apis/examples_python/transport8a_8py_source.html

import os

import sys

from gams import GamsWorkspace, GamsModifier

from multiprocessing import Lock, Process, Queue


GAMS_MODEL = """

Sets

    i   canning plants   / seattle, san-diego /

    j   markets          / new-york, chicago, topeka / ;


Parameters

    a(i)  capacity of plant i in cases

    /   seattle     350

        san-diego   600  /


    b(j)  demand at market j in cases

    /   new-york    325

        chicago     300

        topeka      275  / ;


Table d(i,j)  distance in thousands of miles

                  new-york       chicago      topeka

    seattle          2.5           1.7          1.8

    san-diego        2.5           1.8          1.4  ;


Scalar f      freight in dollars per case per thousand miles  /90/ ;

Scalar bmult  demand multiplier /1/;


Parameter c(i,j)  transport cost in thousands of dollars per case ;

    c(i,j) = f * d(i,j) / 1000 ;


Variables

    x(i,j)  shipment quantities in cases

    z       total transportation costs in thousands of dollars ;


Positive Variable x ;


Equations

    cost        define objective function

    supply(i)   observe supply limit at plant i

    demand(j)   satisfy demand at market j ;


cost ..        z  =e=  sum((i,j), c(i,j)*x(i,j)) ;


supply(i) ..   sum(j, x(i,j))  =l=  a(i) ;


demand(j) ..   sum(i, x(i,j))  =g=  bmult*b(j) ;


Model transport /all/ ;

"""


def scen_solve(cp_file, bmult_queue, queue_lock, io_lock):

    ws = GamsWorkspace()

    cp = ws.add_checkpoint(cp_file)

    mi = cp.add_modelinstance()

    bmult = mi.sync_db.add_parameter("bmult", 0, "demand multiplier")

    opt = ws.add_options()

    opt.all_model_types = "cplex"


    # instantiate the GAMSModelInstance and pass a model definition and GAMSModifier to declare bmult mutable

    mi.instantiate("transport use lp min z", GamsModifier(bmult), opt)

    bmult.add_record().value = 1.0


    while True:

        # dynamically get a bmult value from the queue instead of passing it to the different processes at creation time

        queue_lock.acquire()

        if bmult_queue.empty():

            queue_lock.release()

            return

        b = bmult_queue.get()

        queue_lock.release()

        bmult.first_record().value = b

        mi.solve()


        # we need to make the ouput a critical section to avoid messed up report informations

        io_lock.acquire()

        print("Scenario bmult=" + str(b) + ":")

        print("  Modelstatus: " + str(mi.model_status))

        print("  Solvestatus: " + str(mi.solver_status))

        print("  Obj: " + str(mi.sync_db.get_variable("z").find_record().level))

        io_lock.release()


if __name__ == "__main__":

    sys_dir = sys.argv[1] if len(sys.argv) > 1 else None

    ws = GamsWorkspace(system_directory=sys_dir)

    cp = ws.add_checkpoint()


    # initialize a GamsCheckpoint by running a GamsJob

    job = ws.add_job_from_string(GAMS_MODEL)

    job.run(checkpoint=cp)

    cp_file = os.path.join(ws.working_directory, cp.name) + ".g00"


    bmult_queue = Queue()

    bmult = [0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3]

    for b in bmult:

        bmult_queue.put(b)


    # solve multiple model instances in parallel

    io_lock = Lock()

    queue_lock = Lock()


    nr_workers = 4

    processes = {}

    for i in range(nr_workers):

        processes[i] = Process(

            target=scen_solve, args=(cp_file, bmult_queue, queue_lock, io_lock)

        )

        processes[i].start()

    for i in range(nr_workers):

        processes[i].join()