Data Prefetch

Another way to address problems related to I/O communication and volume is with data prefetch. Data prefetch is a way of preloading data in anticipation of client requests. Data prefetch loads data from a device onto the server even if no clients are requesting the data. Because the data is preloaded onto the server, the data is available when a client requests it; the client doesn’t have to wait for the I/O from the device. Data prefetch is useful with slower devices or communication networks where I/Os between clients and devices are typically delayed because, for example:

Your older or slower devices don’t have the capacity to fulfill their I/O requirements.
Geographical distance separates clients from the server, or the server from the device.

For example, a client requests second-by-second polling data from a device that is located on the other side of the continent, and your limited hardware device cannot accommodate the requests in a timely way. In this case, data prefetch might provide the solution you are looking for.

Restrict Your Use of Data Prefetch

There is a downside to using data prefetch. The ongoing loading of data from the device to the server creates a continuous stream of traffic between the device and the server. For that reason, it is important that you use restraint when choosing the data points you use prefetch for; this is not a solution you want to overuse. You will have to determine whether the volume of traffic that prefetch creates works to your advantage. For example, even if a hardware device has 30 data points, there may be only one data point on that device where prefetch benefits you.

Also, keep in mind that any register that does data prefetch consumes a license point because the input/output tag is in constant use by the register.

Configuring Data Prefetch

To configure a register that prefetches data:

Click Start > All Programs > ICONICS > Tools > Unified Data Manager.
Expand the folder for the host, then expand the Unified Data Manager configuration database.
Right-click the Registers folder and select Register.
On the Register Properties tab, put a check mark in the Writable check box, and set the Propagation Style to Disabled.
Go to the Register Input/Output tab, and enter the OPC tag for the data point you are aggregating in both the Input OPC Data Tag field and the Output Tag field.

Note. If the OPC tag is read-only, you can leave the Output Tag field empty and leave the Writable check box unchecked.

Remove the check mark from the Release tags when not in use check box.
Click Apply to save the register.