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Queue Example

In this example, we introduce another Py-DES component called Queue. Queues are an essential part of almost every system in real life.

pydes.Queue(sim, capacity=inf)

Queues are used to acumulate objects in a buffer and retrieved them from it.

Parameters:

Name Type Description Default
sim Simulator

The simulator instance.

 required
capacity float | int

The maximun lenght of the queue.

 inf

Methods:

Name Description
put

tries to insert a new member into the queue and waits if the queue is full.
get

tries to get one member from the queue and waits if the queue is empty.

Constructor for Queue class.

Parameters:

Name Type Description Default
sim Simulator

The simulator instance.

 required

get()

Get an item from the queue.

Waits until there is an item available in the queue.

Returns:

Name Type Description
Any Any

The item retrieved from the queue.

put(member)

Put an item into the queue or waits if the queue is full.

Parameters:

Name Type Description Default
member Any

The item to be put into the queue.

 required

size()

Get the size of the queue. Its equivalent to the number of member inside.

Returns:

Name Type Description
int int

The number of items in the queue.

Imports

First of all we import the necessary objects from pydes

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from dataclasses import dataclass
import random
from pydes import Component, Queue, Simulator

Define Model

For this example we define a first process that is going to put elements into the Queue with a time interval.

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@dataclass
class Element:
    id: int


class Process1(Component):
    def __init__(self, sim: Simulator, queue: Queue):
        self.sim = sim
        self.queue = queue

    def main(self):
        # generating elements:
        i = 0
        while True:
            e = Element(i)
            self.sim.record(self.id, f"{e} created")
            self.sim.sleep(random.randint(5, 10))
            self.queue.put(e)
            self.sim.record(self.id, "inserted element into queue")
            self.sim.record(self.id, f"{self.queue.size()} elements in queue")
            i += 1

There is another process running that will try get elements from the same Queue and wait if it is empty.

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class Process2(Component):
    def __init__(self, sim: Simulator, queue: Queue):
        self.sim = sim
        self.queue = queue

    def main(self):
        while True:
            self.sim.record(self.id, "requesting an element from queue")
            e = self.queue.get()
            self.sim.record(self.id, f"got {e} from queue")
            self.sim.record(self.id, "waiting before getting next element")
            self.sim.sleep(random.randint(10, 20))

Run Simulation

With all this defined. We build the Simulator and the components involved. We schedule both processes and then start the simulation.

sim = Simulator()
queue = Queue(sim)
p1 = Process1(sim, queue)
p2 = Process2(sim, queue)
sim.schedule(p1.main)
sim.schedule(p2.main)
sim.run(until=30)

The simulation output would resemble the following

| ------------------------------ | --------------- | ---------------------------------------- | ------------------------------ |
| time                           | component       | value                                    | description                    |
| ------------------------------ | --------------- | ---------------------------------------- | ------------------------------ |
|                                |                 |                                          |                                |
| 0                              | Process1.0      | Element(id=0) created                    | None                           |
| 0                              | Process2.0      | requesting an element from queue         | None                           |
| 6                              | Process1.0      | inserted element into queue              | None                           |
| 6                              | Process1.0      | 1 elements in queue                      | None                           |
| 6                              | Process1.0      | Element(id=1) created                    | None                           |
| 6                              | Process2.0      | got Element(id=0) from queue             | None                           |
| 6                              | Process2.0      | waiting before getting next element      | None                           |
| 13                             | Process1.0      | inserted element into queue              | None                           |
| 13                             | Process1.0      | 1 elements in queue                      | None                           |
| 13                             | Process1.0      | Element(id=2) created                    | None                           |
| 19                             | Process1.0      | inserted element into queue              | None                           |
| 19                             | Process1.0      | 2 elements in queue                      | None                           |
| 19                             | Process1.0      | Element(id=3) created                    | None                           |
| 26                             | Process2.0      | requesting an element from queue         | None                           |
| 26                             | Process2.0      | got Element(id=1) from queue             | None                           |
| 26                             | Process2.0      | waiting before getting next element      | None                           |
| 28                             | Process1.0      | inserted element into queue              | None                           |
| 28                             | Process1.0      | 2 elements in queue                      | None                           |
| 28                             | Process1.0      | Element(id=4) created                    | None                           |
| 38                             | Process1.0      | inserted element into queue              | None                           |
| 38                             | Process1.0      | 3 elements in queue                      | None                           |
| 38                             | Process1.0      | Element(id=5) created                    | None                           |