Hot, warm, and cold failover modes

The failover mode specifies how the backup interface instance handles connecting to a data source and adding points when failover occurs. The sooner the backup interface can take over data collection, the less data is lost. However, increasing the failover level also increases data source load and system resource usage.

To determine which mode to use, consider how much data you can afford to lose and how much workload your system can handle. Be prepared to experiment, and consult your data source documentation and vendor as needed.

UniInt provides three levels of failover: cold, warm and hot. Higher ("hotter") levels preserve more data in the event of failover, but impose increasing workload on the system.

Hot failover

Hot failover is the most resource-intensive mode. Both the primary and backup interface instances collect data. No data is lost during failover (unless both the primary and backup interface nodes fail together), but the data source carries a double workload.

Warm failover

In a warm failover configuration, the backup interface does not actively collect data. The backup interface loads the list of PI points and waits to collect data until the primary interface fails or stops collecting data for any reason. If the backup interface assumes the role of primary, it starts collecting data. Some data loss can occur in a warm failover configuration.

Cold failover

In cold failover, the backup instance does not connect with the data source or load the list of PI points until it becomes primary. This delay almost always causes some data loss but imposes no additional load on the data source. Cold failover is required for the following cases:

• A data source can support only one client.

• You are using redundant data sources and the backup data source cannot accept connections.

  Note: Upon initial startup of the interface, the device status health point is set to a 0 | Good even though the interface is not connected with the data source. This is to provide the backup interface instance an opportunity to assume the primary role if the current primary interface device status is not good. Once an interface instance attempts to assume the primary role, the device status health point reflects the correct status of the data source connection.