Applications written using the data flow system are composed of components which implement the data flow component interface. A component's logic is executed by a scheduler by at most one thread at any given time.
Components communicate with other components using events. All events
are sent using DATA-FLOW:ENQUEUE-EVENT and the component's logic is
scheduled for execution if and only if the component is /stopped/
i.e. it is not executing/running nor scheduled.
When scheduling a component for execution, a component implementation
must schedule the closure returned by
DATA-FLOW:MAKE-COMPONENT-LAMBDA rather than scheduling the component
itself.
Upon invocation, the closure first processes all of the events
received by the component using DATA-FLOW:PROCESS-ALL-EVENTS. It
then applies DATA-FLOW:RUN to the component. The component's
DATA-FLOW:RUN method must call DATA-FLOW:PROCESS-ALL-EVENTS to
obtain any new data sent to the component during an invocation of
DATA-FLOW:RUN.
Upon completion of the component's DATA-FLOW:RUN method, the closure
must call the predicate DATA-FLOW:REQUIRES-EXECUTION-P to check if
any new events have not been processed by the component. The component
will be rescheduled for execution if the predicate returns non NIL.
Each event object received by a component must have an applicable
DATA-FLOW:PROCESS-EVENT method. This generic function is applied to
each received event by DATA-FLOW:PROCESS-ALL-EVENTS
The generic functions DATA-FLOW:EXECUTION-STATE and
DATA-FLOW:COMPARE-AND-CHANGE-EXECUTION-STATE ensure that at most one
thread can execute the logic of the component. The execution state of
a component can be one of :STOPPED, :SCHEDULED or :RUNNING. The
function DATA-FLOW:EXECUTION-STATE returns the current state of the
component, and the function
DATA-FLOW:COMPARE-AND-CHANGE-EXECUTION-STATE will change the state
of the component if and only if the component's current state is equal
to a specific value.
The scheduler used by a component can be accessed using the
DATA-FLOW:SCHEDULER function.
Users are able to use the dynamic variable DATA-FLOW:*SCHEDULER* to
specify the default scheduler for a component.
Implementations of DATA-FLOW:ENQUEUE-EVENT,
DATA-FLOW:EXECUTION-STATE and
DATA-FLOW:COMPARE-AND-CHANGE-EXECUTION-STATE must linearize all
concurrent access to the component. All other operators are assumed to
be invoked during an invocation of the closure returned by
DATA-FLOW:MAKE-COMPONENT-LAMBDA.
An object which implements the component protocol must be a subclass
of DATA-FLOW:COMPONENT.
The data flow system provides the class DATA-FLOW:BASIC-COMPONENT
which implements the component protocol above.
The data flow system provides the class DATA-FLOW:STANDARD-COMPONENT
which is a subclass of DATA-FLOW:BASIC-COMPONENT and has support for
all of the other functionality provided by the data flow system
(e.g. ports.md).