There are several elements that make this architecture complex to implement:
- on failover, switching the shared storage requires low level instructions which are storage manufacturer dependent,
- recovery procedure on the file system (FS) must be passed before restarting the application,
- if both file systems on both nodes access the same raw disk at the same time, the full file system will be corrupted,
- to avoid a double access, a quorum disk must be configured.
- There are several elements that make this architecture simple to implement:
- on failover, switching the shared storage means only remounting the external file system,
- no recovery procedure on the file system must be passed before restarting the application,
- if both nodes access the same shared file system at the same time, the full file system will be not be corrupted,
- however, there is still the possibility of a double execution of the same application corrupting its data in the shared storage when nodes are isolated.
There are no such issues with SafeKit because its replication and failover solution does not require a shared storage.
However, if SafeKit must manage a shared storage:
- use a NAS SMB shared storage or a NAS NFS shared storage,
- put in the restart scripts the mount/umount of the external file system,
- configure the SafeKit split brain checker to avoid a double execution of the same application accessing the shared storage when nodes are isolated.
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.
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.
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 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.
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.
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.
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 mirror 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...).
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.
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.
- Solution for a new application (no restart script to write): Windows/Hyper-V, Linux/KVM
No shared disk - 
Shared disk and specific extenal bay of disk
