A mirror cluster for critical database applications

A SafeKit mirror cluster with file replication at byte level provides a simple high availability solution to critical database applications. The SafeKit software implementing a mirror cluster runs either on Windows or Linux (even Windows editions for PCs). It implements synchronous real-time byte-level file replication. The resulting solution is working like a cluster connected to a replicated mirror SAN but without the costs and the complexity of hardware clustering solutions.

A mirror cluster: file replication at byte level and failover

The mirror cluster is a primary-backup high availability solution. The application runs on a primary server and is restarted automatically on a secondary server if the primary server fails. The software data replication is configured at the file level with the name of the file directories to replicate. The directory can contain database files or flat files. With synchronous byte-level file replication, this architecture is particularly suited to providing high availability for back-end applications with critical data to protect against failure. SafeKit provides a generic mirror module on Windows and Linux to build a mirror cluster as presented in the following video. You can write your own mirror module for your application. Microsoft SQL Server, MySQL, Oracle, PostgreSQL, Firebird are examples of mirror modules. And from a mirror module, you can also replicate a full Virtual Machine with automatic failover inside an Hyper-V cluster. Note that this article explains the difference between VM HA vs Application HA.

How the SafeKit mirror cluster works?

Step 1. File replication at byte level in a mirror cluster

This step corresponds to the following figure. Server 1 (PRIM) runs the application. Users are connected to the virtual IP address of the mirror cluster. SafeKit replicates files opened by the application in real time. Only changes made by the application in the files are replicated across the network, thus limiting traffic (byte-level file replication).

File replication at byte level in a mirror cluster

With a software data replication at the file level, only names of file directories are configured in SafeKit. There are no pre-requisites on disk organization for the two servers. Directories to replicate may be located in the system disk. SafeKit implements synchronous replication with no data loss on failure contrary to asynchronous replication.

Step 2. Failover

When Server 1 fails, Server 2 takes over. SafeKit switches the cluster's virtual IP address and restarts the application automatically on Server 2. The application finds the files replicated by SafeKit uptodate on Server 2, thanks to the synchronous replication between Server 1 and Server 2. The application continues to run on Server 2 by locally modifying its files that are no longer replicated to Server 1.

Failover in a mirror cluster

The failover time is equal to the fault-detection time (set to 30 seconds by default) plus the application start-up time. Unlike disk replication solutions, there is no delay for remounting file system and running file system recovery procedures.

Step 3. Failback and reintegration

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. This reintegration takes place without disturbing the applications, which can continue running on Server 2. 

Failback in a mirror cluster

If SafeKit was cleanly stopped on server 1, then at its restart, only the modified zones inside files are reintegrated, according to modification tracking bitmaps.

If server 1 crashed (power off), the modification bitmaps are not reliable and not used. All the files bearing a modification timestamp more recent than the last known synchronization point between both servers (minus a graceful delay, typically one hour) are reintegrated.

Step 4. Return to byte-level file replication in the mirror cluster

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

Passive active mirror cluster with data replication

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.

SafeKit High Availability Differentiators against Competition

Key differentiators of a mirror cluster with replication and failover

Evidian SafeKit mirror cluster with real-time file replication and failover

All clustering features All clustering features

Like  A SafeKit cluster runs on Windows and Linux without the need for expensive shared or replicated disk bays

Like  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

Dislike  This is not the case with replication-only solutions like replication at the database level which implements only replication

Like   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

Like  SafeKit implements quick application restart in case of failure: around 1 mn or less (see RTO/RPO here)

Dislike  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 Synchronous replication

Like  The real-time replication is synchronous with no data loss on failure

Dislike  This is not the case with asynchronous replication

Fully automated failback procedure Automatic failback

Like  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

Dislike  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

Like  The replication is working for databases but also for any files which shall be replicated

Dislike  This not the case for replication at the database level

File replication vs disk replication File replication vs disk replication

Like  The replication is based on file directories that can be located anywhere (even in the system disk)

Disike  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 File replication vs shared disk

Like  The servers can be put in two remote sites

Dislike  This is not the case with shared disk solutions

Remote sites Remote sites

Like  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

Like  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

Like  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 Quorum

Like  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

Like  This is not the case for most clustering solutions where a 3rd server is required for the quorum

