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# Simulation with Event Stepping

Authors:

- Klaus Muller <Muller@users.sourceforge.net>
- Tony Vignaux <Vignaux@users.sourceforge.net>

SimPy release: | 2.3
Web-site: | http://simpy.sourceforge.net/
Python-Version: | 2.6+
Date: | December 2011

Contents

- Simulation with Event Stepping
  - Introduction
  - SimulationStep overview
  - The SimulationStep API

This manual describes SimulationStep, a SimPy module which supports stepping through a simulation model event by event.

## Introduction

SimulationStep can assist with debugging models, interacting with them on an event-by-event basis, getting event-by-event output from a model (e.g. for plotting purposes), etc.

SimulationStep is a derivative of the Simulation module. Over and above the capabilities provided by Simulation, SimulationStep supports stepping through a simulation model event by event. It caters for:

> - running a simulation model, with calling a user-defined procedure after every event,
> - running a simulation model one event at a time by repeated calls,
> - starting and stopping the event stepping mode under program control.

## SimulationStep overview

Here is a simple program which shows basic event stepping capabilities:

    ## Stepping1.py
    from __future__ import generators
    from SimPy.SimulationStep import *                  # (1)

    def callbackTimeTrace():                            # (2)
        """Prints event times
        """
        print "at time=%s"%now()

    class Man(Process):
        def walk(self):
            print "got up"
            yield hold,self,1
            print "got to door"
            yield hold,self,10
            print "got to mail box"
            yield hold,self,10
            print "got home again"

    #trace event times
    initialize()
    otto=Man()
    activate(otto,otto.walk())
    startStepping()                                     # (3)
    simulate(callback=callbackTimeTrace,until=100)      # (4)

A trivial simulation model, but with event stepping:

> 1. import the stepping version of Simulation
> 2. define a procedure which gets called after every event
> 3. switch into event stepping mode
> 4. run the model with event callback to the procedure defined at (2); `simulate` in SimulationStep has an extra named parameter, `callback`.

Running it produces this output:

    got up
    at time=0
    got to door
    at time=1
    got to mail box
    at time=11
    got home again
    at time=21
    at time=21

The callback outputs the simulation time after every event.

Here is another example, the same model, but now with the user getting control back after every event:

    ## Stepping2.py
    from __future__ import generators
    from SimPy.SimulationStep import *

    def callbackUserControl():
        """Allows user to control stepping
        """
        a=raw_input("[Time=%s] Select one: End run (e), Continue stepping (s), Run to end (r)= "%now())
        if a=="e":
            stopSimulation()
        elif a=="s":
            return
        else:
            stopStepping()

    class Man(Process):
        def walk(self):
            print "got up"
            yield hold,self,1
            print "got to door"
            yield hold,self,10
            print "got to mail box"
            yield hold,self,10
            print "got home again"
    #allow user control
    initialize()
    otto=Man()
    activate(otto,otto.walk())
    startStepping()
    simulate(callback=callbackUserControl,until=100)

Its interactive output looks like this:

    got up
    [Time=0] Select one: End run (e), Continue stepping (s), Run to end (r)= s
    got to door
    [Time=1] Select one: End run (e), Continue stepping (s), Run to end (r)= s
    got to mail box
    [Time=11] Select one: End run (e), Continue stepping (s), Run to end (r)= s
    got home again
    [Time=21] Select one: End run (e), Continue stepping (s), Run to end (r)= s
    [Time=21] Select one: End run (e), Continue stepping (s), Run to end (r)= s

or this (the user stopped stepping mode at time=1):

    got up
    [Time=0] Select one: End run (e), Continue stepping (s), Run to end (r)= s
    got to door
    [Time=1] Select one: End run (e), Continue stepping (s), Run to end (r)= r
    got to mail box
    got home again

If one wants to run a tested/debugged model full speed, i.e. without stepping, one can write a program as follows:

    ## Stepping2Fast.py
    from __future__ import generators
    if __debug__:
            from SimPy.SimulationStep import *
    else:
            from SimPy.Simulation import *

    def callbackUserControl():
        """Allows user to control stepping
        """
        if __debug__:
                    a=raw_input("[Time=%s] Select one: End run (e), Continue stepping (s),\
                         Run to end (r)= "%now())
                    if a=="e":
                        stopSimulation()
                    elif a=="s":
                        return
                    else:
                        stopStepping()

    class Man(Process):
        def walk(self):
            print "got up"
            yield hold,self,1
            print "got to door"
            yield hold,self,10
            print "got to mail box"
            yield hold,self,10
            print "got home again"
    #allow user control if debugging
    initialize()
    otto=Man()
    activate(otto,otto.walk())
    if __debug__:
            startStepping()
            simulate(callback=callbackUserControl,until=100)
    else:
            simulate(until=100)

If one runs this with the Python command line option ‘-O’, any statement starting with `if __debug__:` is ignored/skipped by the Python interpreter.

## The SimulationStep API

### Structure

Basically, SimulationStep has the same API as Simulation, but with the following additions and changes:

    def startStepping()         **new**
    def stopStepping()          **new**
    def simulate()              **changed**
    def simulateStep()          **new**

### startStepping

Starts the event-stepping.

Call:

> startStepping()

Mandatory parameters:
  None.
Optional parameters:
  None
Return value:
  None.

### stopStepping

Stops event-stepping.

Call:
  stopStepping()
Mandatory parameters:
  None
Optional parameters:
  None
Return value:
  None

### simulate

Runs a simulation with callback to a user-defined function after each event, if stepping is turned on. By default, stepping is switched off.

Call:
  simulate(callback=<proc>,until=<endtime>)
Mandatory parameters:
  None
Optional parameters:
  - until = 0: the simulation time until which the simulation is to run (positive floating point or integer number)
  - callback = lambda:None: the function to be called after every event (function reference)
Return value:
  The simulation status at exit (string)

### simulateStep

Runs a simulation for one event, with (optional) callback to a user-defined function after the event, if stepping is turned on. By default, stepping is switched off. Thus, to execute the model to completion, simulateStep must be called repeatedly.

Note: it is not yet clear to the developers whether this part of the API offers any advantages or capabilities over and above the *simulate* function. The survival of this function in future versions depends on the feedback from the user community.

Call:
  simulateStep(callback=<proc>,until=<endtime>)
Mandatory parameters:
  None
Optional parameters:
  - until = 0: the simulation time until which the simulation is to run (positive floating point or integer number)
  - callback = lambda:None: the function to be called after every event (function reference)
Return value:
  The tuple (simulation status at exit (string),<resumability flag>). <resumability flag> can have one of two string values: “resumable” if there are more events to be executed, and “notResumable” if all events have been exhausted or an error has occurred. simulateStep should normally only be called if “resumable” is returned.

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