Non-homogeneous arrival process

[augustodarruda]

Is there a way to vary the arrival process as the simulation flows?

In my problem, the arrival process varies with time, at 6am, very few customers joins the queue, but at 11am we have a peak of customers joining the queue, then it starts do decrease.

I'm building a custom arrival function, but I'm starting to wonder if there is another way to do tackle this in simmer.

[Enchufa2]

As usual with simmer, there are several options available to suit your needs. You could simply precompute all your arrivals up to the time you want to simulate into a dataframe, with relative (interarrival) or absolute times, then attach it and run the entire thing. This method is particularly suitable for feeding preexisting experimental data. Example:

library(simmer)

t <- trajectory() %>%
  log_("Here I am")

df_rel <- data.frame(time = rep(2, 5))
df_abs <- data.frame(time = seq(2, 10, 2))

env <- simmer() %>%
  add_dataframe("dummy_rel", t, df_rel) %>%
  add_dataframe("dummy_abs", t, df_abs, time="absolute")

run(env)
#> 2: dummy_rel0: Here I am
#> 2: dummy_abs0: Here I am
#> 4: dummy_rel1: Here I am
#> 4: dummy_abs1: Here I am
#> 6: dummy_rel2: Here I am
#> 6: dummy_abs2: Here I am
#> 8: dummy_rel3: Here I am
#> 8: dummy_abs3: Here I am
#> 10: dummy_rel4: Here I am
#> 10: dummy_abs4: Here I am
#> simmer environment: anonymous | now: 10 | next: 
#> { Monitor: in memory }
#> { Source: dummy_rel | monitored: 1 | n_generated: 5 }
#> { Source: dummy_abs | monitored: 1 | n_generated: 5 }

On the other hand, generator functions can be as complex as you need. You can access the simulation time using now(), so you may apply e.g. a different rate depending on the time of the day:

env <- simmer() %>%
  add_generator("dummy", t, function() {
    t <- now(env) %% 24
    rate <- if (t > 8 && t < 18) 0.5 else 0.1
    rexp(1, rate)
  })

run(env, 48) # 48 hours
#> 0.394713: dummy0: Here I am
#> 7.78957: dummy1: Here I am
#> 19.9987: dummy2: Here I am
#> 27.249: dummy3: Here I am
#> 27.2921: dummy4: Here I am
#> 27.9629: dummy5: Here I am
#> 31.8869: dummy6: Here I am
#> 41.9274: dummy7: Here I am
#> 43.7204: dummy8: Here I am
#> simmer environment: anonymous | now: 48 | next: 59.4903857030956
#> { Monitor: in memory }
#> { Source: dummy | monitored: 1 | n_generated: 10 }

[augustodarruda]

Thanks for the help. But why am I getting this error when I try to run the second part of your answer?

Error: Invalid input type, expected 'character' actual 'environment'

Am I missing something?

[Enchufa2]

Not sure. Copy-pasting works for me. Did you modify something?

[augustodarruda]

Didn't change anything. Will restart R and runit again.

[augustodarruda]

worked, something was bugging my session