Gas Station

This example expands upon SimPy’s Gas Station Refueling example, demonstrating various desmod features.

Note

Desmod’s goal is to support large-scale modeling. Thus this example is somewhat larger-scale than the SimPy model it expands upon.

"""Model refueling at several gas stations.

Each gas station has several fuel pumps and a single, shared reservoir. Each
arrving car pumps gas from the reservoir via a fuel pump.

As the gas station's reservoir empties, a request is made to a tanker truck
company to send a truck to refill the reservoir. The tanker company maintains a
fleet of tanker trucks.

This example demonstrates core desmod concepts including:
 - Modeling using Component subclasses
 - The "batteries-included" simulation environment
 - Centralized configuration
 - Logging

"""
from itertools import count, cycle

from simpy import Resource

from desmod.component import Component
from desmod.dot import generate_dot
from desmod.pool import Pool
from desmod.queue import Queue
from desmod.simulation import simulate


class Top(Component):
    """Every model has a single top-level Component.

    For this gas station model, the top level components are gas stations and a
    tanker truck company.

    """

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # The simulation configuration is available everywhere via the
        # simulation environment.
        num_gas_stations = self.env.config.get('gas_station.count', 1)

        # Instantiate GasStation components. An index is passed so that each
        # child gas station gets a unique name.
        self.gas_stations = [GasStation(self, index=i) for i in range(num_gas_stations)]

        # There is just one tanker company.
        self.tanker_company = TankerCompany(self)

    def connect_children(self):
        # This function is called during the elaboration phase, i.e. after all
        # of the components have been instantiated, but before the simulation
        # phase.
        for gas_station in self.gas_stations:
            # Each GasStation instance gets a reference to (is connected to)
            # the tanker_company instance. This demonstrates the most
            # abbreviated way to call connect().
            self.connect(gas_station, 'tanker_company')

    def elab_hook(self):
        generate_dot(self)


class TankerCompany(Component):
    """The tanker company owns and dispatches its fleet of tanker trunks."""

    # This base_name is used to build names and scopes of component instances.
    base_name = 'tankerco'

    def __init__(self, *args, **kwargs):
        # Many Component subclasses can simply forward *args and **kwargs to
        # the superclass initializer; although Component subclasses may also
        # have custom positional and keyword arguments.
        super().__init__(*args, **kwargs)
        num_tankers = self.env.config.get('tanker.count', 1)

        # Instantiate the fleet of tanker trucks.
        trucks = [TankerTruck(self, index=i) for i in range(num_tankers)]

        # Trucks are dispatched in a simple round-robin fashion.
        self.trucks_round_robin = cycle(trucks)

    def request_truck(self, gas_station, done_event):
        """Called by gas stations to request a truck to refill its reservior.

        Returns an event that the gas station must yield for.

        """
        truck = next(self.trucks_round_robin)

        # Each component has debug(), info(), warn(), and error() log methods.
        # Log lines are automatically annotated with the simulation time and
        # the scope of the component doing the logging.
        self.info(f'dispatching {truck.name} to {gas_station.name}')
        return truck.dispatch(gas_station, done_event)


class TankerTruck(Component):
    """Tanker trucks carry fuel to gas stations.

    Each tanker truck has a queue of gas stations it must visit. When the
    truck's tank becomes empty, it must go refill itself.

    """

    base_name = 'truck'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.pump_rate = self.env.config.get('tanker.pump_rate', 10)
        self.avg_travel = self.env.config.get('tanker.travel_time', 600)
        tank_capacity = self.env.config.get('tanker.capacity', 200)
        self.tank = Pool(self.env, tank_capacity)

        # This auto_probe() call uses the self.tank Pool get/put hooks so that
        # whenever it's level changes, the new level is noted in the log.
        self.auto_probe('tank', log={})

        # The parent TankerCompany enqueues instructions to this queue.
        self._instructions = Queue(self.env)

        # Declare a persistant process to be started at simulation-time.
        self.add_process(self._dispatch_loop)

    def dispatch(self, gas_station, done_event):
        """Append dispatch instructions to the truck's queue."""
        return self._instructions.put((gas_station, done_event))

    def _dispatch_loop(self):
        """This is the tanker truck's main behavior. Travel, pump, refill..."""
        while True:
            if not self.tank.level:
                self.info('going for refill')

