Send historical data between Application Server and Historian

Application Server communicates with the Historian through an interface called the Historian Client Access Layer (HCAL).

HCAL can establish and maintain a connection to one or more historians either synchronously or asynchronously. If a disconnection from the historian occurs, HCAL attempts to restore the connection.

If HCAL cannot communicate with the historian, all data currently being processed can be stored locally on the computer running HCAL. This hard drive location is called the store-and-forward path. Historical data is stored until the threshold capacity of the path is reached or communication to the historian is restored. In the event that all store forward disk capacity is used to store historical data, no more data is stored. An error message is logged. Remote store-and-forward paths are not supported.

HCAL can go into store-and-forward mode even if it has never been connected to the historian. After the tags are configured successfully on the historian, current data will start to be sent, along with the locally-stored data.

You can specify how disconnects between Application Server and the Historian should be reflected in the data until the disconnect period can be backfilled with store-and-forward data.

• If you enable the Reconnect as soon as possible & do not mark disconnects option for an AppEngine, client-side trends during the disconnect interval are filled in with the last-received value before the disconnect, and trends do not show a gap.

• If you disable the option, a gap appears in the client-side trends during the disconnect interval. NULL values are injected during the disconnect interval to create the gap.

For more information about HCAL, see the Historian documentation. For more information about the Reconnect as soon as possible & do not mark disconnects AppEngine option, see AppEngine reconnect configuration for objects linked to the telemetry server.