The basic mechanism for synchronizing two servers and detecting server failures is the heartbeat, which is a monitoring data flow on a network shared by a pair of servers.
The SafeKit software supports as many heartbeats as there are networks shared by two servers. The heartbeat mechanism is used to implement Windows and Linux clusters. It is integrated within the SafeKit mirror cluster with real-time file replication and failover.
In normal operation, the two servers exchange their states (PRIM, SECOND, the resource states) through the heartbeat channels and synchronize their application start and stop procedures. In particular, in case of an application failover because of a software failure or a manual operation, the stop script which stops the application is first executed on the primary server, before executing the start script on the secondary server. Thus, replicated data on the secondary server are in a safe state corresponding to a clean stop of the application.
If all heartbeats are lost, it is interpreted as if the other server was down, and the local server switches to the ALONE state. If it is the SECOND server which goes to the ALONE state, then there is an application failover with restart of the application on the secondary server. Although not mandatory, it is better to have two heartbeat channels on two different networks for synchronizing the two servers in order to separate the network failure case from the server failure one.
Most often, a HA cluster securing a critical application in a data center is implemented with two servers in two geographically remote computer rooms to support the disaster of a full room.
In situation of transient network isolation between both computer rooms, the split brain problem arises. Both servers may start the critical application.
With a hardware failover cluster, this situation must not arise because a double execution means a concurrent access on a shared storage and a potential corruption of the critical application data. That's why a cluster quorum is implemented with a third quorum server or a special quorum disk or even a remote hardware reset when possible to avoid this concurrent execution of the critical application.
Unfortunately this new quorum devices add cost and complexity to the overall clustering architecture. And the system is not immune to a freeze of an OS: when the OS resumes from the freeze, there are a double execution of the application, even with the aforementioned mechanisms and potentially with corruption of data on the shared storage.
With the SafeKit HA software, the quorum within a Windows or Linux cluster requires no third quorum server, no quorum disk and no remote hardware reset. A simple split brain checker is sufficient for the SafeKit quorum to avoid the double execution of an application.
The split brain checker, on the the loss of all heartbeats between servers, selects only one server to become the primary. The other server is not up-to-date anymore and goes into the WAIT state, until it receives the other server's heartbeats again, in which case it automatically resynchronizes the replicated data from the other server.
The primary server election is based on the ping of an IP address, called the witness. The witness is typically a router that does not crash. In case of network isolation, only the server with access to the witness will be primary ALONE, the other will go to WAIT. The witness is not tested permanently but only when the system switches over. If at the time of failover, the witness is down, the cluster goes into the WAIT-WAIT state and an administrator can choose to restart one of the nodes as primary through the SafeKit web interface.
Consider the critical case of an OS freeze or a network isolation without a split brain checker configured. A SafeKit high-availability cluster supports dual execution of a critical application without data corruption. In this case, the primary server continues to run the application in the ALONE state. And the secondary server restarts the application and also goes into the ALONE state. The replicated directories are isolated and each application is working on its own data in its own directory.
When the network is reconnected, a sacrifice must be made by shutting down the application on one of the two servers. This sacrifice shutdowns the application on one server and causes data reintegration from the primary one. After this reintegration, the data are once again in mirror mode between a primary and a secondary server.
All these operations are automatic. The complexity of the heartbeat, failover and quorum management within the cluster is integrated inside the SafeKit product and transparent for users of SafeKit. Thus, people deploying SafeKit without specific skill can do it on two standard servers in any configuration, local or remote. In addition, the configuration is the same for a Windows or a Linux cluster.
Important: if you choose another solution based on a shared or replicated disk, make sure that after an OS freeze, the server that comes out of the freeze can no longer access the shared or replicated disk, because two servers accessing the same disk via its file system leads to data corruption.
Click on the Windows or Linux module to understand and try the solution
Mirror modules (replication and failover)
|Microsoft SQL Server||-|
|Milestone XProtect (based on Microsoft SQL Server)||-|
|Hanwha SSM (based on PostgreSQL)||-|
|Generic mirror module|
Farm modules (load balancing and failover)
|Generic farm module|
Click on the mirror/farm architecture to understand and try the solution
Real-time replication and failover cluster
Load balancing and failover cluster
In video surveillance systems, Evidian SafeKit implements high availability with synchronous replication and failover of
Harmonic is using SafeKit as a software OEM high availability solution and deploys it with its TV broadcasting solutions over satellites, terrestrials, cable, IPTV. Over 80 SafeKit clusters are deployed on Windows for replication of Harmonic database and automatic failover of the critical application. Philippe Vidal, Product Manager, Harmonic says: “SafeKit is the ideal application clustering solution for a software publisher looking for a simple and economical high availability software. We are deploying SafeKit worldwide and we currently have more than 80 SafeKit clusters on Windows with our critical TV broadcasting application through terrestrial, satellite, cable and IP-TV. SafeKit implements the continuous and real-time replication of our database as well as the automatic failover of our application for software and hardware failures. Without modifying our application, it was possible for us to customize the installation of SafeKit. Since then, the time of preparation and implementation has been significantly reduced.”