                # Desmod simulation environments come equipped with a
                # random.Random() instance seeded based on the 'sim.seed'
                # configuration key.
                travel_time = self.env.rand.expovariate(1 / self.avg_travel)
                yield self.env.timeout(travel_time)

                self.info('refilling')
                pump_time = self.tank.capacity / self.pump_rate
                yield self.env.timeout(pump_time)

                yield self.tank.put(self.tank.capacity)
                self.info(f'refilled {self.tank.capacity}L in {pump_time:.0f}s')

            gas_station, done_event = yield self._instructions.get()
            self.info(f'traveling to {gas_station.name}')
            travel_time = self.env.rand.expovariate(1 / self.avg_travel)
            yield self.env.timeout(travel_time)
            self.info(f'arrived at {gas_station.name}')
            while self.tank.level and (
                gas_station.reservoir.level < gas_station.reservoir.capacity
            ):
                yield self.env.timeout(1 / self.pump_rate)
                yield gas_station.reservoir.put(1)
                yield self.tank.get(1)
            self.info('done pumping')
            done_event.succeed()


class GasStation(Component):
    """A gas station has a fuel reservoir shared among several fuel pumps.

    The gas station has a traffic generator process that causes cars to arrive
    to fill up their tanks.

    As the cars fill up, the reservoir's level goes down. When the level goes
    below a critical threshold, the gas station makes a request to the tanker
    company for a tanker truck to refill the reservoir.

    """

    base_name = 'station'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        config = self.env.config
        self.add_connections('tanker_company')
        self.arrival_interval = config.get('gas_station.arrival_interval', 60)

        station_capacity = config.get('gas_station.capacity', 200)
        self.reservoir = Pool(
            self.env, capacity=station_capacity, init=station_capacity
        )
        self.auto_probe('reservoir', log={})

        threshold_pct = config.get('gas_station.threshold_pct', 10)
        self.reservoir_low_water = threshold_pct * station_capacity / 100

        self.pump_rate = config.get('gas_station.pump_rate', 2)
        num_pumps = config.get('gas_station.pumps', 2)
        self.fuel_pumps = Resource(self.env, capacity=num_pumps)
        self.auto_probe('fuel_pumps', log={})

        self.car_capacity = config.get('car.capacity', 50)
        self.car_level_range = config.get('car.level', [5, 25])

        # A gas station has two persistent processes. One to monitor the
        # reservoir level and one that models the arrival of cars at the
        # station. Desmod starts these processes before simulation phase.
        self.add_processes(self._monitor_reservoir, self._traffic_generator)

    @property
    def reservoir_pct(self):
        return self.reservoir.level / self.reservoir.capacity * 100

    def _monitor_reservoir(self):
        """Periodically monitor reservoir level.

        The a request is made to the tanker company when the reservoir falls
        below a critical threshold.

        """
        while True:
            yield self.reservoir.when_at_most(self.reservoir_low_water)
            done_event = self.env.event()
            yield self.tanker_company.request_truck(self, done_event)
            yield done_event

    def _traffic_generator(self):
        """Model the sporadic arrival of cars to the gas station."""
        for i in count():
            interval = self.env.rand.expovariate(1 / self.arrival_interval)
            yield self.env.timeout(interval)
            self.env.process(self._car(i))

    def _car(self, i):
        """Model a car transacting fuel."""
        with self.fuel_pumps.request() as pump_req:
            self.info(f'car{i} awaiting pump')
            yield pump_req
            self.info(f'car{i} at pump')
            car_level = self.env.rand.randint(*self.car_level_range)
            amount = self.car_capacity - car_level
            t0 = self.env.now
            for _ in range(amount):
                yield self.reservoir.get(1)
                yield self.env.timeout(1 / self.pump_rate)
            pump_time = self.env.now - t0
            self.info(f'car{i} pumped {amount}L in {pump_time:.0f}s')


# Desmod uses a plain dictionary to represent the simulation configuration.
# The various 'sim.xxx' keys are reserved for desmod while the remainder are
# application-specific.
config = {
    'car.capacity': 50,
    'car.level': [5, 25],
    'gas_station.capacity': 200,
    'gas_station.count': 3,
    'gas_station.pump_rate': 2,
    'gas_station.pumps': 2,
    'gas_station.arrival_interval': 60,
    'sim.dot.enable': True,
    'sim.dot.colorscheme': 'blues5',
    'sim.duration': '500 s',
    'sim.log.enable': True,
    'sim.log.file': 'sim.log',
    'sim.log.format': '{level:7} {ts:.3f} {ts_unit}: {scope:<16}:',
    'sim.log.level': 'INFO',
    'sim.result.file': 'results.yaml',
    'sim.seed': 42,
    'sim.timescale': 's',
    'sim.workspace': 'workspace',
    'tanker.capacity': 200,
    'tanker.count': 2,
    'tanker.pump_rate': 10,
    'tanker.travel_time': 100,
}

if __name__ == '__main__':
    # Desmod takes responsibility for instantiating and elaborating the model,
    # thus we only need to pass the configuration dict and the top-level
    # Component class (Top) to simulate().
    simulate(config, Top)

The model hierarchy is captured during elaboration as a DOT graph. See the desmod.dot documentation for more detail on DOT output.

