pydevs

This is the documentation of pydevs, a Python wrapper of adevs, a C++ library implementing the Discrete Event System Specification (DEVS).

Contents

pydevs Quickstart

See Modeling and simulation with Adevs for a comprehensive introduction into modeling and simulation with adevs/pydevs.

Here we merely demonstrate how to set up a simple DEVS network model (adevs Digraph model) in Python.

import logging
import devs
logger = logging.getLogger('quickstart')
logger.setLevel(logging.DEBUG)
#logging.getLogger('devs').setLevel(logging.WARNING)

A Source – Processor – Observer model (M/M/1 Queue)

import collections
import random
class Source(devs.AtomicBase):
    arrival_port = 0

    def __init__(self, arrival_rate=1.0, **kwds):
        super().__init__(**kwds)
        self.logger = logging.getLogger('quickstart.Source')
        self.logger.info('Initialize source with arrival rate {}'.format(arrival_rate))
        self.arrival_rate = arrival_rate
        self.inter_arrival_time = random.expovariate(self.arrival_rate)
        self.job_id = 0

    def ta(self):
        self.logger.debug('Next arrival in {} time units'.format(self.inter_arrival_time))
        return self.inter_arrival_time

    def delta_int(self):
        self.job_id += 1
        self.inter_arrival_time = random.expovariate(self.arrival_rate)

    def output_func(self):
        self.logger.info('Generate job {}'.format(self.job_id))
        return self.arrival_port, self.job_id


class Server(devs.AtomicBase):
    arrival_port = 0
    departure_port = 1

    def __init__(self, service_rate=1.0, **kwds):
        super().__init__(**kwds)
        self.logger = logging.getLogger('quickstart.Server')
        self.logger.info('Initialize server with service rate {}'.format(service_rate))
        self.service_rate = service_rate
        self.remaining_service_time = devs.infinity
        self.queue = collections.deque()
        self.job_in_service = None

    def ta(self):
        if self.job_in_service is None:
            self.logger.debug('Server is idle')
            return devs.infinity

        return self.remaining_service_time

    def start_next_job(self):
        self.job_in_service = self.queue.popleft()
        self.remaining_service_time = random.expovariate(self.service_rate)
        self.logger.info('Start processing job {} with service time {}'.format(self.job_in_service, self.remaining_service_time))

    def delta_int(self):
        # service finished
        self.logger.info('Finished processing job {}'.format(self.job_in_service))
        if len(self.queue):
            # jobs waiting, start to process immediately
            self.start_next_job()
        else:
            # no more jobs, switch to idle
            self.logger.info('Queue empty, server turns idle')
            self.job_in_service = None

    def delta_ext(self, e, xb):
        if self.job_in_service is not None:
            self.remaining_service_time -= e

        # new job(s) arriving
        for port, job_id in xb:
            self.logger.info('New job {} arrives'.format(job_id))
            self.queue.append(job_id)
            if self.job_in_service is None:
                # queue empty, start immediately
                self.start_next_job()
            else:
                # server busy
                self.logger.debug('Server busy, enqueueing job {}'.format(job_id))

        self.logger.debug('Remaining service time for job {}: {} time units'.format(self.job_in_service, self.remaining_service_time))

    def delta_conf(xb):
        # treat incoming jobs first
        self.delta_ext(self.ta(), xb)
        self.delta_int()

    def output_func(self):
        # service finished
        return self.departure_port, self.job_in_service


class Observer(devs.AtomicBase):
    arrival_port = 0
    departure_port = 1

    def __init__(self, time=0.0, **kwds):
        super().__init__(**kwds)
        self.logger = logging.getLogger('quickstart.Observer')
        self.logger.info('Initialize observer at time {}'.format(time))
        self.time = time
        self.arrivals = list()
        self.departures = list()

