Kahn Process Networks and a Reactive Extension - Signal Processing Systems - page 1053

Fig. 13 Streaming transactions

are produced. Events should only be accepted in those states where no internal
or output actions are available. In Fig. 13 , these are all states on the lower-left
boundary, such as states s 1 and s 3 . In state s 2 , some remaining output is still pending
and an event cannot be processed yet. Sequences of actions starting from a state
without enabled output or internal actions and going back to such a state represent
complete reactions of the network to input stimuli. These sequences of actions are
grouped together to form atomic transitions, the streaming transactions , that end in
states where events can safely be processed. For example, the sequence of transitions
with thick arrows in the picture from state s 1 to s 3 forms a streaming transaction;
from the end state, only reading of new input is possible, all received input has been
fully processed. (These states correspond to end-points of maximal executions for
a particular finite input, and closely resemble the quiescent states of [ 40 ] .) Some of
the possible streaming transactions are shown in the picture on the right. The bold
one corresponds to the bold path on the left. In this new LTS, the state space consist
of only quiescent states.
The streaming transactions of an RPN are determined in two steps. Individual
streaming transactions of the constituent processes are determined as well as the
processing of events by these processes. Executions of these actions are then
taken together to form maximal streaming transactions of the whole network. Such
streaming transactions may not consume input events . However, they may include
occurrences of internal events and hence they can be indeterminate.
Figure 14 shows a part of a possible execution of the 3D game example. The
first transition is a streaming transaction, consisting of streaming actions of the
processes, but also internal events ( nrObj

( )

). Note that we have abstracted from
internal actions of for instance the rendering component, which might also be visible
in these transactions. The second transition is an event; the player has reached the
end of a level, and the game is reconfigured from 3D mode to the video mode.

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