Mirror cluster with file replication at byte level and application failover
Evidian SafeKit
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.
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).
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.
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.
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.
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
SafeKit high availability software on Windows and Linux saves on 1/ costly external shared or replicated storage, 2/ load balancing boxes, 3/ enterprise editions of OS and databases
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
The cluster configuration is very simple and made
by means of application modules.
New services and new replicated directories can be added to an existing application module
to complete a high availability solution
All the configuration of clusters is made using a simple centralized web administration console
There is no domain controller or active directory to configure as with Microsoft cluster
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
All SafeKit clustering features are working for 2 servers in remote sites. Replication requires an extended LAN type network (latency = performance of synchronous replication, bandwidth = performance of resynchronization after failure).
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 with the "healh check" of SafeKit.
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
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
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
Key differentiators of a farm cluster with load balancing and failover
Evidian SafeKit farm cluster with load balancing and failover
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
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
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 with the help of the SafeKit health check.
Thus you can implement load balancing but also all the clustering features of SafeKit, in particular monitoring and automatic recovery of the critical application on application servers
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
Key differentiators of the SafeKit high availability technology
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
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
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
SafeKit Modules for Plug&Play High Availability Solutions
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SafeKit high availability software on Windows and Linux saves on 1/ costly external shared or replicated storage, 2/ load balancing boxes, 3/ enterprise editions of OS and databases
This is not the case with asynchronous replication
