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Transaction groups offer several advanced settings that affect how execution occurs. These settings can be found under the Options tab for a group.
This setting modifies how the group receives data from OPC.
| • | Subscribe - Data points are registered with the OPC server, and data is received by the group "on-change". This is the default setting and generally offers the best performance, as it reduces unnecessary data flow and allows the OPC server to optimize reads. However, it's important to note that data is received by the group asynchronously, meaning that it can arrive at any time. When the group executes, it "snapshots" the last values received and uses those during evaluation. If some values arrive after execution begins, they will not be used until the following execution cycle. |
| • | Read - Each time the group executes it will first read the values of OPC items from the server. This operation takes more time and involves more overhead than subscribed evaluation, but ensures that all values are updated together with the latest values. It is therefore commonly used with batching situations, where all of the data depends on each other and must be updated together. It's worth noting that when using an OPC item as the trigger, the item will be subscribed, and the rest of the values read when the trigger condition occurs. Note: This option was previously referred to as "polled reads" in earlier versions of the software. |
Only applicable to groups that insert rows into the database. Causes groups to target the database directly instead of going through the store-and-forward system. If the connection becomes unavailable, the group will report errors instead of logging data to the cache.
Specifies the rate at which OPC items in the group will be subscribed. These items are normally subscribed at the rate of the group, but by modifying this setting it is possible to request updates at a faster or slower rate.
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