The European Society of Warranties and Guarantees in Natixis uses SafeKit as a high availability solution for its applications.
Over 30 SafeKit clusters are deployed on Unix and Windows in Natixis.
Fives Syleps, the Sydel software editor implements high availability of its ERP with SafeKit and deploys the solution in the food industry.
Tony Myers, Director of Business Development says:
“By developing applications for air traffic control, Copperchase is in one of the most critical business activities. We absolutely need our applications to be available all the time. We have found with SafeKit a simple and complete clustering solution for our needs. This software combines in a single product load balancing, real time data replication with no data loss and automatic failover. This is why, Copperchase deploys SafeKit for air traffic control in airports in the UK and the 30 countries where we are present.”
Software vendor Wellington IT deploys SafeKit high availability with its banking application for Credit Unions in Ireland and UK.
Peter Knight, Sales Manager says:
“Business continuity and disaster recovery are a major concern for our Locus banking application deployed in numerous Credit Unions around Ireland and the UK. We have found with SafeKit a simple and robust solution for high availability and synchronous replication between two servers with no data loss. With this software solution, we are not dependent on a specific and costly hardware clustering solution. It is a perfect tool to provide a software high availability option to an application of a software vendor.”
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.
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.”
The software integrator Systel deploys SafeKit high-availability solution in firefighter and emergency medical call centers.
Marc Pellas, CEO says:
“SafeKit perfectly meets the needs of a software vendor. Its main advantage is that it brings in high availability through a software option that is added to our own multi-platform software suite. This way, we are not dependent on a specific and costly hardware clustering solution that is not only difficult to install and maintain, but also differs according to client environments. With SafeKit, our firefighter call centers are run with an integrated software clustering solution, which is the same for all our customers, is user friendly and for which we master the installation up to after-sales support.”
ERP high availability and load balancing of the French army (DGA) are made with SafeKit.
Alexandre Barth, Systems administrator says:
“Our production team implemented the SafeKit solution without any difficulty on 14 Windows and Linux clusters. Our critical activity is thus secure, with high-availability and load balancing functions. The advantages of this product are easy deployment and administration of clusters, on the one hand, and uniformity of the solution in the face of heterogeneous operating systems, on the other hand.”
|OEM Software||Distributed Enterprise||Remote Sites|
|A software publisher uses SafeKit as an OEM software for high availability of its application||A distributed enterprise deploys SafeKit in many branches without specific IT skills||SafeKit is deployed in two remote sites without the need for replicated bays of disks through a SAN|
1 - The ideal product for a software publisher
“SafeKit is the ideal application clustering solution for a software publisher. We currently have deployed more than 80 SafeKit clusters worldwide with our critical TV broadcasting application.”
2 - The product very easy to deploy for a reseller
“WithNCompany has deployed in South Korea many SafeKit high availability solutions with the Samsung Video Surveillance Platform. SafeKit is appreciated because the product is easy to install and very quickly deployed.”
3 - The product to gain time for a system integrator
“Thanks to a simple and powerful product, we gained time in the integration and validation of our critical projects like the supervision of Paris and Marseille metro lines (the control rooms).”
High availability architectures comparison
(click on the feature for more information)
|Feature||SafeKit cluster||Other clusters|
|Software clustering vs hardware clustering|| |
A simple software cluster with the SafeKit package 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|| |
SafeKit application HA supports hardware failure, software failure, human errors with quick recovery time
Full virtual machines HA supports only hardware failure with a VM reboot and an unknown recovery time if the OS reboot does not work
|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 byte-level file replication and is simply configured with 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
|Network load balancing|| |
No special network configuration is required in a SafeKit cluster for network load balancing
Special network configuration is required in other clusters for network load balancing
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.
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.
RTO is the time during which the application is unavailable in case of failure. RTO of the SafeKit farm solution is in the order of a few seconds on hardware failure.
For a hardware failure, RTO = failure detection timeout through monitoring channels (default a few seconds): after the timeout the load balancing filters are reconfigured.
For a software failure or an administrator restart, RTO = time to (cleanly) stop services + time to restart them.