Codels are callbacks to the actual code implementing services in a
component. There are two kind of codels: simple codels and finite state
machine codels.
Codels are normally functions that execute in a short amount of time
compared to the time characteristics of the component. During the execution
of a codel, the component is typically blocked. If a codel is
expected to last much longer than the desired reaction time of the
component, or block until some external event arrives, it can be labelled
async.
Simple codels are functions associated to service validation callbacks or
implementing component functions.
The identifier is the name of the external function implementing the codel. parameters are described in the third section.
Finite state machine (FSM) codels are functions associated to the execution
of activities.
In addition to the identifier and parameters described in the previous
section, FSM codels define the state machine of their corresponding
activity. The state machine is defined with a list of states and possible
transitions.
The bracketed event list, before the codel identifier defines one (or more) state of the FSM, and associates the codel with this state (or list of states). When the FSM reaches the corresponding state, the codel is executed. There are two predefined states : start and stop. The start state is required: this is the state in which the FSM begins its execution. The stop state is optional: the FSM goes into this state when it receives an external interruption request.
The event list specified after the yield keyword statically defines all
possible transitions from this state to other states. For each possible
transition, a corresponding state must exist in the activity FSM. There is
one predefined transition called ether: this is the final state of the FSM
and it is used to indicate the regular termination of the activity. There
must always exist a path from the start state to the ether state. If a
stop state is defined, a path from stop to ether must also exist.
The actual transition executed by the FSM depends on the codel function
return value.
Transitions normally occur immediately, so that the FSM executes as fast as
possible. This execution can still be explicitely paused, for instance to
wait until the next period or until something new happens in the
component. For this purpose, a transition to a new state s can optionally
be defined in the pause namespace, by specifying the transition as
pause::s. A paused transition means that the FSM will pause its
execution either until the next period (within periodic tasks), or until a
new internal event occurs (within tasks with no period). Internal events
correspond to a non-paused transition of another activity within the
component, or the execution of an attribute or function.
This would declare a FSM with one main state, typically looping until some condition is met, and a stop state in case of premature interruption.
codel<start> do_start() yield main;
codel<main> do_main(in condition) yield pause::main, ether;
codel<stop> do_stop() yield ether;In this example, the main loop will be paced according to the context in
which the codels are defined (a periodic task, for instance). Without the
paused transition, the loop would typically eat all the available CPU.