History Transporter - Other Options
The History Transporter has settings that give you full control over the history transfer process. When making large history transfers it is important to ensure that the transfer itself does not overwhelm regular traffic on a network. By way of scheduling and limiting the amount of data transfer in each cycle, it is possible to limit any disruption that might occur during large data transfers.
Depth, Slicing, Hedging and Throttling
The time period for history retrieval is configured using the Depth property. A relative time period from the current time backwards can be configured, for example to transfer the last 100 days of history *-100days can be set.
The Slicing property determines what percentage of the alloted schedule time will be used for the history transfer. This is important to limit the amount of data transfer on the network and the relative historians that are being queried and subsequently written to (the max Slicing setting is 75%).
The amount of data transfer can also be controlled using the Hedging property. This will limit the number of data points that can be transferred in the scheduled time period (max Hedging value is one million points). The hedging value will overrule the slicing setting when reached.
The Throttling setting allows another level of control by yielding another fragment of CPU time after each each tag data transfer. As this occurs with each tag, there is a dependance on the number of tags that are added to the Tag Configuration table. This should be kept in mind before using the property as a control mechanism as the throttling will increase considerably when making data transfers for large numbers of tags.
Time Shift
Time Shift is used when transferring data using the Continuous processing mode. In this processing mode, near time data is transferred through history calls. Time Shift will provide an offset (positive or negative) for the end time of the history calls. This can be used to suppress unwanted data or as a balancing mechanism to even out time syncronicity differences between multiple computers. A negative Time Shift value will move the query end time backwards (towards history) whereas a positive value moves the end time forwards (towards the future).