[Question]: What happens when service discipline returns None?

[galenseilis]

I am contemplating what would happen if I had a service discipline that sometimes returns None. As an extreme case to get a sense of the behaviour of Ciw I created a service discipline that always returns None. Here is the minimum reproducible example:

import ciw
import pandas as pd

def just_none(individuals):
    return None

N = ciw.create_network(
    arrival_distributions=[ciw.dists.Exponential(rate=5)],
    service_distributions=[ciw.dists.Exponential(rate=5)],
    service_disciplines=[just_none],
    number_of_servers=[1]
)

ciw.seed(2018)
Q = ciw.Simulation(N)
Q.simulate_until_max_time(1)

df = pd.DataFrame(
    Q.get_all_records()
    )

print(df.to_markdown(index=False))

The output (below) suggests that customers are being served. I had assumed the opposite would occur.

id_number	customer_class	original_customer_class	node	arrival_date	waiting_time	service_start_date	service_time	service_end_date	time_blocked	exit_date	destination	queue_size_at_arrival	queue_size_at_departure	server_id	record_type
1	Customer	Customer	1	0.152353	0	0.152353	0.00561602	0.157969	0	0.157969	-1	0	0	1	service
2	Customer	Customer	1	0.349852	0	0.349852	0.141531	0.491383	0	0.491383	-1	0	1	1	service
4	Customer	Customer	1	0.5283	0	0.5283	0.0537462	0.582046	0	0.582046	-1	1	2	1	service

What is the behaviour of Ciw when a service discipline returns None?

[geraintpalmer]

Good questions! I did not anticipate this the service disciplines being used like this.

Service discipline functions gives the next customer be be served from the queue.

So any customer that arrives when the server is free always gets served. They are never selected from the queue.

In your case, when customers 1, 2 and 4 arrived there was no one in service, and so they all began service straight away. When customer 3 arrived there was someone in service, they joined the queue, but never left because of the service discipline returning None. When customer 4 arrived, customer 3 was waiting, but not in service, and the server was free, to they began service immediately.

Hope that helps.