VM HA vs Application HA - Evidian

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VM HA vs Application HA

Virtual Machine HA (VM HA) implements high availability at the hypervisor level by replicating and restarting full virtual machines, while Application HA implements high availability at the application level by replicating only application data and restarting the application. The two solutions do not offer the same level of high availability as explained in this article.

Application HA with SafeKit

SafeKit offers a complete solution for application HA that solves hardware failures (20% of problems) including the complete failure of a computer room, software failures (40% of problems) including smooth upgrade server by server and human errors (40% of problems) thanks to simplicity of its administration.

Although there is an integration per application (restart scripts), the integration is simple and is made through application modules. Many examples of HA modules are given here.

VM HA with SafeKit

VM HA solves only problems of hardware failures (20% of problems).

SafeKit offers a simple and economical solution to build virtual machines high availability on Hyper-V with

2 Windows Hyper-V physical servers,
1 mirror module for replication and failover of 1 full VM,
N mirror modules for N VMs (limited to 25 VMs on 2 physical servers)
an active-active cluster where VMs are distributed on the two servers,
and a centralized web console to manage failover of all VMs.

The architecture does not require a shared disk or a replicated SAN if the two servers are in two remote computer rooms.

VM HA vs Application HA

VM HA with the SafeKit Hyper-V module	Application HA with SafeKit application modules

Replicates too much data (App+OS)	Replicates only application data
Reboot of VM on hypervisor 2 if hypervisor 1 crashes: recovery time depending on the OS reboot	Quick recovery time with restart of App on OS2 if crash of VM1: around 1 mn or less (see RTO/RPO here)
No application checker and software failover	Application checker and software failover
Risk of downtime when fixing OS or App (single image)	Smooth update of OS1 or OS2 or App (two images)
Generic solution for any application / OS	Simple integration with application modules

SafeKit hyperv.safe vs Microsoft Hyper-V Cluster & VMware HA

SafeKit with the Hyper-V module	Microsoft Hyper-V Cluster & VMware HA

No shared disk - synchronous real-time replication instead with no data loss	Shared disk and specific extenal bay of disk
Remote sites = no SAN for replication	Remote sites = replicated bays of disk across a SAN
No specific IT skill to configure the system	Specific IT skills to configure the system

Note that the Hyper-V/SafeKit solution is limited to replication and failover of 25 VMs.

More comparison on high availability solutions

High availability architectures comparison
Feature	SafeKit cluster	Other clusters
Software clustering vs hardware clustering	A simple software cluster with the SafeKit package just installed on two servers	Complex hardware clustering with external storage or network load balancers
Shared nothing vs a shared disk cluster	SafeKit is a shared-nothing cluster: easy to deploy even in remote sites	A shared disk cluster is complex to deploy
Application High Availability vs Full Virtual Machine High Availability	Application HA supports hardware failure and software failure with a quick recovery time (RTO around 1 mn or less). Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)	Full virtual machines HA supports only hardware failure with a VM reboot and a recovery time depending on the OS reboot. Smooth upgrade not possible
High availability vs fault tolerance	No dedicated server with SafeKit. Each server can be the failover server of the other one. Software failure with restart in another OS environment. Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)	Secondary server dedicated to the execution of the same application synchronized at the instruction level. Software exception on both servers at the same time. Smooth upgrade not possible
Synchronous replication vs asynchronous replication	SafeKit implements real-time synchronous replication with no data loss in case of failure	With asynchronous replication, there is data loss on failure
Byte-level file replication vs block-level disk replication	SafeKit implements real-time byte-level file replication and is simply configured with application directories to replicate even in the system disk	Block-level disk replication is complex to configure and requires to put application data in a special disk
Heartbeat, failover and quorum to avoid 2 master nodes	To avoid 2 masters, SafeKit proposes a simple split brain checker configured on a router	To avoid 2 masters, other clusters require a complex configuration with a third machine, a special quorum disk, a special interconnect
Virtual IP address primary/secondary, network load balancing, failover	No dedicated proxy servers and no special network configuration are required in a SafeKit cluster for virtual IP addresses	Special network configuration is required in other clusters for virtual IP addresses. Note that SafeKit offers a health check adapted to load balancers

