By default, the correlator operates in a serial manner. In a monitor, you have the option of implementing contexts for parallel processing.

During serial correlator operation, the correlator processes events in the order in which they arrive. Each external event matches zero or more listeners. The correlator executes a matching event's associated listeners in a rigid order. The correlator completes the processing related to a particular event before it examines the next event.

For some applications, this serial behavior might not be necessary. In this case, you might be able to improve performance by implementing parallel processing. Parallel processing lets the correlator concurrently process the EPL in multiple monitor instances. To implement parallel processing, you create one or more contexts.

Parallel processing in the correlator is quite different from the parallel processing provided by Java, C++, and other languages. These languages allow shared state, and rely on mutexes, conditions, semaphores, monitors, and so on, to enforce correct behavior. The correlator does not automatically provide shared state. Data sharing happens by sending events between contexts and by using the MemoryStore. See Using the MemoryStore. Parallel processing in the correlator is a message-passing system.