Trends Server Redundancy

It is possible to configure two trend servers as a redundant pair consisting of a primary server and a standby server (see the Mode property description in Add a Trend Server Process).

When both trend servers are in operation, trends are processed in parallel. Each trend server subscribes to the I/O server for data and stores the values on disk, or in its own private area on the file server.

When a trend server starts, it establishes a connection with the redundant trend server. If there is a gap in its trend files, it requests the missing data from the redundant trend server and backfills the gap if the data is available. This typically occurs after a restart, but can also happen when the trend server misses samples for other reasons (such as a temporary loss of communication with the I/O server or a performance issue).

An exception to this occurs with pure event trends that do not have a trigger defined. Samples for this type of trend are populated by either a Cicode function or certain types of I/O drivers (see Configure Event Trends for a DRI Driver). When one trend server receives new values from a pure event trend, it will try to synchronize those values with its redundant peer.

The backfilling mechanism includes built-in resilience to help ensure that backfill requests currently in progress are not lost if a trend server is shut down gracefully. When the trend server restarts, backfilling resumes from the point at which it stopped.

If you want to confirm that backfilling has completed before you shut down a trend server, Cicode can be used to determine backfill progress. Use the ServerGetProperty function with the SyncStatus and SyncProgress modes to retrieve information about the current backfill status.

See Also

File Server Redundancy