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Active active cluster with real-time replication

Evidian SafeKit

How the active active cluster with real-time replication works?

Active- active cluster with real-time replication

Applications are running on two servers in mutual failover. Each server is backup of the other server. And each application works on its own replicated file directories.

SafeKit active active cluster with real-time replication

If one server fails in an active active cluster, all applications will be active on the same server. After restart of the failed server, applications will return to run on their default primary server.

How it works?

The SafeKit active active cluster is simply configured by deploying two mirror modules with real-time replication. Each mirror module contains its own configuration of virtual IP address, real-time replicated directories and restart scripts.

Note that:

  • the 2 applications Appli1 and Appli2 must be installed on each server for application failover,
  • this architecture is not reduced to 2 applications: N application modules can be deployed on 2 servers with a maximum of 25 modules,
  • each mirror module will have its own virtual IP address, its own replicated file directories and its own recovery scripts.

Example of an active active cluster

The Hyper-V or KVM solution with replication, automatic failover and load balancing of VMs is an example of an active-active cluster with N mirror modules replicating N virtual machines between two physical servers.

In this configuration, a mirror module replicates a full virtual machine and restarts the virtual machine in case of failure.

There is no virtual IP address in such a module because restarting a VM on a server makes automatically the failover of the VM physical IP address.

And a critical application is installed inside a VM in this architecture.

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