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Evidian > Products > High Availability Software - Zero Extra Hardware > High Availability and Disaster Recovery (HADR) at the same time

High Availability and Disaster Recovery (HADR) at the same time

Evidian SafeKit

When to combine high availability and disaster recovery (HADR)?

HADR architecture with a fast network

Architecture combining High Availability and Disaster Recovery (HADR)

2 nodes are put in 2 remotes sites for implementing at the same time high availability and disaster recovery (HADR).

HADR solution with a fast nework

A solution without shared disk like Evidian SafeKit is required with 2 nodes replicating data synchronously and in real-time.

Because there is no data loss with synchronous replication, an automatic application failover can be implemented in case of failures.

Why an extended LAN/VLAN?

When NOT to combine high availability and disaster recovery (HADR)?

HADR architecture with a slow network

Architecture where High Availability and Disaster Recovery (HADR) are NOT combined

2 nodes are put in the first site for high availability and a backup solution is implemented for disaster recovery.

HADR solution with a slow network

A high availability solution like Evidian SafeKit is implemented in the first site with synchronous real-time replication and automatic failover.

And a backup solution is implemented for asynchronous replication through the slow network to the disaster recovery site.

Failover to the disaster recovery site

  • As there is data loss with a backup solution, the failover is manual and decided by an administrator.
  • Backups are restored on servers at the disaster recovery site. Today, virtual machine backups are often implemented to facilitate this restoration (like Veeam).
  • Rerouting to the disaster recovery site is made at the DNS level. The rerouting time depends on DNS cache timeout. And this may require restarting clients that perform their DNS resolution only once during their initialization.

High availability does not remove the need for a backup solution

Real-time replication of high availability solutions like SafeKit does not remove the need for a backup solution. Some events are recoverable only with a backup solution. For example, a ransomware encrypting replicated data on the primary server will encrypt data on the secondary with a real-time replication. Only a backup solution with a retention policy can solve a ransomware attack.

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

With this solution, restart scripts must be written to restart the application.

We deliver application modules to implement redundancy at the application level (like the Windows module provided in the free trial below). They are preconfigured for well known applications and databases. You can customize them with your own services, data to replicate, application checkers. And you can combine application modules to build advanced multi-level architectures.

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

  • Solution for a new application (restart scripts to write): Windows, Linux

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 is that there is no restart scripts to write per application and no virtual IP address to define. 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.

More comparison between VM HA vs Application HA

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 Overview

SafeKit 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