UniInt failover modes

The failover mode specifies how the backup PI Interface for OPC AE instance handles connecting to an OPCAE server, creating groups, and adding points when failover occurs.

The faster the backup interface instance can take over data collection, the less data is lost. However, the ongoing activities required to maximize the readiness of the backup interface instance increase OPCAE server load and system resources. To determine which mode to use, consider how long failover takes and how much workload your system can handle. Be prepared to experiment, and consult your OPCAE server documentation and vendor as needed.

PI Interface for OPC AE provides three levels of failover, cold, warm, and hot. Hot failover, is lossless unless both the primary and backup interface instance nodes fail together.

Hot failover

Hot failover is the most resource-intensive mode. Both the primary and backup interface instances are collecting data, possibly from the same OPCAE server. No data is lost during failover, but the OPCAE server carries a double workload, or, if two servers are used, the back-end system must support both OPCAE servers.

Warm failover

An interface for warm failover in the backup role is not actively collecting data from the data source even though it may be configured with PI points and may even have a good connection to the data source.

Cold failover

Cold failover is required if an OPCAE server can support only one client, or if you are using redundant OPCAE servers and the backup OPCAE server cannot accept connections. The backup instance does not connect with the OPCAE server until it becomes primary. At that time, it must create groups, add items to groups, and advise the groups. This delay almost always causes some data loss, but imposes no load at all on the OPCAE server or data source system.

Note: In addition to UniInt based failover, PI Interface for OPC AE also supports OPCAE server-level failover. OPCAE server-level failover is based on information received from the OPCAE Server; there is no PI point to control which OPCAE Server is active or which connection is preferred..