Best practices of a mirror cluster with replication and failover

Evidian SafeKit mirror cluster with real-time file replication and failover
All clustering features		A SafeKit cluster runs on Windows and Linux without the need for expensive shared or replicated disk bays SafeKit includes all clustering features: synchronous real-time file replication, monitoring of server/network/software failures, automatic application restart, virtual IP address switched in case of failure to reroute clients This is not the case with replication-only solutions like replication at the database level which implements only replication The cluster configuration is very simple and made by means of application modules. There is no domain controller or active directory to configure as with Microsoft cluster SafeKit implements quick application restart in case of failure: around 1 mn or less (see RTO/RPO here) Quick application restart is not ensured with full virtual machines replication. In case of hypervisor failure, a full VM must be rebooted on a new hypervisor with a recovery time depending on the OS reboot as with VMware HA or Hyper-V cluster
Synchronous replication		The real-time replication is synchronous with no data loss on failure This is not the case with asynchronous replication
Fully automated failback procedure		After a failure when a server reboots, the replication failback procedure is fully automatic and the failed server reintegrates the cluster without stopping the application on the only remaining server This is not the case with most replication solutions particularly with replication at the database level. Manual operations are required for resynchronizing a failed server. The application may even be stopped on the only remaining server during the resynchonization of the failed server
Replication of any type of data		The replication is working for databases but also for any files which shall be replicated This not the case for replication at the database level
File replication vs disk replication		The replication is based on file directories that can be located anywhere (even in the system disk) This is not the case with disk replication where special application configuration must be made to put the application data in a special disk
File replication vs shared disk		The servers can be put in two remote sites This is not the case with shared disk solutions
Remote sites and virtual IP address		All SafeKit clustering features are working for 2 servers in remote sites. Performances of replication depends on the interconnect latency for real-time synchronous replication and on the bandwidth for resynchronizing data on a failed server If both servers are connected to the same IP network through an extended LAN between two remote sites, the virtual IP address of SafeKit is working with rerouting at level 2 If both servers are connected to two different IP networks between two remote sites, the virtual IP address can be configured at the level of a load balancer. SafeKit offers a health check: the load balancer is configured with a URL managed by SafeKit which returns OK on the primary server and NOT FOUND else. This solution is implemented for SafeKit in the Cloud but it can be also implemented with a load balancer on premise
Quorum		With remote sites, the solution works with only 2 servers and for the quorum (network isolation), a simple split brain checker to a router is offered to support a single execution This is not the case for most clustering solutions where a 3^rd server is required for the quorum
Active/active cluster		The secondary server is not dedicated to the restart of the primary server. The cluster can be active-active by running 2 different mirror modules This is not the case with a fault-tolerant system where the secondary is dedicated to the execution of the same application synchronized at the instruction level
Uniform high availability solution		SafeKit implements a mirror cluster with replication and failover. But it imlements also a farm cluster with load balancing and failover. Thus a N-tiers architecture can be made highly available and load balanced with the same solution on Windows and Linux (same installation, configuration, administration with the SafeKit console or with the command line interface). This is unique on the market This is not the case with an architecture mixing different technologies for load balancing, replication and failover

Best practices of a farm cluster with load balancing and failover

Evidian SafeKit farm cluster with load balancing and failover
No load balancer or dedicated proxy servers or special multicast Ethernet address		The solution does not require load balancers or dedicated proxy servers above the farm for imlementing load balancing. SafeKit is installed directly on the application servers in the farm. The load balancing is based on a standard virtual IP address/Ethernet MAC address and is working with physical servers or virtual machines on Windows and Linux without special network configuration This is not the case with network load balancers This is not the case with dedicated proxies on Linux This is not the case with a specific multicast Ethernet address on Windows
All clustering features		The solution includes all clustering features: virtual IP address, load balancing on client IP address or on sessions, monitoring of server/network/software failures, automatic application restart with a quick revovery time and a replication option with a mirror module This is not the case with other load balancing solutions. They are able to make load balancing but they do not include a full clustering solution with restart scripts and automatic application restart in case of failure. They do not offer a replication option The cluster configuration is very simple and made by means of application modules. There is no domain controller or active directory to configure on Windows. The solution works on Windows and Linux
Remote sites and virtual IP address		If servers are connected to the same IP network through an extended LAN between remote sites, the virtual IP address of SafeKit is working with load balancing at level 2 If servers are connected to different IP networks between remote sites, the virtual IP address can be configured at the level of a load balancer. SafeKit offers a health check: the load balancer is configured with a URL managed by SafeKit which returns OK on the UP servers and NOT FOUND else. This solution is implemented for SafeKit in the Cloud but it can be also implemented with a load balancer on premise. Thus you can implement load balancing but also all the clustering features of SafeKit including an easy administration of the cluster through the SafeKit web console
Uniform high availability solution		SafeKit imlements a farm cluster with load balancing and failover. But it implements also a mirror cluster with replication and failover. Thus a N-tiers architecture can be made highly available and load balanced with the same solution on Windows and Linux (same installation, configuration, administration with the SafeKit console or with the command line interface). This is unique on the market This is not the case with an architecture mixing different technologies for load balancing, replication and failover

Real-time file replication and failover: click on the blue buttons
Mirror modules	Windows	Linux
Microsoft SQL Server		-
Oracle
MySQL
PostgreSQL
Firebird
Hyper-V		-
Milestone XProtect		-
Hanwha Wisenet SSM		-
Nedap AEOS		-
New application
Amazon AWS mirror
Microsoft Azure mirror
Google GCP mirror
Cloud generic mirror