How Can We Increase the Availability of Kepserver (OPC Server) with EverRun?

This post is focused on determining what redundancy architectures are available from Kepware to increase availability. The architectures offered by the software itself will be determined, and it will also be seen how they can be complemented jointly with EverRun from Stratus Technologies to increase the degree of availability.

First, Kepware offers two ways to achieve redundancy between OPC servers: the software “Redundancy Master” and the “Media Level Redundancy Plug-in”.

REDUNDANCY MASTER
The first of these is the software “Redundancy Master”. As can be seen in the image, the “Redundancy Master” software is installed on the PC where the SCADA server that needs the OPC data is located.

It is important to note that for this architecture the communications are OPC-DA, with all that this implies (configuring the Windows DCOM, not being able to create specific firewall rules, among others).

Another point to keep in mind is that if you work with static tags (which means that we register the PLC tags on the OPC server) you have to maintain two tag databases, one on each of the OPC servers. That is, if a change is made on the primary OPC server, it must also be made on the secondary server.
Finally, the OPC tag structure of the two servers must also match for the redundancy to function correctly.

MEDIA LEVEL REDUNDANCY PLUG-IN
The second corresponds to the use of the “Media Level Redundancy Plugin” for OPC-UA. The Media-Level Redundancy Plug-In manages the transition between a primary device and a secondary device, in the communications between the OPC and KepServerEx clients. The use of this functionality allows the secondary system to take over communications in the event of a failure in the primary system.

In the case of using the OPC-UA Client driver, we must have two devices created, one pointing to each of the OPC UA Servers.

The Media Level Redundancy Plugin is installed on the PC where the Kepserver with the OPC-UA Client driver is located. In the same way as in the previous architecture, it must be taken into account that if you work with static tags, you have to maintain two tag databases, one on each of the OPC servers.

Also, just as with the software “Redundancy Master”, the OPC tag structure of the two servers has to match.

KEPSERVER EVERRUN-HA
The proposal to increase availability and simplify the redundancy architecture is the joint use of Kepserver and EverRun HA from Stratus Technologies.

EverRun HA is a hypervisor that is installed on two PCs and makes them work as one. At the operating level, only one virtual machine with a Kepserver will be available, which will have to be configured, and it will be everRun who will balance this virtual machine from one host to the other if one of the hosts becomes unusable.

The advantage of this architecture is that at the operating level there is only one Kepserver, with a single license and a single configuration. The advantage of EverRun compared to other systems is that it is capable of creating a single hardware image, with which, and especially with Kepserver that uses parameters of the physical machine to activate the license, there are no license problems when balancing between hosts.

At the downtime level, the everRun HA solution manages to restart the machine on the secondary server with a small pass through zero. That means that we can have a downtime of a few seconds if one of the hosts fails. To further improve this operation, it is possible to use the EverRun Enterprise license, which converts the entire set into a fault-tolerant system. The difference is that, with a fault-tolerant system, there is no pass through zero since it manages to synchronize the two servers in real time.

In short, what EverRun allows us is to have a high availability solution that simplifies the architecture to a single Kepserver machine and therefore a single configuration and a single license.