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 the communication layer, and emitting output events (Figure 2).
S4 routes each event to PNs 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 the S4 cluster. An event listener in the PN passes incoming events to the processing element container (PEC) which invokes the appropriate PEs in the appropriate order