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The CEMView High Availability Edition system builds on the basic robust and scalable CEMView architecture, increasing the overall system availability with the addition of a second CEMView system operating in parallel.
In order to operate two CEMView systems in parallel, i.e. the High Availability mode, the basic CEMView system must be enhanced with the following additions.
The first is a hardware requirement which requires all instrument communication sub-systems to be accessible to both computer systems running the CEMView software. This is typically accomplished by using Ethernet Serial Device Server(s) for connecting to the instruments.
The second additional requirement is for the CEMView software to be extended to support the High Availability operation mode. The High Availability operation mode is accomplished with the standard CEMView software application. It is not a different or separate application. Instead, it relies on several configuration changes and add-on components to accomplish the task of operating two CEMView systems in parallel.
There are naturally two computers required for our CEMView High Availability system, which we will refer to as Primary and Secondary. Although they are in fact peer systems, one must be designated to be the Primary and the other the Secondary system.
If both Primary and Secondary computer systems are operating, the primary computer's CEMView Server loads its instrument OPC Servers and communicates to the instruments, operating as a 'standard' CEMView system. The secondary computer CEMView Server does not load its OPC Servers, but instead connects to the OPC Servers of the primary computer over the Ethernet network. The OPC Server in the primary computer effectively polls the instrument(s) and sends the information to both the primary and secondary CEMView systems. This ensures that the instrument is not polled twice for the same information and it also minimizes the differences that would naturally occur in the datasets of the two CEMView systems if the polls were done independently.
The CEMView Client applications in both the primary and secondary computer connects to the CEMView Server in the primary computer. Again, this is done to minimize or, in fact, eliminate the differences that naturally arise from the two systems operating in parallel. It is possible to view the secondary system from the CEMView client application at any time, but the default configuration is for all CEMView clients, including any Remote CEMView clients to connect to the primary computer CEMView Server if it is available. The primary and secondary CEMView servers are configured with watchdog handshake logic to ensure that each system is operating and fully aware of the health of the other system.
See also: Additional information on High Availability Operating Scenarios.
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