    def delta_ext(self, e, xb):
        self.time += e
        for port, job_id in xb:
            if port == self.arrival_port:
                self.logger.info('Job {} arrives at time {}'.format(job_id, self.time))
                self.arrivals.append(self.time)
            elif port == self.departure_port:
                self.logger.info('Job {} departs at time {}'.format(job_id, self.time))
                self.departures.append(self.time)
source = Source(1.0)
server = Server(1.0)
observer = Observer()
INFO:quickstart.Source:Initialize source with arrival rate 1.0
INFO:quickstart.Server:Initialize server with service rate 1.0
INFO:quickstart.Observer:Initialize observer at time 0.0
digraph = devs.Digraph()
digraph.add(source)
digraph.add(server)
digraph.add(observer)
digraph.couple(source, source.arrival_port, server, server.arrival_port)
digraph.couple(source, source.arrival_port, observer, observer.arrival_port)
digraph.couple(server, server.departure_port, observer, observer.departure_port)
simulator = devs.Simulator(digraph)
DEBUG:quickstart.Server:Server is idle
WARNING:devs.devs.AtomicBase:ta not implemented, return devs.infinity
-c:1: UserWarning: ta not implemented, return devs.infinity
DEBUG:quickstart.Source:Next arrival in 0.1652953524349517 time units
simulator.execute_until(5.0)
INFO:quickstart.Source:Generate job 0
INFO:quickstart.Server:New job 0 arrives
INFO:quickstart.Server:Start processing job 0 with service time 3.5509846975085804
DEBUG:quickstart.Server:Remaining service time for job 0: 3.5509846975085804 time units
INFO:quickstart.Observer:Job 0 arrives at time 0.1652953524349517
DEBUG:quickstart.Source:Next arrival in 1.0903431091204843 time units
WARNING:devs.devs.AtomicBase:ta not implemented, return devs.infinity
INFO:quickstart.Source:Generate job 1
INFO:quickstart.Server:New job 1 arrives
DEBUG:quickstart.Server:Server busy, enqueueing job 1
DEBUG:quickstart.Server:Remaining service time for job 0: 2.4606415883880963 time units
INFO:quickstart.Observer:Job 1 arrives at time 1.255638461555436
DEBUG:quickstart.Source:Next arrival in 2.3049818738267307 time units
WARNING:devs.devs.AtomicBase:ta not implemented, return devs.infinity
INFO:quickstart.Source:Generate job 2
INFO:quickstart.Server:New job 2 arrives
DEBUG:quickstart.Server:Server busy, enqueueing job 2
DEBUG:quickstart.Server:Remaining service time for job 0: 0.15565971456136563 time units
INFO:quickstart.Observer:Job 2 arrives at time 3.5606203353821666
DEBUG:quickstart.Source:Next arrival in 2.937090534560785 time units
WARNING:devs.devs.AtomicBase:ta not implemented, return devs.infinity
INFO:quickstart.Server:Finished processing job 0
INFO:quickstart.Server:Start processing job 1 with service time 0.9782561195217124
INFO:quickstart.Observer:Job 0 departs at time 3.7162800499435322
WARNING:devs.devs.AtomicBase:ta not implemented, return devs.infinity
INFO:quickstart.Server:Finished processing job 1
INFO:quickstart.Server:Start processing job 2 with service time 0.404069818122655
INFO:quickstart.Observer:Job 1 departs at time 4.694536169465245
WARNING:devs.devs.AtomicBase:ta not implemented, return devs.infinity
observer.arrivals, observer.departures
([0.1652953524349517, 1.255638461555436, 3.5606203353821666],
 [3.7162800499435322, 4.694536169465245])
logger.setLevel(logging.ERROR)
logging.getLogger('devs').setLevel(logging.ERROR)
simulator.execute_until(100000.0)
len(observer.arrivals), len(observer.departures), len(server.queue)
(100015, 99697, 317)

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devs

devs package

Submodules
devs.devs module
class devs.devs.AtomicBase

Bases: object

Python extension type, base type for DEVS Atomic Model

Python modules subclass this type and overwrite the methods

When initialized, the constructor (__init__) creates a new instance of the underlying C++ wrapper class Atomic (defined in the C++ header file). The C++ wrapper class Atomic inherits from adevs::Atomic and implements all the virtual functions. The C++ wrapper instance receives the function pointers to the cy_* helper functions defined here, as well as a pointer to the Python extension type instance. Whenever adevs calls one of the virtual functions of the C++ wrapper instance, the C++ wrapper instance routes it via the function pointer to the corresponding cy_* helper function. The cy_* helper function calls the corresponding method of the instance of the Python extension type.

