Processing Nodes (PNs) are the logical hosts to PEs
They are responsible for listening to events, executing operations on the incoming events, dispatching events with the assistance of communication layer and emitting output events
S4 routes each event to PN based on a hash function of the values of all known keyed attributes in that event
A single event may be routed to multiple PNs
The set of all possible keying attributes is known from the configuration of S4 cluster
An event listener in the PN passes incoming events to the processing element container (PEC) which invokes the appropriate PEs in appropriate order