Every PE consumes exactly those events which correspond to the value on which it is keyed and can produce output events
PE is instantiated for each value of the key attribute Special class of PEs is the set of keyless PEs with no keyed attribute or value.
These PEs consume all events of the type with which they are associated
Keyless PEs are typically used at the input layer of an S4 cluster where events are assigned a key
"Garbage Collection" of PEs represents a major challenge for the platform
State of the PE is lost after the "cleanup