Snapshot actions

A snapshot action logs into dedicated SQL Server tables the data values for selected analog, discrete, or string tags that have the same timestamp as the detected event. Quality is also logged. Value snapshots are stored in tables according to the tag type, either AnalogSnapshot, DiscreteSnapshot, or StringSnapshot.

A snapshot action requires an expensive SQL join between the extension tables and the snapshot tag table. The process of performing the join and logging the retrieved results to the snapshot tables can be very slow. This is because most of the tables used for event snapshots are normal SQL Server tables, subject to the data processing limitations of Microsoft SQL Server. Thus, the higher the number of snapshots that are being taken by the event system, the higher the transaction load on the Microsoft SQL Server.

Important: The Classic Event subsystem is not a data acquisition system. DO NOT attempt to use snapshot actions to move data stored in the extension tables to normal SQL Server tables. This type of misapplication is guaranteed to result in exceedingly poor throughput and storage rates.

When trying to determine how many snapshots can be made by the system, you should execute the intended snapshot queries to the server using a batch file, leaving the Classic Event subsystem out of the exercise. By executing repeated snapshot queries at the server as fast as the computer will allow, you can better determine how many snapshots can be performed on a system over a given time period. Using this result and applying a safety factor may provide a good guideline for assessing how much your system can safely handle. Keep in mind that discrete snapshots are many times slower than analog snapshots.