Active active cluster with real-time replication
Applications in mutual failover with real-time crossed data replication
The SafeKit software provides a simple active active cluster with real-time replication on Windows or Linux. Applications are running on two servers in mutual failover. Each server is backup of the other server. And each application works on its own replicated file directories.
If one server fails in an active active cluster, all applications will be active on the same server. After restart of the failed server, applications will return to run on their default primary server.
How works an active active cluster with real-time replication?
The SafeKit active active cluster is simply configured by deploying two mirror modules with real-time replication. Each mirror module contains its own configuration of virtual IP address, real-time replicated directories and restart scripts.
Note that:
- the 2 applications Appli1 and Appli2 must be installed on each server for application failover,
- this architecture is not reduced to 2 applications: N application modules can be deployed on 2 servers with a maximum of 25 modules,
- each mirror module will have its own virtual IP address, its own replicated file directories and its own recovery scripts.
Example of an active active cluster with real-time replication
The Hyper-V replication automatic failover and load balancing solution is an example of an active-active cluster with N mirror modules replicating N virtual machines between two physical servers. In this configuration, a mirror module replicates a full virtual machine and restarts the virtual machine in case of failure. There is no virtual IP in such a module because restarting a VM on a server makes automatically the IP failover of the VM. And an application is installed in one single VM in this architecture.
As presented in the Hyper-V cluster demonstration, an active active cluster can implement real-time data replication for directories inside the operating system disk. Note that replicating data inside the system disk is possible with byte-level file replication and impossible with block-level disk replication.
Another way to implement an active active cluster with SafeKit is the network load balancing cluster described here.
FAQ on Evidian SafeKit
Best use cases [+]
Customers [+]
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Video Surveillance (CCTV) [+]
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Transport [+]
Paris transport company (RATP) chose the SafeKit high availability and load balancing solution for the centralized control room of line 1 of the Paris subway.
20 SafeKit clusters are deployed on Windows and Linux.
Stéphane Guilmin, RATP, Project manager says:
“Automation of line 1 of the Paris subway is a major project for RATP, requiring a centralized command room (CCR) designed to resist IT failures. With SafeKit, we have three distinct advantages to meet this need. Firstly, SafeKit is a purely software solution that does not demand the use of shared disks on a SAN and network boxes for load balancing. It is very simple to separate our servers into separate machine rooms. Moreover, this clustering solution is homogeneous for our Windows and Unix platforms. SafeKit provides the three functions that we needed: load balancing between servers, automatic failover after an incident and real time data replication.”
And also, Philippe Marsol, Atos BU Transport, Integration Manager says:
“SafeKit is a simple and powerful product for application high availability. We have integrated SafeKit in our critical projects like the supervision of Paris metro Line 4 (the control room) or Marseille Line 1 and Line 2 (the operations center). Thanks to the simplicity of the product, we gained time for the integration and validation of the solution and we had also quick answers to our questions with a responsive Evidian team.”
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Application high availability modules [+]
SafeKit Webinar [+]
Pricing - Free trial [+]
Comparison with other solutions [+]
Demonstration of a mirror cluster [+]
What are the advantages of the mirror cluster [+]
- Low Complexity
- Plug&Play deployment with no specific skills
- Suitable for large deployments in many sites (very simple to deploy)
- 2 nodes (3 nodes replication possibility)
- No shared storage requirement
- No Domain Controller requirement
- Well documented API and support
- Synchronous data replication (no data loss in case of failure)
- Replicated directories can be in the system disk
- Supports multiple heartbeats and vitual IP addresses
- Offers configurable software, hardware and network checkers
- For the quorum, does not require a special disk or a third machine or a dedicated link between both servers
- Automatic failover of services with a recovery time in the order of one minute
- Automatic failback when a server comes back after a failure (no manual operation)
- A very simple console to deploy the solution and to maintain it afterwards for end-customer
- Supports human errors (40% of causes of unavailability) thanks to its simplicity
- Supports software failures (40% of causes of unavailability): regression on software update (version N and N+1 can coexist), Operating System frozen, software bug
- Supports hardware and environment failures (20% of causes of unavailability), including the complete failure of a computer room with 2 nodes in two remote sites
What is the recovery time (RTO) in a mirror cluster [+]
RTO is the time during which the application is unavailable in case of failure. RTO of the SafeKit mirror solution is in the order of 1 mn.
For a hardware failure, RTO = heartbeat timeout (default 30 s, can be changed in userconfig.xml) + time to restart services.
For a software failure or an administrator restart, RTO = time to (cleanly) stop services + time to restart them.
Be careful, with solutions that reboot a full virtual machine in case of failure, the RTO is unpredictable as manual operations may be required after a hardware crash to reboot the virtual machine.
What is the data loss (RPO) in a mirror cluster [+]
RPO reflects the data loss in case of failure. RPO of the SafeKit mirror solution is 0 as the replication is synchronous and real-time.
Be careful, with asynchronous replication, RPO is not 0 and there is data loss in case of failure when the application restarts on the secondary server.