The simulation log, sim.log, shows what happened during the simulation:

INFO    0.000 s: tankerco.truck0 : going for refill
INFO    0.000 s: tankerco.truck1 : going for refill
INFO    1.520 s: station1        : car0 awaiting pump
INFO    1.520 s: station1        : car0 at pump
INFO    15.520 s: station1        : car0 pumped 28L in 14s
INFO    19.297 s: station2        : car0 awaiting pump
INFO    19.297 s: station2        : car0 at pump
INFO    24.755 s: station2        : car1 awaiting pump
INFO    24.755 s: station2        : car1 at pump
INFO    25.259 s: tankerco.truck0 : refilling
INFO    26.693 s: station2        : car2 awaiting pump
INFO    35.297 s: station2        : car0 pumped 32L in 16s
INFO    35.297 s: station2        : car2 at pump
INFO    41.496 s: station2        : car3 awaiting pump
INFO    45.259 s: tankerco.truck0 : refilled 200L in 20s
INFO    46.255 s: station2        : car1 pumped 43L in 22s
INFO    46.255 s: station2        : car3 at pump
INFO    49.797 s: station2        : car2 pumped 29L in 14s
INFO    60.255 s: station2        : car3 pumped 28L in 14s
INFO    61.204 s: station0        : car0 awaiting pump
INFO    61.204 s: station0        : car0 at pump
INFO    69.270 s: station1        : car1 awaiting pump
INFO    69.270 s: station1        : car1 at pump
INFO    73.704 s: station0        : car0 pumped 25L in 13s
INFO    74.505 s: station0        : car1 awaiting pump
INFO    74.505 s: station0        : car1 at pump
INFO    85.270 s: station1        : car1 pumped 32L in 16s
INFO    88.005 s: station0        : car1 pumped 27L in 14s
INFO    89.447 s: station0        : car2 awaiting pump
INFO    89.447 s: station0        : car2 at pump
INFO    96.777 s: station2        : car4 awaiting pump
INFO    96.777 s: station2        : car4 at pump
INFO    109.006 s: station0        : car3 awaiting pump
INFO    109.006 s: station0        : car3 at pump
INFO    111.947 s: station0        : car2 pumped 45L in 22s
INFO    116.777 s: station2        : car4 pumped 40L in 20s
INFO    126.506 s: station0        : car3 pumped 35L in 18s
INFO    133.359 s: tankerco.truck1 : refilling
INFO    141.774 s: station1        : car2 awaiting pump
INFO    141.774 s: station1        : car2 at pump
INFO    153.359 s: tankerco.truck1 : refilled 200L in 20s
INFO    161.274 s: station1        : car2 pumped 39L in 20s
INFO    161.307 s: station1        : car3 awaiting pump
INFO    161.307 s: station1        : car3 at pump
INFO    178.807 s: station1        : car3 pumped 35L in 18s
INFO    180.874 s: station0        : car4 awaiting pump
INFO    180.874 s: station0        : car4 at pump
INFO    182.106 s: station2        : car5 awaiting pump
INFO    182.106 s: station2        : car5 at pump
INFO    185.606 s: tankerco        : dispatching truck0 to station2
INFO    185.606 s: tankerco.truck0 : traveling to station2
INFO    187.343 s: station0        : car5 awaiting pump
INFO    187.343 s: station0        : car5 at pump
INFO    188.209 s: station2        : car6 awaiting pump
INFO    188.209 s: station2        : car6 at pump
INFO    195.843 s: tankerco        : dispatching truck1 to station0
INFO    195.843 s: tankerco.truck1 : traveling to station0
INFO    197.374 s: station0        : car4 pumped 33L in 16s
INFO    202.843 s: station0        : car5 pumped 31L in 16s
INFO    204.051 s: tankerco.truck1 : arrived at station0
INFO    223.651 s: tankerco.truck1 : done pumping
INFO    234.311 s: station2        : car7 awaiting pump
INFO    255.130 s: station2        : car8 awaiting pump
INFO    278.171 s: tankerco.truck0 : arrived at station2
INFO    285.271 s: station2        : car5 pumped 34L in 103s
INFO    285.271 s: station2        : car7 at pump
INFO    286.087 s: station0        : car6 awaiting pump
INFO    286.087 s: station0        : car6 at pump
INFO    290.871 s: station2        : car6 pumped 33L in 103s
INFO    290.871 s: station2        : car8 at pump
INFO    298.171 s: tankerco.truck0 : done pumping
INFO    298.171 s: tankerco.truck0 : going for refill
INFO    302.271 s: station2        : car7 pumped 34L in 17s
INFO    307.587 s: station0        : car6 pumped 43L in 22s
INFO    312.871 s: station2        : car8 pumped 44L in 22s
INFO    314.565 s: station2        : car9 awaiting pump
INFO    314.565 s: station2        : car9 at pump
INFO    336.065 s: station2        : car9 pumped 43L in 22s
INFO    337.760 s: station0        : car7 awaiting pump
INFO    337.760 s: station0        : car7 at pump
INFO    350.399 s: station1        : car4 awaiting pump
INFO    350.399 s: station1        : car4 at pump
INFO    358.760 s: station0        : car7 pumped 42L in 21s
INFO    365.899 s: station1        : car4 pumped 31L in 16s
INFO    379.093 s: station1        : car5 awaiting pump
INFO    379.093 s: station1        : car5 at pump
INFO    386.093 s: tankerco        : dispatching truck0 to station1
INFO    396.093 s: station1        : car5 pumped 34L in 17s
INFO    403.551 s: station2        : car10 awaiting pump
INFO    403.551 s: station2        : car10 at pump
INFO    406.424 s: tankerco.truck0 : refilling
INFO    413.051 s: tankerco        : dispatching truck1 to station2
INFO    413.051 s: tankerco.truck1 : traveling to station2
INFO    414.202 s: station2        : car11 awaiting pump
INFO    414.202 s: station2        : car11 at pump
INFO    426.424 s: tankerco.truck0 : refilled 200L in 20s
INFO    426.424 s: tankerco.truck0 : traveling to station1
INFO    432.837 s: station2        : car12 awaiting pump
INFO    433.832 s: tankerco.truck0 : arrived at station1
INFO    436.561 s: tankerco.truck1 : arrived at station2
INFO    436.961 s: tankerco.truck1 : done pumping
INFO    436.961 s: tankerco.truck1 : going for refill
INFO    436.961 s: tankerco        : dispatching truck0 to station2
INFO    439.677 s: station1        : car6 awaiting pump
INFO    439.677 s: station1        : car6 at pump
INFO    453.753 s: station0        : car8 awaiting pump
INFO    453.753 s: station0        : car8 at pump
INFO    453.832 s: tankerco.truck0 : done pumping
INFO    453.832 s: tankerco.truck0 : going for refill
INFO    455.177 s: station1        : car6 pumped 31L in 16s
INFO    456.524 s: station1        : car7 awaiting pump
INFO    456.524 s: station1        : car7 at pump
INFO    466.253 s: station0        : car8 pumped 25L in 12s
INFO    471.402 s: station1        : car8 awaiting pump
INFO    471.402 s: station1        : car8 at pump
INFO    474.024 s: station1        : car7 pumped 35L in 18s
INFO    482.382 s: station0        : car9 awaiting pump
INFO    482.382 s: station0        : car9 at pump
INFO    493.305 s: station2        : car13 awaiting pump
INFO    493.402 s: station1        : car8 pumped 44L in 22s
INFO    497.990 s: station0        : car10 awaiting pump
INFO    497.990 s: station0        : car10 at pump