Active/active cluster Active active mirror cluster

Like  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

Dislike  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 Uniform high availability solution

Like  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

Dislike  This is not the case with an architecture mixing different technologies for load balancing, replication and failover

Evidian SafeKit farm cluster with load balancing and failover

No load balancer or dedicated proxy servers or special multicast Ethernet address No load balancer or dedicated proxy servers

Like   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

Dislike  This is not the case with network load balancers

Dislike  This is not the case with dedicated proxies on Linux

Dislike  This is not the case with a specific multicast Ethernet address on Windows

All clustering features All clustering features

Like  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

Dislike  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

Like   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 Remote sites

Like   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

Like   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 Uniform high availability solution

Like  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

Dislike  This is not the case with an architecture mixing different technologies for load balancing, replication and failover

High availability architectures comparison

Feature

SafeKit cluster

Other clusters

Software clustering vs hardware clustering
More information... 		A software cluster with SafeKit installed on two servers

Like  A simple software cluster with the SafeKit package just installed on two servers		 Hardware clustering with external shared storage Network load balancers or dedicated proxy servers



Dislike  Complex hardware clustering with external storage or network load balancers
Shared nothing vs a shared disk cluster
More information... 		SafeKit shared-nothing cluster: easy to deploy even in remote sites

Like  SafeKit is a shared-nothing cluster: easy to deploy even in remote sites		 Shared disk cluster: complex to deploy

Dislike  A shared disk cluster is complex to deploy
Application High Availability vs Full Virtual Machine High Availability
More information...

Like  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)		 Virtual machines high availability supports only hardware failure with an recovery time depending on the OS reboot

Dislike  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 SafeKit high availability vs fault-tolerance

Like  No dedicated server. 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)		 Fault tolerance system

Dislike  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
More information...

Like  SafeKit implements real-time synchronous replication with no data loss in case of failure		 Asynchronous replication with data loss on failure

Dislike  With asynchronous replication, there is data loss on failure
Byte-level file replication vs block-level disk replication
More information... 		SafeKit cluster with byte-level file replication: simply replicates directories even in the system disk

Like  SafeKit implements real-time byte-level file replication and is simply configured with application directories to replicate even in the system disk		 Cluster with block-level disk replication: complex and require to put application data in a special disk

Dislike  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
More information... 		Simple quorum in a SafeKit cluster with a split brain checker configured on a router

Like  To avoid 2 masters, SafeKit proposes a simple split brain checker configured on a router		 Complex quorum in other clusters: third machine, special quorum disk, remote hardware reset

Dislike  To avoid 2 masters, other clusters require a complex configuration with a third machine, a special quorum disk, a special interconnect
Network load balancing
More information... 		No special network configuration in a SafeKit cluster

Like  No dedicated proxy servers and no special network configuration are required in a SafeKit cluster for network load balancing		 Special network configuration in other clusters

Dislike  Special network configuration is required in other clusters for network load balancing

SafeKit Modules for Plug&Play High Availability Solutions

SafeKit Modules for Plug&Play High Availability Solutions

Network load balancing and failover: click on the blue buttons

Farm modules

Windows

Linux

IIS-
Apache
New application
Amazon AWS farm
Microsoft Azure farm
Google GCP farm
Cloud generic farm

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-
New application
Amazon AWS mirror
Microsoft Azure mirror
Google GCP mirror
Cloud generic mirror

Demonstrations of SafeKit High Availability Software

SafeKit Webinar

This webinar presents in 10 minutes Evidian SafeKit.

In this webinar, you will understand:

  • mirror and farm clusters
  • cost savings against hardware clustering solutions
  • best use cases
  • the integration process for a new application

Microsoft SQL Server Cluster

This video shows a mirror module configuration with synchronous real-time replication and failover.

The file replication and the failover are configured for Microsoft SQL Server but it works in the same manner for other databases.

Free trial here

Apache Cluster

This video shows a farm module configuration with load balancing and failover.

The load balancing and the failover are configured for Apache but it works in the same manner for other web services.

Free trial here

Hyper-V Cluster

This video shows a Hyper-V cluster with full replications of virtual machines.

Virtual machines can run on both Hyper-V servers and they are restarted in case of failure.

Free trial here