Event queue

The event queue serves as a memory and disk buffer between the snapshot subsystem and archive subsystem. PI Snapshot Subsystem adds data to the queue and PI Archive Subsystem removes data from the queue.

Normally the event queue passes events to the archive as quickly as they arrive, but in some circumstances PI Archive Subsystem might be busy or unavailable. When this happens, the event queue stores the data, filling until the archive is again available. This is called archive queuing.

Archive queuing occurs in the following cases:

• Interface nodes or other data sources produce a burst of events. This might occur when an interface recovers data after a network outage.

• PI Archive Subsystem becomes busy storing out-of-order events or processing expensive user queries.

• The operating system or hardware supporting Data Archive has a reduced I/O or CPU capacity. Other applications that run on the Data Archive computer or that share the same hardware host might cause this situation.