This example does not make heavy use of desmod’s result-gathering capability, but we can nonetheless see the minimal results.yaml file generated from the simulation:

config:
  car.capacity: 50
  car.level: [5, 25]
  gas_station.arrival_interval: 60
  gas_station.capacity: 200
  gas_station.count: 3
  gas_station.pump_rate: 2
  gas_station.pumps: 2
  meta.sim.workspace: workspace
  sim.config.file: null
  sim.db.enable: false
  sim.db.persist: true
  sim.dot.all.file: all.dot
  sim.dot.colorscheme: blues5
  sim.dot.conn.file: conn.dot
  sim.dot.enable: true
  sim.dot.hier.file: hier.dot
  sim.duration: 500 s
  sim.log.buffering: -1
  sim.log.enable: true
  sim.log.exclude_pat: []
  sim.log.file: sim.log
  sim.log.format: '{level:7} {ts:.3f} {ts_unit}: {scope:<16}:'
  sim.log.include_pat: [.*]
  sim.log.level: INFO
  sim.log.persist: true
  sim.progress.enable: false
  sim.progress.max_width: null
  sim.progress.update_period: 1 s
  sim.result.file: results.yaml
  sim.seed: 42
  sim.timescale: s
  sim.vcd.enable: false
  sim.vcd.persist: true
  sim.workspace: workspace
  sim.workspace.overwrite: false
  tanker.capacity: 200
  tanker.count: 2
  tanker.pump_rate: 10
  tanker.travel_time: 100
sim.exception: null
sim.now: 500.0
sim.runtime: 0.039155590000000004
sim.time: 500