source: branches/OKBJavaConnector/ECJClient/src/ec/steadystate/README @ 9449

This package implements steady state evolution and, by extension, asynchronous
evolution when used in combination with the master/slave evaluation package.
The steady state evolution facility in ECJ is fairly simple: each iteration
the facility breeds a single individual, evaluates it, and reinserts it into
the population (selecting an individual to die and be replaced with it).  In
asynchronous evolution, the system breeds individuals and ships them off to
remote slaves to evaluated whenever a remote slave is available; when an 
individual is completed from a remote slave, if the initial population has not
yet been filed up, the individual is inserted directly in the population, else
the system selects an individual to die and be replaced by it.

To do this magic requires several replacement classes: a new top-level
evolution state object provides the evolutionary loop, plus a custom breeder
and evaluator.  Because steady-state isn't generational, certain ECJ classes
are required to implement special interfaces in order to be compatible with
it: breeding sources (breeding pipelines, selection methods) must be of
SteadyStateBSourceform -- notably many selection methods don't work right,
we suggest using Tournament Selection; the statistics facility must implement
a different non-generational collection of statistics hooks embodied in 
SteadyStateStatisticsForm; and exchangers must implement 
SteadyStateExchangerForm.

Steady state evolution doesn't have a notion of generations per se, as no
entire generation is replaced each iteration.  Instead, the evolution state
facility defines a "generation" as when a population's worth of individuals
has just been introduced into the system.  Most statistics counts are run
not off of these pseudo-generations but rather are based on the number of
*evaluations* (a variable in SteadyStateEvolutionState) which have been
done so far.  As such you have the option of either stating the number
of evaluations that the system should run for, OR the number of "generations"
(so to speak) the system should run for.  See the file steadystate.params
for an example of how to set up steady state evolution.

Classes of relevance:



ec.steady.SteadyStateEvolutionState

The top-level EvolutionState which performs steady-state and asynchronous
evolution loops.


ec.steady.SteadyStateBreeder

The steady-state breeder.   A drop-in replacement for Breeder, except that
it also requires an additional parameter 'deselector' which indicates the
SelectionMethod used to pick individuals to be replaced with new incoming
children.  This could be a random selector, or a Tournament Selection
method picking the worst individuals, etc.  See steadystate.params for
an example.


ec.steady.SteadyStateEvaluator

The steady-state evaluator.  A drop-in replacement for Evaluator.


ec.steady.QueueIndividual

A wrapper which holds an Individual, plus the subpopulation the Individual
is stored in.  This class is used by the SteadyStateEvaluator and also by
the master-slave evaluation system to maintain some additional information 
about the Individual for purposes of Asynchronous Evolution.  It's an 
internal class and shouldn't be fooled around with; it's public because
the master-slave facility must also be able to use it.  Ignore it.


ec.steady.SteadyStateStatisticsForm

Statistics objects which implement this interface will receive additional
statistics hooks appropriate for the steady state loop (based largely on
individuals being evaluated rather than based around generations).  This
is an optional interface and your Statistics object can always be used
even if it doesn't implement this interface; it just won't receive the
new method calls.


ec.steady.SteadyStateExchangerForm

All exchangers must implement the SteadyStateExchangerForm in order to be
used with the steady state facility.  This is a "gold star" interface which
has no methods but merely is implemented by exchangers which have agreed
to perform certain special tasks (see the code of the class).


ec.steady.SteadyStateBSourceForm

All breeding sources (BreedingPipelines, SelectionMethods) must implement
this interface in order to be used with the steady state facility.  The
interface largely informs breeding sources that an individual has replaced
another in the population  Some breeding sources (such as 
FitnessProportionateSelection) must update their distributions each time
this happens, which is quite expensive, and so they do not implement this
interface.  We suggest you use Tournament Selection, which is effective
and fast.