What are the configuration differences between Hot, Warm, and Cold redundancy in OPC Expert?
OPC Expert supports three redundancy modes: Hot, Warm, and Cold. Each mode represents a different approach to managing the connection between OPC Expert and its primary and standby OPC servers. The right choice depends on your PLC or control system architecture, your tolerance for data loss during a failover, and the constraints of your OPC server vendor.
Hot Redundancy: Zero Data Loss, Maximum Load
In Hot mode, OPC Expert maintains active subscriptions to both the primary and the standby OPC server simultaneously. Both servers are actively requesting data from the underlying control system at all times. Because OPC Expert is already receiving a live data stream from both sources, there is no switchover delay and no data loss when the primary fails. The standby server is already delivering data, so OPC Expert simply stops using the primary path and continues with the secondary without interruption.
The tradeoff is that both OPC servers are generating traffic to the PLC or DCS at the same time. Some control systems cannot handle dual connections gracefully, and certain vendor architectures explicitly restrict the number of concurrent OPC server connections. If your hardware or firmware supports dual active connections and data continuity is critical, Hot mode is the recommended configuration.
Warm Redundancy: Monitored Standby with a Brief Transition
In Warm mode, OPC Expert connects to both the primary and the standby OPC server, but only instructs the standby to actively collect data from the control system when a failure occurs. OPC Expert monitors the standby connection continuously, so the communication path is established and ready. When the primary fails, OPC Expert signals the standby server to begin data collection. This transition takes a short amount of time, typically in the range of a few seconds, because the standby server must initiate its subscription cycle with the underlying hardware.
The benefit is that only one OPC server is placing a data demand on the PLC or DCS at any given time, reducing overall system load. The cost is that there is a brief window of potential data loss during the transition. If even a few seconds of lost data is unacceptable, Hot mode is the better choice. If the PLC can only support a single active OPC connection at a time but can still accept a pre-established network session from the standby, Warm mode is viable.
Cold Redundancy: Full Standby with Longer Transition
In Cold mode, the standby OPC server is not running when the primary is active. It may not be connected, configured, or even powered on. When the primary server fails, OPC Expert waits for the standby server to become available, then establishes a connection and begins collecting data. Because the standby server must start from a fully inactive state, the transition time is longer than either Hot or Warm mode.
Cold mode is appropriate for control system architectures that do not permit more than one OPC server to run simultaneously. Some Honeywell systems, for example, enforce this constraint by design. In those environments, Cold mode is the only viable approach. The data loss window is the longest of the three modes and is largely determined by how quickly the standby server can start and register with the control system.
Configuration in OPC Expert
Switching between modes in OPC Expert requires no complex scripting. After establishing connections to a primary and a standby OPC server in the interface, users set up a redundancy object by dragging the primary server icon onto the standby server icon. A configuration dialog then presents the Hot, Warm, and Cold options along with the option to enable built-in tunneling. The entire setup process is visual and requires no changes to the Windows Registry or any OPC server software.
Related Tools
OPC Redundancy is the specific feature within OPC Expert that manages failover between primary and standby OPC servers. It handles continuous connection monitoring, automatic switchover, optional failback, and real-time status reporting for all three redundancy modes.
OPC Tunneler, also part of OPC Expert, can be combined with any of the three redundancy modes to route OPC traffic across networks without DCOM configuration, making it easier to connect to remote primary and standby servers within the same redundancy setup.
Conclusion
Hot redundancy eliminates data loss but requires the control system to support dual active OPC server connections. Warm redundancy reduces system load at the cost of a brief switchover delay. Cold redundancy accommodates architectures where only one OPC server can be active at a time. OPC Expert supports all three modes and lets you configure the right one through a drag-and-drop interface without modifying your existing OPC infrastructure.