Hour for 1 Tera-bytes or less depending on disk write performances.

Alternative

Why a replication < 1,000,000 files?

  • Resynchronization time performance after a failure (step 3).
  • Time to check each file between both nodes.

Alternative

  • Put the many files to replicate in a virtual hard disk / virtual machine.
  • Only the files representing the virtual hard disk / virtual machine will be replicated and resynchronized in this case.

Why a failover ≤ 32 replicated VMs?

  • Each VM runs in an independent mirror module.
  • Maximum of 32 mirror modules running on the same cluster.

Alternative

  • Use an external shared storage and another VM clustering solution.
  • More expensive, more complex.

Why a LAN/VLAN network between remote sites?

Alternative

  • Use a load balancer for the virtual IP address if the 2 nodes are in 2 subnets (supported by SafeKit, especially in the cloud).
  • Use backup solutions with asynchronous replication for high latency network.

SafeKit Solutions and Quick Installation Guides

New application (real-time replication and failover)

New application (network load balancing and failover)

Database (real-time replication and failover)

Web (network load balancing and failover)

Full VM or container real-time replication and failover

Amazon AWS

Google GCP

Microsoft Azure

Other clouds

Physical security (real-time replication and failover)

Siemens (real-time replication and failover)

SafeKit High Availability Differentiators