http://stackoverflow.com/a/12700121/2366781 https://bitbucket.org/binet/cy-cxxfwk/src

When initialized, the constructor (__init__) creates a new instance of the underlying C++ wrapper class Atomic (defined in the C++ header file). Upon adding the model to a Digraph, the Digraph increases the reference count to this Python object, and decreases the reference count upon destruction. Note that the adevs C++ Digraph instance assumes ownership of the C++ wrapper instances. The C++ Digraph instance deletes all C++ wrapper instances upon destruction. So the Python object might still exist even though the C++ wrapper instance is long gone. When adevs deletes the C++ wrapper instance, the Python object is not deleted, when it is still referenced in the Python scope, but we can live with that.

The port type is integer. The value type is a generic Python object. This Python wrapper class abstracts away the underlying adevs C++ PortValue type.

adevs creates (copies) the C++ PortValue instance. https://github.com/smiz/adevs/blob/aae196ba660259ac32fc254bad810f4b4185d52f/include/adevs_digraph.h#L194 https://github.com/smiz/adevs/blob/aae196ba660259ac32fc254bad810f4b4185d52f/include/adevs_bag.h#L156

The only interface we need is to iterate over input (InputBag) in delta_ext and delta_conf, and to add output events (OutputBag) in output_func. Adding output events, the instance of this Python wrapper class increases the reference counter of the value Python object. The C++ wrapper class decreases the reference counter upon adevs’ call to the gc_output garbage collection function.

We deliberately break the adevs interface for the output_func method. In adevs, a reference to a Bag is supplied to the method returning void. Here, we choose the Pythonic way and take the return value of the method as the output bag. This is converted automatically by the cy_output_func helper function. output_func can either return

None (no output), a tuple (of length 2: port, value), or an iterable (of tuples of length 2: port, value).

For example, output_func can be implemented as a generator expression.

Similarly, the cy_delta_ext and cy_delta_conf helper functions convert the input bag to a Python list of port, value tuples.

delta_conf()
delta_ext()
delta_int()
output_func()
ta()
class devs.devs.Digraph

Bases: object

Python extension type that wraps the C++ wrapper class for the adevs Digraph class

For now, we only provide Atomic models. I.e. nested network models are not supported yet.

An instance of the C++ Digraph class takes ownership of added components, i.e. deletes the components at the end of its lifetime. This is why we increase the reference count to the Python object as soon as we add it to the Digraph. Upon deletion of the Digraph, the reference count is decreased. https://github.com/smiz/adevs/blob/aae196ba660259ac32fc254bad810f4b4185d52f/include/adevs_digraph.h#L205

add()
couple()
class devs.devs.IOBag

Bases: object

Python extension base type that wraps an existing C++ I/O bag

For constant bags, only the internal pointer _thisconstptr is used. For non-const bags, both internal pointers _thisconstptr and _thisptr are used.

empty()
size()
class devs.devs.InputBag

Bases: devs.devs.IOBag

Python extension type that wraps an existing C++ I/O bag

class devs.devs.OutputBag

Bases: devs.devs.IOBag

Python extension type that wraps an existing C++ I/O bag

To construct an instance in Cython, use the CreateOutputBag factory function.

When inserting port/values, increase the reference counter for Python objects.

insert()
class devs.devs.Simulator

Bases: object

Python extension type that wraps the adevs C++ Simulator class

Note that the adevc C++ Simulator class does not assume ownership of the model. Hence, when using a Python wrapper Simulator instance, we need to keep the Python wrapper Digraph or AtomicBase-subclassed instance in scope as well. When the model Python instance goes out of scope, the internal C++ pointer gets deleted, rendering the Simulator defunct.

execute_next_event()
execute_until()
next_event_time()
Module contents

Copyright 2014 The pydevs Developers

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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